source: trunk/libs/newlib/src/libgloss/doc/porting.texi @ 514

\input texinfo        @c                    -*- Texinfo -*-
@setfilename porting.info
@settitle Embed with GNU

@c
@c This file documents the process of porting the GNU tools to an
@c embedded environment.
@c

@finalout
@setchapternewpage off
@iftex
@raggedbottom
@global@parindent=0pt
@end iftex

@titlepage
@title Embed With GNU
@subtitle Porting The GNU Tools To Embedded Systems
@sp 4
@subtitle Spring 1995
@subtitle Very *Rough* Draft
@author Rob Savoye - Cygnus Support
@page

@vskip 0pt plus 1filll
Copyright @copyright{} 1993, 1994, 1995 Cygnus Support

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions.
@end titlepage

@ifnottex
@format
START-INFO-DIR-ENTRY
* Embed with GNU: (porting-).         Embed with GNU
END-INFO-DIR-ENTRY
@end format
Copyright (c) 1993, 1994, 1995 Cygnus Support

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions.

@node Top
@top Embed with GNU

@end ifnottex
@strong{Rough Draft}

The goal of this document is to gather all the information needed to
port the GNU tools to a new embedded target in one place. This will
duplicate some info found in the other manual for the GNU tools, but
this should be all you'll need. 

@menu
* Libgloss::            Libgloss, a library of board support packages.
* GCC::                 Porting GCC/G++ to a new embedded target.
* Libraries::           Making Newlib run on an new embedded target.
* GDB::                 Making GDB understand a new back end.
* Binutils::            Using the GNU binary utilities.
* Code Listings::       Listings of the commented source code from the
                        text.
@end menu

@node Libgloss, GCC, Top, Top
@chapter Libgloss
Libgloss is a library for all the details that usually get glossed over.
This library refers to things like startup code, and usually I/O support
for @code{gcc} and @code{C library}. The C library used through out
this manual is @code{newlib}. Newlib is a ANSI conforming C library
developed by Cygnus Support. Libgloss could easily be made to
support other C libraries, and it can be used standalone as well. The
standalone configuration is typically used when bringing up new
hardware, or on small systems. 

For a long time, these details were part of newlib. This approach worked
well when a complete tool chain only had to support one system. A tool
chain refers to the series of compiler passes required to produce a
binary file that will run on an embedded system. For C, the passes are
cpp, gcc, gas, ld. Cpp is the preprocessor, which process all the header
files and macros. Gcc is the compiler, which produces assembler from the
processed C files. Gas assembles the code into object files, and then ld
combines the object files and binds the code to addresses and produces
the final executable image. 

Most of the time a tool chain does only have to support one target
execution environment. An example of this would be a tool chain for the
AMD 29k processor family. All of the execution environments for this
processor have the same interface, the same memory map, and the same
I/O code. In this case all of the support code is under newlib/libc/sys.
Libgloss's creation was forced initially because of the @code{cpu32}
processor family. There are many different execution environments for
this line, and they vary wildly. newlib itself has only a few
dependencies that it needs for each target. These are explained later in
this doc. The hardware dependent part of newlib was reorganized into a
separate directory structure within newlib called the stub dirs. It was
initially called this because most of the routines newlib needs for a
target were simple stubs that do nothing, but return a value to the
application. They only exist so the linker can produce a final
executable image. This work was done during the early part of 1993.

After a while it became apparent that this approach of isolating the
hardware and systems files together made sense. Around this same time
the stub dirs were made to run standalone, mostly so it could also be
used to support GDB's remote debugging needs. At this time it was
decided to move the stub dirs out of newlib and into it's own separate
library so it could be used standalone, and be included in various other
GNU tools without having to bring in all of newlib, which is large. The
new library is called Libgloss, for Gnu Low-level OS support. 

@menu
* Supported targets::           What targets libgloss currently
                                supports.
* Building libgloss::           How to configure and built libgloss
                                for a target.
* Board support::               How to add support for a new board.
@end menu

@node Supported targets, Building libgloss, Libgloss, Libgloss
@section Supported Targets
Currently libgloss is being used for the following targets:

@menu
* Sparclite::                   Fujitsu's sparclite.
* CPU32::                       Various m68k based targets.
* Mips::                        Mips code based targets.
* PA-RISC::                     Precision Risc Organization..
@end menu

@node Sparclite, CPU32, , Supported targets
@subsection Sparclite Targets Supported
@c FIXME: put links to the docs in etc/targetdoc
This is for the Fujitsu Sparclite family of processors. Currently this 
covers the ex930, ex931, ex932, ex933, and the ex934. In addition to the
I/O code a startup file, this has a GDB debug-stub that gets linked into
your application. This is an exception handler style debug stub. For
more info, see the section on Porting GDB. @ref{GDB,,Porting GDB}.

The Fujitsu eval boards use a host based terminal program to load and
execute programs on the target. This program, @code{pciuh} is relatively
new (in 1994) and it replaced the previous ROM monitor which had the
shell in the ROM. GDB uses the the GDB remote protocol, the relevant
source files from the gdb sources are remote-sparcl.c. The debug stub is
part of libgloss and is called sparcl-stub.c.

@node CPU32, Mips, Sparclite, Supported targets
@subsection Motorola CPU32 Targets supported
This refers to Motorola's m68k based CPU32 processor family. The crt0.S
startup file should be usable with any target environment, and it's
mostly just the I/O code and linker scripts that vary. Currently there
is support for the Motorola MVME line of 6U VME boards and IDP
line of eval boards. All of the
Motorola VME boards run @code{Bug}, a ROM based debug monitor.
This monitor has the feature of using user level traps to do I/O, so
this code should be portable to other MVME boards with little if any
change. The startup file also can remain unchanged. About the only thing
that varies is the address for where the text section begins. This can
be accomplished either in the linker script, or on the command line
using the @samp{-Ttext [address]}.

@c FIXME: Intermetrics or ISI wrote rom68k ?
There is also support for the @code{rom68k} monitor as shipped on
Motorola's IDP eval board line. This code should be portable across the
range of CPU's the board supports. There is also GDB support for this
target environment in the GDB source tree. The relevant files are
gdb/monitor.c, monitor.h, and rom58k-rom.c. The usage of these files is
discussed in the GDB section.

@node Mips, PA-RISC, CPU32, Supported targets
@subsection Mips core Targets Supported
The Crt0 startup file should run on any mips target that doesn't require
additional hardware initialization. The I/O code so far only supports a
custom LSI33k based RAID disk controller board. It should easy to
change to support the IDT line of eval boards. Currently the two
debugging protocols supported by GDB for mips targets is IDT's mips
debug protocol, and a customized hybrid of the standard GDB remote
protocol and GDB's standard ROM monitor support. Included here is the
debug stub for the hybrid monitor. This supports the LSI33k processor,
and only has support for the GDB protocol commands @code{g}, @code{G},
@code{m}, @code{M}, which basically only supports the register and
memory reading and writing commands. This is part of libgloss and is
called lsi33k-stub.c.

The crt0.S should also work on the IDT line of eval boards, but has only
been run on the LSI33k for now. There is no I/O support for the IDT eval
board at this time. The current I/O code is for a customized version of
LSI's @code{pmon} ROM monitor. This uses entry points into the monitor,
and should easily port to other versions of the pmon monitor. Pmon is
distributed in source by LSI.

@node PA-RISC, , Mips, Supported targets
@subsection PA-RISC Targets Supported
This supports the various boards manufactured by the HP-PRO consortium.
This is a group of companies all making variations on the PA-RISC
processor. Currently supported are ports to the WinBond @samp{Cougar}
board based around their w89k version of the PA. Also supported is the
Oki op50n processor.

There is also included, but never built an unfinished port to the HP 743
board. This board is the main CPU board for the HP700 line of industrial
computers. This target isn't exactly an embedded system, in fact it's
really only designed to load and run HP-UX. Still, the crt0.S and I/O
code are fully working. It is included mostly because their is a barely
functioning exception handler GDB debug stub, and I hope somebody could
use it. The other PRO targets all use GDB's ability to talk to ROM
monitors directly, so it doesn't need a debug stub. There is also a
utility that will produce a bootable file by HP's ROM monitor. This is
all included in the hopes somebody else will finish it. :-)

Both the WinBond board and the Oki board download srecords. The WinBond
board also has support for loading the SOM files as produced by the
native compiler on HP-UX. WinBond supplies a set of DOS programs that
will allow the loading of files via a bidirectional parallel port. This
has never been tested with the output of GNU SOM, as this manual is
mostly for Unix based systems. 

@node Building libgloss, Board support, Supported targets, Libgloss
@section Configuring and building libgloss.

Libgloss uses an autoconf based script to configure. Autoconf scripts
are portable shell scripts that are generated from a configure.in file.
Configure input scripts are based themselves on m4. Most configure
scripts run a series of tests to determine features the various
supported features of the target. For features that can't be determined
by a feature test, a makefile fragment is merged in. The configure
process leaves creates a Makefile in the build directory. For libgloss,
there are only a few configure options of importance. These are --target
and --srcdir. 

Typically libgloss is built in a separate tree just for objects. In this
manner, it's possible to have a single source tree, and multiple object
trees. If you only need to configure for a single target environment,
then you can configure in the source tree. The argument for --target is
a config string. It's usually safest to use the full canonical opposed
to the target alias. So, to configure for a CPU32 (m68k) with a separate
source tree, use:

@smallexample
../src/libgloss/configure --verbose --target m68k-coff
@end smallexample

The configure script is in the source tree. When configure is invoked
it will determine it's own source tree, so the --srcdir is would be
redundant here.

Once libgloss is configured, @code{make} is sufficient to build it. The
default values for @code{Makefiles} are typically correct for all
supported systems. The test cases in the testsuite will also built
automatically as opposed to a @code{make check}, where test binaries
aren't built till test time. This is mostly cause the libgloss
testsuites are the last thing built when building the entire GNU source
tree, so it's a good test of all the other compilation passes.

The default values for the Makefiles are set in the Makefile fragment
merged in during configuration. This fragment typically has rules like

@smallexample
CC_FOR_TARGET = `if [ -f $$@{OBJROOT@}/gcc/xgcc ] ; \
        then echo $@{OBJROOT@}/gcc/xgcc -B$@{OBJROOT@}/gcc/ ; \
        else t='$@{program_transform_name@}'; echo gcc | sed -e '' $$t ; fi`
@end smallexample

Basically this is a runtime test to determine whether there are freshly
built executables for the other main passes of the GNU tools. If there
isn't an executable built in the same object tree, then
@emph{transformed}the generic tool name (like gcc) is transformed to the
name typically used in GNU cross compilers. The  names are
typically based on the target's canonical name, so if you've configured
for @code{m68k-coff} the transformed name is @code{m68k-coff-gcc} in
this case. If you install with aliases or rename the tools, this won't
work, and it will always look for tools in the path. You can force the a
different name to work by reconfiguring with the
@code{--program-transform-name} option to configure. This option takes a
sed script like this @code{-e s,^,m68k-coff-,} which produces tools
using the standard names (at least here at Cygnus). 

The search for the other GNU development tools is exactly the same idea. 
This technique gets messier when build options like @code{-msoft-float}
support are used. The Makefile fragments set the @code{MUTILIB}
variable, and if it is set, the search path is modified. If the linking
is done with an installed cross compiler, then none of this needs to be
used. This is done so libgloss will build automatically with a fresh,
and uninstalled object tree. It also makes it easier to debug the other
tools using libgloss's test suites.

@node Board support, , Building libgloss, Libgloss
@section Adding Support for a New Board

This section explains how to add support for a new board to libgloss.
In order to add support for a board, you must already have developed a
toolchain for the target architecture.

All of the changes you will make will be in the subdirectory named
after the architecture used by your board.  For example, if you are
developing support for a new ColdFire board, you will modify files in
the @file{m68k} subdirectory, as that subdirectory contains support
for all 68K devices, including architecture variants like ColdFire.

In general, you will be adding three components: a @file{crt0.S} file
(@pxref{Crt0}), a linker script (@pxref{Linker Scripts}), and a
hardware support library.  Each should be prefixed with the name of
your board.  For example, if you ard adding support for a new Surf
board, then you will be adding the assembly @file{surf-crt0.S} (which
will be assembled into @file{surf-crt0.o}), the linker script
@file{surf.ld}, and other C and assembly files which will be combined
into the hardware support library @file{libsurf.a}.

You should modify @file{Makefile.in} to define new variables
corresponding to your board.  Although there is some variation between
architectures, the general convention is to use the following format:

@example
# The name of the crt0.o file.
SURF_CRT0    = surf-crt0.o
# The name of the linker script.
SURF_SCRIPTS = surf.ld
# The name of the hardware support library.
SURF_BSP     = libsurf.a
# The object files that make up the hardware support library.
SURF_OBJS    = surf-file1.o surf-file2.o 
# The name of the Makefile target to use for installation.
SURF_INSTALL = install-surf
@end example

Then, you should create the @code{$@{SURF_BSP@}} and
@code{$@{SURF_INSTALL@}} make targets.  Add @code{$@{SURF_CRT0@}} to
the dependencies for the @code{all} target and add
@code{$@{SURF_INSTALL@}} to the dependencies for the @code{install}
target.  Now, when libgloss is built and installed, support for your
BSP will be installed as well.

@node GCC, Libraries, Libgloss, Top
@chapter Porting GCC

Porting GCC requires two things, neither of which has anything to do
with GCC. If GCC already supports a processor type, then all the work in
porting GCC is really a linker issue. All GCC has to do is produce
assembler output in the proper syntax. Most of the work is done by the
linker, which is described elsewhere.

Mostly all GCC does is format the command line for the linker pass. The
command line for GCC is set in the various config subdirectories of gcc.
The options of interest to us are @code{CPP_SPEC} and
@code{STARTFILE_SPEC}. CPP_SPEC sets the builtin defines for your
environment. If you support multiple environments with the same
processor, then OS specific defines will need to be elsewhere. 
@c FIXME: Check these names

@code{STARTFILE_SPEC}

Once you have linker support, GCC will be able to produce a fully linked
executable image. The only @emph{part} of GCC that the linker wants is a
crt0.o, and a memory map. If you plan on running any programs that do
I/O of any kind, you'll need to write support for the C library, which
is described elsewhere. 

@menu
* Overview::            An overview as to the compilation passes.
* Options::             Useful GCC options for embedded systems.
@end menu

@node Overview, Options, , GCC
@section Compilation passes

GCC by itself only compiles the C or C++ code into assembler. Typically
GCC invokes all the passes required for you. These passes are cpp, cc1,
gas, ld. @code{cpp} is the C preprocessor. This will merge in the
include files, expand all macros definitions, and process all the
@code{#ifdef} sections. To see the output of ccp, invoke gcc with the
@code{-E} option, and the preprocessed file will be printed on the
stdout. cc1 is the actual compiler pass that produces the assembler for
the processed file. GCC is actually only a driver program for all the
compiler passes. It will format command line options for the other passes.
The usual command line GCC uses for the final link phase will have LD
link in the startup code and additional libraries by default.

GNU AS started it's life to only function as a compiler pass, but
these days it can also be used as a source level assembler. When used as
a source level assembler, it has a companion assembler preprocessor
called @code{gasp}. This has a syntax similar to most other assembler
macros packages. GAS emits a relocatable object file from the assembler
source. The object file contains the executable part of the application,
and debug symbols.

LD is responsible for resolving the addresses and symbols to something
that will be fully self-contained. Some RTOS's use relocatable object
file formats like @code{a.out}, but more commonly the final image will
only use absolute addresses for symbols. This enables code to be burned
into PROMS as well. Although LD can produce an executable image, there
is usually a hidden object file called @code{crt0.o} that is required as
startup code.  With this startup code and a memory map, the executable
image will actually run on the target environment. @ref{Crt0,,Startup
Files}.

The startup code usually defines a special symbol like @code{_start}
that is the default base address for the application, and the first
symbol in the executable image. If you plan to use any routines from the
standard C library, you'll also need to implement the functions that
this library is dependent on. @ref{Libraries,,Porting Newlib}.

@node Options, , Overview, GCC
@c FIXME: Need stuff here about -fpic, -Ttext, etc...

Options for the various development tools are covered in more detail
elsewhere. Still, the amount of options can be an overwhelming amount of
stuff, so the options most suited to embedded systems are summarized
here. If you use GCC as the main driver for all the passes, most of the
linker options can be passed directly to the compiler. There are also
GCC options that control how the GCC driver formats the command line
arguments for the linker.

@menu
* GCC Options::         Options for the compiler.
* GAS Options::         Options for the assembler.
* LD Options::          Options for the linker.
@end menu

@node GCC Options, GAS Options, , Options
Most of the GCC options that we're interested control how the GCC driver
formats the options for the linker pass. 

@c FIXME: this section is still under work.
@table @code
@item -nostartfiles
@item -nostdlib
@item -Xlinker
Pass the next option directly to the linker.

@item -v
@item -fpic
@end table

@node GAS Options, LD Options, GCC Options, Options
@c FIXME: Needs stuff here

@node LD Options, , GAS Options, Options
@c FIXME: Needs stuff here


@node Libraries, GDB, GCC, Top
@chapter Porting newlib

@menu
* Crt0::                Crt0.S.
* Linker Scripts::      Linker scripts for memory management.
* What to do now::      Tricks for manipulating formats.
* Libc::                Making libc work.
@end menu

@node Crt0, Linker Scripts, , Libraries
@section Crt0, the main startup file
       
To make a program that has been compiled with GCC to run, you
need to write some startup code. The initial piece of startup code is
called a crt0. (C RunTime 0) This is usually written in assembler, and
it's object gets linked in first, and bootstraps the rest of the
application when executed. This file needs to do the following things.

@enumerate
@item
Initialize anything that needs it. This init section varies. If you are
developing an application that gets download to a ROM monitor, then
there is usually no need for any special initialization. The ROM monitor
handles it for you.

If you plan to burn your code in a ROM, then the crt0 typically has to
do all the hardware initialization that is required to run an
application. This can include things like initializing serial ports or
run a memory check. It all depends on the hardware. 
    
@item
Zero the BSS section. This is for uninitialized data. All the addresses in
this section need to be initialized to zero so that programs that forget
to check new variables default value will get unpredictable results.

@item
Call main()
This is what basically starts things running. If your ROM monitor
supports it, then first setup argc and argv for command line arguments
and an environment pointer. Then branch to main(). For G++ the the main
routine gets a branch to __main inserted by the code generator at the
very top.  __main() is used by G++ to initialize it's internal tables.
__main() then returns back to your original main() and your code gets
executed.

@item
Call exit()
After main() has returned, you need to cleanup things and return control
of the hardware from the application. On some hardware, there is nothing
to return to, especially if your program is in ROM.  Sometimes the best
thing to do in this case is do a hardware reset, or branch back to the
start address all over again.

When there is a ROM monitor present, usually a user trap can be called
and then the ROM takes over. Pick a safe vector with no side
effects. Some ROMs have a builtin trap handler just for this case.
@end enumerate
portable between all the m68k based boards we have here.
@ref{crt0.S,,Example Crt0.S}. 


@smallexample
/* ANSI concatenation macros.  */

#define CONCAT1(a, b) CONCAT2(a, b)
#define CONCAT2(a, b) a ## b
@end smallexample
These we'll use later.

@smallexample
/* These are predefined by new versions of GNU cpp.  */

#ifndef __USER_LABEL_PREFIX__
#define __USER_LABEL_PREFIX__ _
#endif

/* Use the right prefix for global labels.  */
#define SYM(x) CONCAT1 (__USER_LABEL_PREFIX__, x)

@end smallexample

These macros are to make this code portable between both @emph{COFF} and
@emph{a.out}. @emph{COFF} always has an @var{_ (underline)} prepended on
the front of all global symbol names. @emph{a.out} has none.

@smallexample
#ifndef __REGISTER_PREFIX__
#define __REGISTER_PREFIX__
#endif

/* Use the right prefix for registers.  */
#define REG(x) CONCAT1 (__REGISTER_PREFIX__, x)

#define d0 REG (d0)
#define d1 REG (d1)
#define d2 REG (d2)
#define d3 REG (d3)
#define d4 REG (d4)
#define d5 REG (d5)
#define d6 REG (d6)
#define d7 REG (d7)
#define a0 REG (a0)
#define a1 REG (a1)
#define a2 REG (a2)
#define a3 REG (a3)
#define a4 REG (a4)
#define a5 REG (a5)
#define a6 REG (a6)
#define fp REG (fp)
#define sp REG (sp)
@end smallexample

This is for portability between assemblers. Some register names have a
@var{%} or @var{$} prepended to the register name.

@smallexample
/*
 * Set up some room for a stack. We just grab a chunk of memory.
 */
        .set    stack_size, 0x2000
        .comm   SYM (stack), stack_size
@end smallexample

Set up space for the stack. This can also be done in the linker script,
but it typically gets done here.

@smallexample
/*
 * Define an empty environment.
 */
        .data
        .align 2
SYM (environ):
        .long 0
@end smallexample

Set up an empty space for the environment. This is bogus on any most ROM 
monitor, but we setup a valid address for it, and pass it to main. At
least that way if an application checks for it, it won't crash.

@smallexample
        .align  2
        .text
        .global SYM (stack)

        .global SYM (main)
        .global SYM (exit)
/* 
 * This really should be __bss_start, not SYM (__bss_start).
 */
        .global __bss_start
@end smallexample

Setup a few global symbols that get used elsewhere. @var{__bss_start}
needs to be unchanged, as it's setup by the linker script.

@smallexample
/*
 * start -- set things up so the application will run.
 */
SYM (start):
        link    a6, #-8
        moveal  #SYM (stack) + stack_size, sp

/*
 * zerobss -- zero out the bss section
 */
        moveal  #__bss_start, a0
        moveal  #SYM (end), a1
1:
        movel   #0, (a0)
        leal    4(a0), a0
        cmpal   a0, a1
        bne     1b
@end smallexample

The global symbol @code{start} is used by the linker as the default
address to use for the @code{.text} section. then it zeros the
@code{.bss} section so the uninitialized data will all be cleared. Some
programs have wild side effects from having the .bss section let
uncleared. Particularly it causes problems with some implementations of
@code{malloc}. 

@smallexample
/*
 * Call the main routine from the application to get it going.
 * main (argc, argv, environ)
 * We pass argv as a pointer to NULL.
 */
        pea     0
        pea     SYM (environ)
        pea     sp@@(4)
        pea     0
        jsr     SYM (main)
        movel   d0, sp@@-
@end smallexample

Setup the environment pointer and jump to @code{main()}. When
@code{main()} returns, it drops down to the @code{exit} routine below.

@smallexample
/*
 * _exit -- Exit from the application. Normally we cause a user trap
 *          to return to the ROM monitor for another run.
 */
SYM (exit):
        trap    #0
@end smallexample

Implementing @code{exit} here is easy. Both the @code{rom68k} and @code{bug}
can handle a user caused exception of @code{zero} with no side effects.
Although the @code{bug} monitor has a user caused trap that will return
control to the ROM monitor, this solution has been more portable.

@node Linker Scripts, What to do now, Crt0, Libraries
@section Linker scripts for memory management

The linker script sets up the memory map of an application. It also
sets up default values for variables used elsewhere by sbrk() and the
crt0. These default variables are typically called @code{_bss_start} and
@code{_end}.

For G++, the constructor and destructor tables must also be setup here.
The actual section names vary depending on the object file format. For
@code{a.out} and @code{coff}, the three main sections are @code{.text},
@code{.data}, and @code{.bss}.

Now that you have an image, you can test to make sure it got the
memory map right. You can do this by having the linker create a memory
map (by using the @code{-Map} option), or afterwards by using @code{nm} to
check a few critical addresses like @code{start}, @code{bss_end}, and
@code{_etext}.

Here's a breakdown of a linker script for a m68k based target board.
See the file @code{libgloss/m68k/idp.ld}, or go to the appendixes in
the end of the manual. @ref{idp.ld,,Example Linker Script}. 

@smallexample
STARTUP(crt0.o)
OUTPUT_ARCH(m68k)
INPUT(idp.o)
SEARCH_DIR(.)
__DYNAMIC  =  0;
@end smallexample

The @code{STARTUP} command loads the file specified so that it's
first. In this case it also doubles to load the file as well, because
the m68k-coff configuration defaults to not linking in the crt0.o by
default. It assumes that the developer probably has their own crt0.o.
This behavior is controlled in the config file for each architecture.
It's a macro called @code{STARTFILE_SPEC}, and if it's set to
@code{null}, then when @code{gcc} formats it's command line, it doesn't
add @code{crto.o}. Any file name can be specified here, but the default
is always @code{crt0.o}.

Course if you only use @code{ld} to link, then the control of whether or
not to link in @code{crt0.o} is done on the command line. If you have
multiple crto files, then you can leave this out all together, and link
in the @code{crt0.o} in the makefile, or by having different linker
scripts. Sometimes this is done for initializing floating point
optionally, or to add device support.

The @code{OUTPUT_ARCH} sets architecture the output file is for.

@code{INPUT} loads in the file specified. In this case, it's a relocated
library that contains the definitions for the low-level functions need
by libc.a.  This could have also been specified on the command line, but
as it's always needed, it might as well be here as a default.
@code{SEARCH_DIR} specifies the path to look for files, and
@code{_DYNAMIC} means in this case there are no shared libraries. 

@c FIXME: Check the linker manual to make sure this is accurate.
@smallexample
/*
 * Setup the memory map of the MC68ec0x0 Board (IDP)
 * stack grows up towards high memory. This works for
 * both the rom68k and the mon68k monitors.
 */
MEMORY
@{
  ram     : ORIGIN = 0x10000, LENGTH = 2M
@}
@end smallexample

This specifies a name for a section that can be referred to later in the 
script. In this case, it's only a pointer to the beginning of free RAM
space, with an upper limit at 2M. If the output file exceeds the upper
limit, it will produce an error message.

@smallexample
/*
 * stick everything in ram (of course)
 */
SECTIONS
@{
  .text :
  @{
    CREATE_OBJECT_SYMBOLS
    *(.text)
     etext  =  .;
     __CTOR_LIST__ = .;
     LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)
    *(.ctors)
     LONG(0)
     __CTOR_END__ = .;
     __DTOR_LIST__ = .;
     LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)
    *(.dtors)
     LONG(0)
     __DTOR_END__ = .;
    *(.lit)
    *(.shdata)
  @}  > ram
  .shbss SIZEOF(.text) + ADDR(.text) :  @{
    *(.shbss)
  @}
@end smallexample

Set up the @code{.text} section. In a @code{COFF} file, .text is where
all the actual instructions are. This also sets up the @emph{CONTRUCTOR}
and the @emph{DESTRUCTOR} tables for @code{G++}. Notice that the section
description redirects itself to the @emph{ram} variable setup earlier.

@smallexample
  .talias :      @{ @}  > ram
  .data  : @{
    *(.data)
    CONSTRUCTORS
    _edata  =  .;
  @} > ram
@end smallexample

Setup the @code{.data} section. In a @code{coff} file, this is where all
he initialized data goes. @code{CONSTRUCTORS} is a special command used
by @code{ld}.

@smallexample
  .bss SIZEOF(.data) + ADDR(.data) :
  @{
   __bss_start = ALIGN(0x8);
   *(.bss)
   *(COMMON)
      end = ALIGN(0x8);
      _end = ALIGN(0x8);
      __end = ALIGN(0x8);
  @}
  .mstack  : @{ @}  > ram
  .rstack  : @{ @}  > ram
  .stab  . (NOLOAD) : 
  @{
    [ .stab ]
  @}
  .stabstr  . (NOLOAD) :
  @{
    [ .stabstr ]
  @}
@}
@end smallexample

Setup the @code{.bss} section. In a @code{COFF} file, this is where
unitialized data goes. The symbols @code{_bss_start} and @code{_end}
are setup here for use by the @code{crt0.o} when it zero's the
@code{.bss} section. 


@node What to do now, Libc, Linker Scripts, Libraries
@section What to do when you have a binary image

A few ROM monitors load binary images, typically @code{a.out}, but most all
will load an @code{srecord}. An srecord is an ASCII representation of a binary 
image. At it's simplest, an srecord is an address, followed by a byte
count, followed by the bytes, and a 2's compliment checksum. A whole
srecord file has an optional @emph{start} record, and a required @emph{end} 
record. To make an srecord from a binary image, the GNU @code{objcopy} program
is used. This will read the image and make an srecord from it. To do
this, invoke objcopy like this: @code{objcopy -O srec infile outfile}. Most
PROM burners also read srecords or a similar format. Use @code{objdump -i} to
get a list of support object files types for your architecture. 

@node Libc, , What to do now, Libraries
@section Libraries

This describes @code{newlib}, a freely available libc replacement. Most
applications use calls in the standard C library. When initially linking
in libc.a, several I/O functions are undefined. If you don't plan on
doing any I/O, then you're OK, otherwise they need to be created. These
routines are read, write, open, close. sbrk, and kill. Open & close
don't need to be fully supported unless you have a filesystems, so
typically they are stubbed out. Kill is also a stub, since you can't do
process control on an embedded system. 

Sbrk() is only needed by applications that do dynamic memory
allocation. It's uses the symbol @code{_end} that is setup in the linker
script. It also requires a compile time option to set the upper size
limit on the heap space. This leaves us with read and write, which are
required for serial I/O. Usually these two routines are written in C,
and call a lower level function for the actual I/O operation. These two
lowest level I/O primitives are inbyte() and outbyte(), and are also
used by GDB back ends if you've written an exception handler. Some
systems also implement a havebyte() for input as well. 

Other commonly included functions are routines for manipulating
LED's on the target (if they exist) or low level debug help. Typically a
putnum() for printing words and bytes as a hex number is helpful, as
well as a low-level print() to output simple strings. 

As libg++ uses the I/O routines in libc.a, if read and write work,
then libg++ will also work with no additional changes. 

@menu
* I/O Support::         Functions that make serial I/O work.
* Memory Support::      Memory support.
* Misc Support::        Other needed functions.
* Debugging::            Useful Debugging Functions
@end menu

@node I/O Support, Memory Support, , Libc
@subsection Making I/O work

@node Memory Support, Misc Support, I/O Support, Libc
@subsection Routines for dynamic memory allocation
To support using any of the memory functions, you need to implement
sbrk(). @code{malloc()}, @code{calloc()}, and @code{realloc()} all call
@code{sbrk()} at there lowest level. @code{caddr_t} is defined elsewhere
as @code{char *}. @code{RAMSIZE} is presently a compile time option. All
this does is move a pointer to heap memory and check for the upper
limit. @ref{glue.c,,Example libc support code}. @code{sbrk()} returns a
pointer to the previous value before more memory was allocated.

@smallexample
/* _end is set in the linker command file *
extern caddr_t _end;/

/* just in case, most boards have at least some memory */
#ifndef RAMSIZE
#  define RAMSIZE             (caddr_t)0x100000
#endif

/*
 * sbrk -- changes heap size size. Get nbytes more
 *         RAM. We just increment a pointer in what's
 *         left of memory on the board.
 */
caddr_t
sbrk(nbytes)
     int nbytes;
@{
  static caddr_t heap_ptr = NULL;
  caddr_t        base;

  if (heap_ptr == NULL) @{
    heap_ptr = (caddr_t)&_end;
  @}

  if ((RAMSIZE - heap_ptr) >= 0) @{
    base = heap_ptr;
    heap_ptr += nbytes;
    return (base);
  @} else @{
    errno = ENOMEM;
    return ((caddr_t)-1);
  @}
@}
@end smallexample

@node Misc Support, Debugging, Memory Support, Libc
@subsection Misc support routines

These are called by @code{newlib} but don't apply to the embedded
environment. @code{isatty()} is self explanatory. @code{kill()} doesn't
apply either in an environment withno process control, so it justs
exits, which is a similar enough behavior. @code{getpid()} can safely
return any value greater than 1. The value doesn't effect anything in
@code{newlib} because once again there is no process control.

@smallexample
/*
 * isatty -- returns 1 if connected to a terminal device,
 *           returns 0 if not. Since we're hooked up to a
 *           serial port, we'll say yes and return a 1.
 */
int
isatty(fd)
     int fd;
@{
  return (1);
@}

/*
 * getpid -- only one process, so just return 1.
 */
#define __MYPID 1
int
getpid()
@{
  return __MYPID;
@}

/*
 * kill -- go out via exit...
 */
int
kill(pid, sig)
     int pid;
     int sig;
@{
  if(pid == __MYPID)
    _exit(sig);
  return 0;
@}
@end smallexample

@node Debugging, , Misc Support, Libc
@subsection Useful debugging functions

There are always a few useful functions for debugging your project in
progress. I typically implement a simple @code{print()} routine that
runs standalone in liblgoss, with no @code{newlib} support. The I/O
function @code{outbyte()} can also be used for low level debugging. Many
times print will work when there are problems that cause @code{printf()} to
cause an exception. @code{putnum()} is just to print out values in hex
so they are easier to read.

@smallexample
/*
 * print -- do a raw print of a string
 */ 
int
print(ptr)
char *ptr;
@{
  while (*ptr) @{
    outbyte (*ptr++);
  @}
@}

/*
 * putnum -- print a 32 bit number in hex
 */
int
putnum (num)
unsigned int num;
@{
  char  buffer[9];
  int   count;
  char  *bufptr = buffer;
  int   digit;
  
  for (count = 7 ; count >= 0 ; count--) @{
    digit = (num >> (count * 4)) & 0xf;
    
    if (digit <= 9)
      *bufptr++ = (char) ('0' + digit);
    else
      *bufptr++ = (char) ('a' - 10 + digit);
  @}

  *bufptr = (char) 0;
  print (buffer);
  return;
@}
@end smallexample

If there are LEDs on the board, they can also be put to use for
debugging when the serial I/O code is being written. I usually implement
a @code{zylons()} function, which strobes the LEDS (if there is more
than one) in sequence, creating a rotating effect. This is convenient
between I/O to see if the target is still alive. Another useful LED
function is @code{led_putnum()}, which takes a digit and displays it as
a bit pattern or number. These usually have to be written in assembler
for each target board. Here are a number of C based routines that may be
useful.

@code{led_putnum()} puts a number on a single digit segmented
LED display. This LED is set by setting a bit mask to an address, where
1 turns the segment off, and 0 turns it on. There is also a little
decimal point on the LED display, so it gets the leftmost bit. The other
bits specify the segment location. The bits look like:

@smallexample
        [d.p | g | f | e | d | c | b | a ] is the byte.
@end smallexample

The locations are set up as:

@smallexample 
             a
           -----
        f |     | b
          |  g  |
           -----
          |     |
        e |     | c
           -----
             d
@end smallexample

This takes a number that's already been converted to a string, and
prints it. 

@smallexample
#define LED_ADDR        0xd00003

void
led_putnum ( num )
char num;
@{
    static unsigned char *leds = (unsigned char *)LED_ADDR;
    static unsigned char num_bits [18] = @{
      0xff,                                             /* clear all */
      0xc0, 0xf9, 0xa4, 0xb0, 0x99, 0x92, 0x82, 0xf8, 0x80, 0x98, /* numbers 0-9 */
      0x98, 0x20, 0x3, 0x27, 0x21, 0x4, 0xe             /* letters a-f */
    @};

    if (num >= '0' && num <= '9')
      num = (num - '0') + 1;

    if (num >= 'a' && num <= 'f')
      num = (num - 'a') + 12;

    if (num == ' ')
      num = 0;

    *leds = num_bits[num];
@}

/*
 * zylons -- draw a rotating pattern. NOTE: this function never returns.
 */
void
zylons()
@{
  unsigned char *leds   = (unsigned char *)LED_ADDR;
  unsigned char curled = 0xfe;

  while (1)
    @{
      *leds = curled;
      curled = (curled >> 1) | (curled << 7);
      delay ( 200 );
    @}
@}
@end smallexample


@node GDB, Binutils, Libraries, Top
@chapter Writing a new GDB backend

Typically, either the low-level I/O routines are used for debugging, or
LEDs, if present. It is much easier to use GDb for debugging an
application. There are several different techniques used to have GDB work
remotely. Commonly more than one kind of GDB interface is used to cober
a wide variety of development needs.

The most common style of GDB backend is an exception handler for
breakpoints. This is also called a @emph{gdb stub}, and is requires the
two additional lines of init code in your @code{main()} routine. The GDB
stubs all use the GDB @emph{remote protocol}. When the application gets a
breakpoint exception, it communicates to GDB on the host.

Another common style of interfacing GDB to a target is by using an
existing ROM monitor. These break down into two main kinds, a similar
protocol to the GDB remote protocol, and an interface that uses the ROM
monitor directly. This kind has GDB simulating a human operator, and all
GDB does is work as a command formatter and parser. 

@menu
* GNU remote protocol::         The standard remote protocol.
* Exception handler::           A linked in exception handler.
* ROM monitors::                Using a ROM monitor as a backend.
* Other remote protocols::      Adding support for new protocols.
@end menu

@node GNU remote protocol, Exception handler, ,GDB
@section The standard remote protocol

The standard remote protocol is a simple, packet based scheme. A debug
packet whose contents are @emph{<data>} is encapsulated for transmission
in the form:

@smallexample
        $ <data> # CSUM1 CSUM2
@end smallexample

@emph{<data>} must be ASCII alphanumeric and cannot include characters
@code{$} or @code{#}.  If @emph{<data>} starts with two characters
followed by @code{:}, then the existing stubs interpret this as a
sequence number. For example, the command @code{g} is used to read the
values of the registers. So, a packet to do this would look like

@smallexample
        $g#67
@end smallexample

@emph{CSUM1} and @emph{CSUM2} are an ascii representation in hex of an
8-bit checksum of @emph{<data>}, the most significant nibble is sent first.
the hex digits 0-9,a-f are used.

A simple protocol is used when communicating with the target. This is
mainly to give a degree of error handling over the serial cable. For
each packet transmitted successfully, the target responds with a
@code{+} (@code{ACK}). If there was a transmission error, then the target
responds with a @code{-} (@code{NAK}). An error is determined when the
checksum doesn't match the calculated checksum for that data record.
Upon reciept of the @code{ACK}, @code{GDB} can then transmit the next
packet. 

Here is a list of the main functions that need to be supported. Each data
packet is a command with a set number of bytes in the command packet.
Most commands either return data, or respond with a @code{NAK}. Commands
that don't return data respond with an @code{ACK}. All data values are
ascii hex digits. Every byte needs two hex digits to represent t. This
means that a byte with the value @samp{7} becomes @samp{07}. On a 32 bit
machine this works out to 8 characters per word. All of the bytes in a
word are stored in the target byte order. When writing the host side of
the GDB protocol, be careful of byte order, and make sure that the code
will run on both big and little endian hosts and produce the same answers.

These functions are the minimum required to make a GDB backend work. All
other commands are optional, and not supported by all GDB backends.

@table @samp
@item  read registers  @code{g}

returns @code{XXXXXXXX...}

Registers are in the internal order for GDB, and the bytes in a register
are in the same order the machine uses. All values are in sequence
starting with register 0. All registers are listed in the same packet. A
sample packet would look like @code{$g#}.

@item   write registers @code{GXXXXXXXX...}
@code{XXXXXXXX} is the value to set the register to.  Registers are in
the internal order for GDB, and the bytes in a register are in the same
order the machine uses. All values are in sequence starting with
register 0. All registers values are listed in the same packet. A sample
packet would look like @code{$G000000001111111122222222...#}

returns @code{ACK} or @code{NAK}

@item   read memory     @code{mAAAAAAAA,LLLL}
@code{AAAAAAAA} is address, @code{LLLL} is length. A sample packet would
look like @code{$m00005556,0024#}. This would request 24 bytes starting
at address @emph{00005556}

returns @code{XXXXXXXX...}
@code{XXXXXXXX} is the memory contents. Fewer bytes than requested will
be returned if only part of the data can be read. This can be determined
by counting the values till the end of packet @code{#} is seen and
comparing that with the total count of bytes that was requested.

@item   write memory    @code{MAAAAAAAA,LLLL:XXXXXXXX}
@code{AAAAAAAA} is the starting address, @code{LLLL} is the number of
bytes to be written, and @code{XXXXXXXX} is value to be written. A
sample packet would look like
@code{$M00005556,0024:101010101111111100000000...#}

returns @code{ACK} or @code{NAK} for an error. @code{NAK} is also
returned when only part of the data is written.

@item   continue        @code{cAAAAAAAAA}
@code{AAAAAAAA} is address to resume execution at. If @code{AAAAAAAA} is
omitted, resume at the curent address of the @code{pc} register.

returns the same replay as @code{last signal}. There is no immediate
replay to @code{cont} until the next breakpoint is reached, and the
program stops executing.

@item   step            sAA..AA
@code{AA..AA} is address to resume
If @code{AA..AA} is omitted, resume at same address.

returns the same replay as @code{last signal}. There is no immediate
replay to @code{step} until the next breakpoint is reached, and the
program stops executing.

@item   last signal     @code{?}

This returns one of the following:

@itemize @bullet
@item @code{SAA}
Where @code{AA} is the number of the last signal.
Exceptions on the target are converted to the most similar Unix style
signal number, like @code{SIGSEGV}. A sample response of this type would
look like @code{$S05#}.

@item TAAnn:XXXXXXXX;nn:XXXXXXXX;nn:XXXXXXXX;
@code{AA} is the signal number.
@code{nn} is the register number.
@code{XXXXXXXX} is the register value.

@item WAA
The process exited, and @code{AA} is the exit status.  This is only
applicable for certains sorts of targets.

@end itemize

These are used in some GDB backends, but not all. 

@item write reg         @code{Pnn=XXXXXXXX}
Write register @code{nn} with value @code{XXXXXXXX}.

returns @code{ACK} or @code{NAK}

@item   kill request    k

@item   toggle debug    d
toggle debug flag (see 386 & 68k stubs)

@item   reset           r
reset -- see sparc stub.

@item   reserved        @code{other}
On other requests, the stub should ignore the request and send an empty
response @code{$#<checksum>}.  This way we can extend the protocol and GDB
can tell whether the stub it is talking to uses the old or the new.

@item   search          @code{tAA:PP,MM}
Search backwards starting at address @code{AA} for a match with pattern
PP and mask @code{MM}. @code{PP} and @code{MM} are 4 bytes.

@item   general query   @code{qXXXX}
Request info about XXXX.

@item   general set     @code{QXXXX=yyyy}
Set value of @code{XXXX} to @code{yyyy}.

@item   query sect offs @code{qOffsets}
Get section offsets.  Reply is @code{Text=xxx;Data=yyy;Bss=zzz}

@item   console output  Otext
Send text to stdout. The text gets display from the target side of the
serial connection.

@end table

Responses can be run-length encoded to save space.  A @code{*}means that
the next character is an ASCII encoding giving a repeat count which
stands for that many repetitions of the character preceding the @code{*}.
The encoding is n+29, yielding a printable character where n >=3 
(which is where run length encoding starts to win). You can't use a
value of where n >126 because it's only a two byte value. An example
would be a @code{0*03} means the same thing as @code{0000}.

@node Exception handler, ROM monitors, GNU remote protocol, GDB
@section A linked in exception handler

A @emph{GDB stub} consists of two parts, support for the exception
handler, and the exception handler itself. The exception handler needs
to communicate to GDB on the host whenever there is a breakpoint
exception. When GDB starts a program running on the target, it's polling
the serial port during execution looking for any debug packets. So when
a breakpoint occurs, the exception handler needs to save state, and send
a GDB remote protocol packet to GDB on the host. GDB takes any output
that isn't a debug command packet and displays it in the command window.

Support for the exception handler varies between processors, but the
minimum supported functions are those needed by GDB. These are functions
to support the reading and writing of registers, the reading and writing
of memory, start execution at an address, single step, and last signal.
Sometimes other functions for adjusting the baud rate, or resetting the
hardware are implemented. 

Once GDB gets the command packet from the breakpoint, it will read a few
registers and memory locations an then wait for the user. When the user
types @code{run} or @code{continue} a @code{continue} command is issued
to the backend, and control returns from the breakpoint routine to the
application.

@node ROM monitors, Other remote protocols, Exception handler, GDB
@section Using a ROM monitor as a backend
GDB also can mimic a human user and use a ROM monitors normal debug
commands as a backend. This consists mostly of sending and parsing
@code{ASCII} strings. All the ROM monitor interfaces share a common set
of routines in @code{gdb/monitor.c}. This supports adding new ROM
monitor interfaces by filling in a structure with the common commands
GDB needs. GDb already supports several command ROM monitors, including
Motorola's @code{Bug} monitor for their VME boards, and the Rom68k
monitor by Integrated Systems, Inc. for various m68k based boards. GDB
also supports the custom ROM monitors on the WinBond and Oki PA based
targets. There is builtin support for loading files to ROM monitors
specifically. GDB can convert a binary into an srecord and then load it
as an ascii file, or using @code{xmodem}.

@c FIXME: do I need trademark somethings here ? Is Integrated the right
@c company? 

@node Other remote protocols, ,ROM monitors, GDB 
@section Adding support for new protocols
@c FIXME: write something here 

@node Binutils, Code Listings, GDB, Top

@node Code Listings, idp.ld, Binutils, Top
@appendix Code Listings

@menu
* idp.ld::              A m68k linker script.
* crt0.S::              Crt0.S for an m68k.
* glue.c::              C based support for for Stdio functions.
* mvme.S::              Rom monitor based I/O support in assembler.
* io.c::                C based for memory mapped I/O.
* leds.c::              C based LED routines.
@end menu

@node idp.ld, crt0.S, Code Listings, Code Listings
@section Linker script for the IDP board

This is the linker script script that is used on the Motorola IDP board.

@example
STARTUP(crt0.o)
OUTPUT_ARCH(m68k)
INPUT(idp.o)
SEARCH_DIR(.)
__DYNAMIC  =  0;
/*
 * Setup the memory map of the MC68ec0x0 Board (IDP)
 * stack grows up towards high memory. This works for
 * both the rom68k and the mon68k monitors.
 */
MEMORY
@{
  ram     : ORIGIN = 0x10000, LENGTH = 2M
@}
/*
 * stick everything in ram (of course)
 */
SECTIONS
@{
  .text :
  @{
    CREATE_OBJECT_SYMBOLS
    *(.text)
     etext  =  .;
     __CTOR_LIST__ = .;
     LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)
    *(.ctors)
     LONG(0)
     __CTOR_END__ = .;
     __DTOR_LIST__ = .;
     LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)
    *(.dtors)
     LONG(0)
     __DTOR_END__ = .;
    *(.lit)
    *(.shdata)
  @}  > ram
  .shbss SIZEOF(.text) + ADDR(.text) :  @{
    *(.shbss)
  @} 
  .talias :      @{ @}  > ram
  .data  : @{
    *(.data)
    CONSTRUCTORS
    _edata  =  .;
  @} > ram

  .bss SIZEOF(.data) + ADDR(.data) :
  @{
   __bss_start = ALIGN(0x8);
   *(.bss)
   *(COMMON)
      end = ALIGN(0x8);
      _end = ALIGN(0x8);
      __end = ALIGN(0x8);
  @}
  .mstack  : @{ @}  > ram
  .rstack  : @{ @}  > ram
  .stab  . (NOLOAD) : 
  @{
    [ .stab ]
  @}
  .stabstr  . (NOLOAD) :
  @{
    [ .stabstr ]
  @}
@}
@end example

@node crt0.S, glue.c, idp.ld, Code Listings
@section crt0.S - The startup file

@example
/*
 * crt0.S -- startup file for m68k-coff
 * 
 */

        .title "crt0.S for m68k-coff"

/* These are predefined by new versions of GNU cpp.  */

#ifndef __USER_LABEL_PREFIX__
#define __USER_LABEL_PREFIX__ _
#endif

#ifndef __REGISTER_PREFIX__
#define __REGISTER_PREFIX__
#endif

/* ANSI concatenation macros.  */

#define CONCAT1(a, b) CONCAT2(a, b)
#define CONCAT2(a, b) a ## b

/* Use the right prefix for global labels.  */

#define SYM(x) CONCAT1 (__USER_LABEL_PREFIX__, x)

/* Use the right prefix for registers.  */

#define REG(x) CONCAT1 (__REGISTER_PREFIX__, x)

#define d0 REG (d0)
#define d1 REG (d1)
#define d2 REG (d2)
#define d3 REG (d3)
#define d4 REG (d4)
#define d5 REG (d5)
#define d6 REG (d6)
#define d7 REG (d7)
#define a0 REG (a0)
#define a1 REG (a1)
#define a2 REG (a2)
#define a3 REG (a3)
#define a4 REG (a4)
#define a5 REG (a5)
#define a6 REG (a6)
#define fp REG (fp)
#define sp REG (sp)

/*
 * Set up some room for a stack. We just grab a chunk of memory.
 */
        .set    stack_size, 0x2000
        .comm   SYM (stack), stack_size

/*
 * Define an empty environment.
 */
        .data
        .align 2
SYM (environ):
        .long 0

        .align  2
        .text
        .global SYM (stack)

        .global SYM (main)
        .global SYM (exit)
/* 
 * This really should be __bss_start, not SYM (__bss_start).
 */
        .global __bss_start

/*
 * start -- set things up so the application will run.
 */
SYM (start):
        link    a6, #-8
        moveal  #SYM (stack) + stack_size, sp

/*
 * zerobss -- zero out the bss section
 */
        moveal  #__bss_start, a0
        moveal  #SYM (end), a1
1:
        movel   #0, (a0)
        leal    4(a0), a0
        cmpal   a0, a1
        bne     1b

/*
 * Call the main routine from the application to get it going.
 * main (argc, argv, environ)
 * We pass argv as a pointer to NULL.
 */
        pea     0
        pea     SYM (environ)
        pea     sp@@(4)
        pea     0
        jsr     SYM (main)
        movel   d0, sp@@-

/*
 * _exit -- Exit from the application. Normally we cause a user trap
 *          to return to the ROM monitor for another run.
 */
SYM (exit):
        trap    #0
@end example

@node glue.c, mvme.S, crt0.S, Code Listings
@section C based "glue" code.

@example

/*
 * glue.c -- all the code to make GCC and the libraries run on
 *           a bare target board. These should work with any
 *           target if inbyte() and outbyte() exist.
 */

#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#ifndef NULL
#define NULL 0
#endif

/* FIXME: this is a hack till libc builds */
__main()
@{
  return;
@}

#undef errno
int errno;

extern caddr_t _end;                /* _end is set in the linker command file */
extern int outbyte();
extern unsigned char inbyte();
extern int havebyte();

/* just in case, most boards have at least some memory */
#ifndef RAMSIZE
#  define RAMSIZE             (caddr_t)0x100000
#endif

/*
 * read  -- read bytes from the serial port. Ignore fd, since
 *          we only have stdin.
 */
int
read(fd, buf, nbytes)
     int fd;
     char *buf;
     int nbytes;
@{
  int i = 0;

  for (i = 0; i < nbytes; i++) @{
    *(buf + i) = inbyte();
    if ((*(buf + i) == '\n') || (*(buf + i) == '\r')) @{
      (*(buf + i)) = 0;
      break;
    @}
  @}
  return (i);
@}

/*
 * write -- write bytes to the serial port. Ignore fd, since
 *          stdout and stderr are the same. Since we have no filesystem,
 *          open will only return an error.
 */
int
write(fd, buf, nbytes)
     int fd;
     char *buf;
     int nbytes;
@{
  int i;

  for (i = 0; i < nbytes; i++) @{
    if (*(buf + i) == '\n') @{
      outbyte ('\r');
    @}
    outbyte (*(buf + i));
  @}
  return (nbytes);
@}

/*
 * open -- open a file descriptor. We don't have a filesystem, so
 *         we return an error.
 */
int
open(buf, flags, mode)
     char *buf;
     int flags;
     int mode;
@{
  errno = EIO;
  return (-1);
@}

/*
 * close -- close a file descriptor. We don't need
 *          to do anything, but pretend we did.
 */
int
close(fd)
     int fd;
@{
  return (0);
@}

/*
 * sbrk -- changes heap size size. Get nbytes more
 *         RAM. We just increment a pointer in what's
 *         left of memory on the board.
 */
caddr_t
sbrk(nbytes)
     int nbytes;
@{
  static caddr_t heap_ptr = NULL;
  caddr_t        base;

  if (heap_ptr == NULL) @{
    heap_ptr = (caddr_t)&_end;
  @}

  if ((RAMSIZE - heap_ptr) >= 0) @{
    base = heap_ptr;
    heap_ptr += nbytes;
    return (base);
  @} else @{
    errno = ENOMEM;
    return ((caddr_t)-1);
  @}
@}

/*
 * isatty -- returns 1 if connected to a terminal device,
 *           returns 0 if not. Since we're hooked up to a
 *           serial port, we'll say yes and return a 1.
 */
int
isatty(fd)
     int fd;
@{
  return (1);
@}

/*
 * lseek -- move read/write pointer. Since a serial port
 *          is non-seekable, we return an error.
 */
off_t
lseek(fd,  offset, whence)
     int fd;
     off_t offset;
     int whence;
@{
  errno = ESPIPE;
  return ((off_t)-1);
@}

/*
 * fstat -- get status of a file. Since we have no file
 *          system, we just return an error.
 */
int
fstat(fd, buf)
     int fd;
     struct stat *buf;
@{
  errno = EIO;
  return (-1);
@}

/*
 * getpid -- only one process, so just return 1.
 */
#define __MYPID 1
int
getpid()
@{
  return __MYPID;
@}

/*
 * kill -- go out via exit...
 */
int
kill(pid, sig)
     int pid;
     int sig;
@{
  if(pid == __MYPID)
    _exit(sig);
  return 0;
@}

/*
 * print -- do a raw print of a string
 */ 
int
print(ptr)
char *ptr;
@{
  while (*ptr) @{
    outbyte (*ptr++);
  @}
@}

/*
 * putnum -- print a 32 bit number in hex
 */
int
putnum (num)
unsigned int num;
@{
  char  buffer[9];
  int   count;
  char  *bufptr = buffer;
  int   digit;
  
  for (count = 7 ; count >= 0 ; count--) @{
    digit = (num >> (count * 4)) & 0xf;
    
    if (digit <= 9)
      *bufptr++ = (char) ('0' + digit);
    else
      *bufptr++ = (char) ('a' - 10 + digit);
  @}

  *bufptr = (char) 0;
  print (buffer);
  return;
@}
@end example

@node mvme.S, io.c, glue.c, Code Listings
@section I/O assembler code sample

@example
/*
 * mvme.S -- board support for m68k
 */

        .title "mvme.S for m68k-coff"

/* These are predefined by new versions of GNU cpp.  */

#ifndef __USER_LABEL_PREFIX__
#define __USER_LABEL_PREFIX__ _
#endif

#ifndef __REGISTER_PREFIX__
#define __REGISTER_PREFIX__
#endif

/* ANSI concatenation macros.  */

#define CONCAT1(a, b) CONCAT2(a, b)
#define CONCAT2(a, b) a ## b

/* Use the right prefix for global labels.  */

#define SYM(x) CONCAT1 (__USER_LABEL_PREFIX__, x)

/* Use the right prefix for registers.  */

#define REG(x) CONCAT1 (__REGISTER_PREFIX__, x)

#define d0 REG (d0)
#define d1 REG (d1)
#define d2 REG (d2)
#define d3 REG (d3)
#define d4 REG (d4)
#define d5 REG (d5)
#define d6 REG (d6)
#define d7 REG (d7)
#define a0 REG (a0)
#define a1 REG (a1)
#define a2 REG (a2)
#define a3 REG (a3)
#define a4 REG (a4)
#define a5 REG (a5)
#define a6 REG (a6)
#define fp REG (fp)
#define sp REG (sp)
#define vbr REG (vbr)

        .align  2
        .text
        .global SYM (_exit)
        .global SYM (outln)
        .global SYM (outbyte)
        .global SYM (putDebugChar)
        .global SYM (inbyte)
        .global SYM (getDebugChar)
        .global SYM (havebyte)
        .global SYM (exceptionHandler)

        .set    vbr_size, 0x400
        .comm   SYM (vbr_table), vbr_size

/*
 * inbyte -- get a byte from the serial port
 *      d0 - contains the byte read in
 */
        .align  2
SYM (getDebugChar):             /* symbol name used by m68k-stub */
SYM (inbyte):
        link    a6, #-8
        trap    #15
        .word   inchr
        moveb   sp@@, d0
        extbl   d0
        unlk    a6
        rts

/*
 * outbyte -- sends a byte out the serial port
 *      d0 - contains the byte to be sent
 */
        .align  2
SYM (putDebugChar):             /* symbol name used by m68k-stub */
SYM (outbyte):
        link    fp, #-4
        moveb   fp@@(11), sp@@
        trap    #15
        .word   outchr
        unlk    fp
        rts

/*
 * outln -- sends a string of bytes out the serial port with a CR/LF
 *      a0 - contains the address of the string's first byte
 *      a1 - contains the address of the string's last byte
 */
        .align  2
SYM (outln):
        link    a6, #-8
        moveml  a0/a1, sp@@
        trap    #15
        .word   outln
        unlk    a6
        rts

/*
 * outstr -- sends a string of bytes out the serial port without a CR/LF
 *      a0 - contains the address of the string's first byte
 *      a1 - contains the address of the string's last byte
 */
        .align  2
SYM (outstr):
        link    a6, #-8
        moveml  a0/a1, sp@@
        trap    #15
        .word   outstr
        unlk    a6
        rts

/*
 * havebyte -- checks to see if there is a byte in the serial port,
 *           returns 1 if there is a byte, 0 otherwise.
 */
SYM (havebyte):
        trap    #15
        .word   instat
        beqs    empty
        movel   #1, d0
        rts
empty:
        movel   #0, d0
        rts

/*
 * These constants are for the MVME-135 board's boot monitor. They
 * are used with a TRAP #15 call to access the monitor's I/O routines.
 * they must be in the word following the trap call.
 */
        .set inchr, 0x0
        .set instat, 0x1
        .set inln, 0x2
        .set readstr, 0x3
        .set readln, 0x4
        .set chkbrk, 0x5

        .set outchr, 0x20
        .set outstr, 0x21
        .set outln, 0x22
        .set write, 0x23
        .set writeln, 0x24
        .set writdln, 0x25
        .set pcrlf, 0x26
        .set eraseln, 0x27
        .set writd, 0x28
        .set sndbrk, 0x29

        .set tm_ini, 0x40
        .set dt_ini, 0x42
        .set tm_disp, 0x43
        .set tm_rd, 0x44

        .set redir, 0x60
        .set redir_i, 0x61
        .set redir_o, 0x62
        .set return, 0x63
        .set bindec, 0x64

        .set changev, 0x67
        .set strcmp, 0x68
        .set mulu32, 0x69
        .set divu32, 0x6A
        .set chk_sum, 0x6B

@end example

@node io.c, leds.c, mvme.S, Code Listings
@section I/O code sample

@example
#include "w89k.h"

/*
 * outbyte -- shove a byte out the serial port. We wait till the byte 
 */
int
outbyte(byte)
     unsigned char byte;
@{
  while ((inp(RS232REG) & TRANSMIT) == 0x0) @{  @} ;
  return (outp(RS232PORT, byte));
@}

/*
 * inbyte -- get a byte from the serial port
 */
unsigned char
inbyte()
@{
  while ((inp(RS232REG) & RECEIVE) == 0x0) @{ @};
  return (inp(RS232PORT));
@}
@end example

@node leds.c, ,io.c, Code Listings
@section Led control sample

@example
/*
 * leds.h -- control the led's on a Motorola mc68ec0x0 board.
 */

#ifndef __LEDS_H__
#define __LEDS_H__

#define LED_ADDR        0xd00003
#define LED_0           ~0x1
#define LED_1           ~0x2
#define LED_2           ~0x4
#define LED_3           ~0x8
#define LED_4           ~0x10
#define LED_5           ~0x20
#define LED_6           ~0x40
#define LED_7           ~0x80
#define LEDS_OFF        0xff
#define LEDS_ON         0x0

#define FUDGE(x) ((x >= 0xa && x <= 0xf) ? (x + 'a') & 0x7f : (x + '0') & 0x7f)

extern void led_putnum( char );

#endif          /* __LEDS_H__ */

/*
 * leds.c -- control the led's on a Motorola mc68ec0x0 (IDP)board.
 */
#include "leds.h"

void zylons();
void led_putnum();

/*
 * led_putnum -- print a hex number on the LED. the value of num must be a char with
 *              the ascii value. ie... number 0 is '0', a is 'a', ' ' (null) clears
 *              the led display.
 *              Setting the bit to 0 turns it on, 1 turns it off.
 *              the LED's are controlled by setting the right bit mask in the base
 *              address. 
 *              The bits are:
 *                      [d.p | g | f | e | d | c | b | a ] is the byte.
 *
 *              The locations are:
 *              
 *                       a
 *                     -----
 *                  f |     | b
 *                    |  g  |
 *                     -----
 *                    |     |
 *                  e |     | c
 *                     -----
 *                       d                . d.p (decimal point)
 */
void
led_putnum ( num )
char num;
@{
    static unsigned char *leds = (unsigned char *)LED_ADDR;
    static unsigned char num_bits [18] = @{
      0xff,                                             /* clear all */
      0xc0, 0xf9, 0xa4, 0xb0, 0x99, 0x92, 0x82, 0xf8, 0x80, 0x98, /* numbers 0-9 */
      0x98, 0x20, 0x3, 0x27, 0x21, 0x4, 0xe             /* letters a-f */
    @};

    if (num >= '0' && num <= '9')
      num = (num - '0') + 1;

    if (num >= 'a' && num <= 'f')
      num = (num - 'a') + 12;

    if (num == ' ')
      num = 0;

    *leds = num_bits[num];
@}

/*
 * zylons -- draw a rotating pattern. NOTE: this function never returns.
 */
void
zylons()
@{
  unsigned char *leds   = (unsigned char *)LED_ADDR;
  unsigned char curled = 0xfe;

  while (1)
    @{
      *leds = curled;
      curled = (curled >> 1) | (curled << 7);
      delay ( 200 );
    @}
@}
@end example

@page
@contents
@c second page break makes sure right-left page alignment works right
@c with a one-page toc, even though we don't have setchapternewpage odd.
@page
@bye