source: soft/giet_vm/libs/stdio.c @ 163

//////////////////////////////////////////////////////////////////////////////////
// File     : stdio.c         
// Date     : 01/04/2010
// Author   : alain greiner & Joel Porquet
// Copyright (c) UPMC-LIP6
///////////////////////////////////////////////////////////////////////////////////
// The stdio.c and stdio.h files are part of the GIET nano-kernel.
// This library contains all user-level functions that contain a system call
// to access protected or shared ressources.
///////////////////////////////////////////////////////////////////////////////////

#include <stdarg.h>
#include <stdio.h>

#define SYSCALL_PROCID          0x00
#define SYSCALL_PROCTIME        0x01
#define SYSCALL_TTY_WRITE       0x02
#define SYSCALL_TTY_READ        0x03
#define SYSCALL_TIMER_WRITE     0x04
#define SYSCALL_TIMER_READ      0x05
#define SYSCALL_GCD_WRITE       0x06
#define SYSCALL_GCD_READ        0x07
#define SYSCALL_TTY_READ_IRQ    0x0A
#define SYSCALL_TTY_WRITE_IRQ   0x0B
#define SYSCALL_CTX_SWITCH      0x0D
#define SYSCALL_EXIT            0x0E
#define SYSCALL_PROC_NUMBER     0x0F
#define SYSCALL_FB_SYNC_WRITE   0x10
#define SYSCALL_FB_SYNC_READ    0x11
#define SYSCALL_FB_WRITE        0x12
#define SYSCALL_FB_READ         0x13
#define SYSCALL_FB_COMPLETED    0x14
#define SYSCALL_IOC_WRITE       0x15
#define SYSCALL_IOC_READ        0x16
#define SYSCALL_IOC_COMPLETED   0x17
#define SYSCALL_VOBJ_GET_VBASE  0x1A

//////////////////////////////////////////////////////////////////////////////////
//      sys_call()
// This generic C function is used to implement all system calls.
//////////////////////////////////////////////////////////////////////////////////
static inline unsigned int sys_call( unsigned int call_no,
                                     unsigned int arg_0, 
                                     unsigned int arg_1, 
                                     unsigned int arg_2, 
                                     unsigned int arg_3)
{
    register unsigned int reg_no_and_output asm("v0") = call_no;
    register unsigned int reg_a0            asm("a0") = arg_0;
    register unsigned int reg_a1            asm("a1") = arg_1;
    register unsigned int reg_a2            asm("a2") = arg_2;
    register unsigned int reg_a3            asm("a3") = arg_3;

    asm volatile(
            "syscall"
            : "=r" (reg_no_and_output)  /* output argument */
            : "r" (reg_a0),             /* input arguments */
              "r" (reg_a1),
              "r" (reg_a2),
              "r" (reg_a3),
              "r" (reg_no_and_output)
            : "memory",
            /* These persistant registers will be saved on the stack by the
             * compiler only if they contain relevant data. */
            "at",
            "v1",
            "ra",
            "t0",
            "t1",
            "t2",
            "t3",
            "t4",
            "t5",
            "t6",
            "t7",
            "t8",
            "t9"
               );
    return reg_no_and_output;
}

/////    MIPS32 related system calls  /////

////////////////////////////////////////////////////////////////////////////////////
//      procid()
// This function returns the processor identifier.
////////////////////////////////////////////////////////////////////////////////////
unsigned int procid()
{
    return sys_call(SYSCALL_PROCID, 
                    0, 0, 0, 0);
}
////////////////////////////////////////////////////////////////////////////////////
//      proctime()
// This function returns the local processor time (clock cycles since boot)
////////////////////////////////////////////////////////////////////////////////////
unsigned int proctime()
{
    return sys_call(SYSCALL_PROCTIME, 
                    0, 0, 0, 0);
}

//////  TTY device related system calls /////

////////////////////////////////////////////////////////////////////////////////////
//      tty_putc()
// This function displays a single ascii character on a terminal.
// The terminal index must be defined in the task context in the boot phase.
// It doesn't use the TTY_PUT_IRQ interrupt, and the associated kernel buffer.
// - Returns 1 if the character has been written, 0 otherwise.
////////////////////////////////////////////////////////////////////////////////////
unsigned int tty_putc(char byte)
{
    return sys_call(SYSCALL_TTY_WRITE,
                    (unsigned int)(&byte),
                    1,
                    0,0);
}
////////////////////////////////////////////////////////////////////////////////////
//       tty_puts()
// This function displays a string on a terminal.
// The terminal index must be defined in the task context in the boot phase.
// The string must be terminated by a NUL character.
// It doesn't use the TTY_PUT_IRQ interrupt, and the associated kernel buffer.
// - Returns the number of written characters.
////////////////////////////////////////////////////////////////////////////////////
unsigned int tty_puts(char *buf)
{
    unsigned int length = 0;
    while (buf[length] != 0)
    {
        length++;
    }
    return sys_call(SYSCALL_TTY_WRITE,
                    (unsigned int)buf,
                    length,
                    0,0);
}
////////////////////////////////////////////////////////////////////////////////////
//      tty_putw()
// This function displays the value of a 32-bit word with decimal characters.
// The terminal index must be defined in the task context in the boot phase.
// It doesn't use the TTY_PUT_IRQ interrupt, and the associated kernel buffer.
// Returns the number of written characters (should be equal to ten).
////////////////////////////////////////////////////////////////////////////////////
unsigned int tty_putw(unsigned int val)
{
    char buf[10];
    unsigned int i;
    for (i = 0; i < 10; i++)
    {
        buf[9-i] = (val % 10) + 0x30;
        val = val / 10;
    }
    return sys_call(SYSCALL_TTY_WRITE,
                    (unsigned int)buf,
                    10,
                    0,0);
}

////////////////////////////////////////////////////////////////////////////////////
// tty_getc()
// This blocking function fetches a single ascii character from a terminal.
// The terminal index must be defined in the task context in the boot phase.
// It doesn't use the IRQ_GET interrupt, and the associated kernel buffer.
// - Returns necessarily 0 when completed.
////////////////////////////////////////////////////////////////////////////////////
unsigned int tty_getc(char *byte)
{
    unsigned int ret = 0;
    while (ret == 0)
    {
        ret = sys_call(SYSCALL_TTY_READ,
                       (unsigned int)byte,
                       1,
                       0,0);
    }
    return 0;
}
////////////////////////////////////////////////////////////////////////////////////
//       tty_getc_irq()
// This blocking function fetches a single ascii character from a terminal.
// The terminal index must be defined in the task context in the boot phase.
// It uses the IRQ_GET interrupt, and the associated kernel buffer.
// - Returns 0 when completed.
////////////////////////////////////////////////////////////////////////////////////
unsigned int tty_getc_irq(char *byte)
{
    unsigned int ret = 0;
    while (ret == 0)
    {
        ret = sys_call(SYSCALL_TTY_READ_IRQ,
                       (unsigned int)byte,
                       1,
                       0,0);
    }
    return 0;
}
////////////////////////////////////////////////////////////////////////////////////
//      tty_gets_irq()
// This blocking function fetches a string from a terminal to a fixed length buffer.
// The terminal index must be defined in the task context in the boot phase.
// It uses the TTY_GET_IRQ interrupt, anf the associated kernel buffer.
// - Returns 0 when completed.
// - Up to (bufsize - 1) characters (including the non printable characters)
//   will be copied into buffer, and the string is always completed by a NUL
//   character.
// - The <LF> character is interpreted, as the function close the string with a
//   NUL character if <LF> is read.
// - The <DEL> character is interpreted, and the corresponding character(s) are
//   removed from the target buffer.
////////////////////////////////////////////////////////////////////////////////////
unsigned int tty_gets_irq(char *buf, unsigned int bufsize)
{
    unsigned int ret;
    unsigned char byte;
    unsigned int index = 0;

    while (index < (bufsize - 1))
    {
        do {
            ret = sys_call(SYSCALL_TTY_READ_IRQ,
                           (unsigned int)(&byte),
                           1,
                           0,0);
        } while (ret != 1);

        if ( byte == 0x0A )
            break; /* LF */
        else if ((byte == 0x7F) && (index > 0))
            index--; /* DEL */
        else
        {
            buf[index] = byte;
            index++;
        }
    }
    buf[index] = 0;
    return 0;
}
////////////////////////////////////////////////////////////////////////////////////
//      tty_getw_irq()
// This blocking function fetches a string of decimal characters (most
// significant digit first) to build a 32-bit unsigned integer.
// The terminal index must be defined in the task context in the boot phase.
// It uses the TTY_GET_IRQ interrupt, anf the associated kernel buffer.
// - Returns necessarily 0 when completed.
//
// - The non-blocking system function _tty_read_irq is called several times,
//   and the decimal characters are written in a 32 characters buffer until a
//   <LF> character is read.
// - The <DEL> character is interpreted, and previous characters can be
//   cancelled. All others characters are ignored.
// - When the <LF> character is received, the string is converted to an
//   unsigned int value. If the number of decimal digit is too large for the 32
//   bits range, the zero value is returned.
////////////////////////////////////////////////////////////////////////////////////
unsigned int tty_getw_irq(unsigned int *val)
{
    unsigned char buf[32];
    unsigned char byte;
    unsigned int save = 0;
    unsigned int dec = 0;
    unsigned int done = 0;
    unsigned int overflow = 0;
    unsigned int max = 0;
    unsigned int i;
    unsigned int ret;

    while (done == 0)
    {
        do {
            ret = sys_call(SYSCALL_TTY_READ_IRQ,
                           (unsigned int)(&byte),
                           1,
                           0,0);
        } while (ret != 1);

        if ((byte > 0x2F) && (byte < 0x3A)) /* decimal character */
        {
            buf[max] = byte;
            max++;
            tty_putc(byte);
        }
        else if ((byte == 0x0A) || (byte == 0x0D)) /* LF or CR character */
        {
            done = 1;
        }
        else if (byte == 0x7F) /* DEL character */
        {
            if (max > 0)
            {
                max--; /* cancel the character */
                tty_putc(0x08);
                tty_putc(0x20);
                tty_putc(0x08);
            }
        }
        if (max == 32) /* decimal string overflow */
        {
            for (i = 0; i < max; i++) /* cancel the string */
            {
                tty_putc(0x08);
                tty_putc(0x20);
                tty_putc(0x08);
            }
            tty_putc(0x30);
            *val = 0; /* return 0 value */
            return 0;
        }
    }

    /* string conversion */
    for (i = 0; i < max; i++)
    {
        dec = dec * 10 + (buf[i] - 0x30);
        if (dec < save)
            overflow = 1;
        save = dec;
    }

    /* check overflow */
    if (overflow == 0)
    {
        *val = dec; /* return decimal value */
    }
    else
    {
        for (i = 0; i < max; i++) /* cancel the string */
        {
            tty_putc(0x08);
            tty_putc(0x20);
            tty_putc(0x08);
        }
        tty_putc(0x30);
        *val = 0; /* return 0 value */
    }
    return 0;
}
////////////////////////////////////////////////////////////////////////////////////
// tty_printf()
// This function is a simplified version of the mutek_printf() function.
// The terminal index must be defined in the calling task context.
// It doesn't use the IRQ_PUT interrupt, and the associated kernel buffer.
// Only a limited number of formats are supported:
//   - %d : signed decimal
//   - %u : unsigned decimal
//   - %x : hexadecimal
//   - %c : char
//   - %s : string
// - Returns 0 if success, > 0 if error.
////////////////////////////////////////////////////////////////////////////////////
unsigned int tty_printf(char *format, ...)
{
    va_list ap;
    va_start(ap, format);
    unsigned int ret;

printf_text:

    while (*format) {
        unsigned int i;
        for (i = 0; format[i] && format[i] != '%'; i++)
            ;
        if (i) {
            ret = sys_call(SYSCALL_TTY_WRITE,
                    (unsigned int)format,
                    i,
                    0,0);
            if (ret != i)
                return 1; /* return error */
            format += i;
        }
        if (*format == '%') {
            format++;
            goto printf_arguments;
        }
    }

    va_end(ap);
    return 0;

printf_arguments:

    {
        int         val = va_arg(ap, long);
        char            buf[20];
        char*           pbuf;
        unsigned int        len = 0;
        static const char   HexaTab[] = "0123456789ABCDEF";
        unsigned int        i;

        switch (*format++) {
            case ('c'):             /* char conversion */
                len = 1;
                buf[0] = val;
                pbuf = buf;
                break;
            case ('d'):             /* decimal signed integer */
                if (val < 0) {
                    val = -val;
                    ret = sys_call(SYSCALL_TTY_WRITE,
                            (unsigned int)"-",
                            1,
                            0,0);
                    if (ret != 1)
                        return 1; /* return error */
                }
            case ('u'):             /* decimal unsigned integer */
                for( i=0 ; i<10 ; i++) {
                    buf[9-i] = HexaTab[val % 10];
                    if (!(val /= 10)) break;
                }
                len =  i+1;
                pbuf = &buf[9-i];
                break;
            case ('x'):             /* hexadecimal integer */
                ret = sys_call(SYSCALL_TTY_WRITE,
                        (unsigned int)"0x",
                        2,
                        0,0);
                if (ret != 2)
                    return 1; /* return error */
                for( i=0 ; i<8 ; i++) {
                    buf[7-i] = HexaTab[val % 16U];
                    if (!(val /= 16U)) break;
                }
                len =  i+1;
                pbuf = &buf[7-i];
                break;
            case ('s'):             /* string */
                {
                    char *str = (char*)val;
                    while ( str[len] ) len++;
                    pbuf = (char*)val;
                }
                break;
            default:
                goto printf_text;
        }

        ret = sys_call(SYSCALL_TTY_WRITE,
                (unsigned int)pbuf,
                len,
                0,0);
        if (ret != len)
            return 1;
        goto printf_text;
    }
}

/////  Timer device related system calls /////

#define TIMER_VALUE     0
#define TIMER_MODE      1
#define TIMER_PERIOD    2
#define TIMER_RESETIRQ  3

/*
 * timer_set_mode()
 *
 * This function defines the operation mode of a timer. The possible values for
 * this mode are:
 * - 0x0 : Timer not activated
 * - 0x1 : Timer activated, but no interrupt is generated
 * - 0x3 : Timer activarted and periodic interrupts generated
 *
 * - Returns 0 if success, > 0 if error.
 */
unsigned int timer_set_mode(unsigned int val)
{
    return sys_call(SYSCALL_TIMER_WRITE,
            TIMER_MODE,
            val,
            0, 0);
}

/*
 * timer_set_period()
 *
 * This function defines the period value of a timer to enable a periodic
 * interrupt.
 * - Returns 0 if success, > 0 if error.
 */
unsigned int timer_set_period(unsigned int val)
{
    return sys_call(SYSCALL_TIMER_WRITE,
            TIMER_PERIOD,
            val,
            0, 0);
}

/*
 * timer_reset_irq()
 *
 * This function resets the interrupt signal issued by a timer.
 * - Returns 0 if success, > 0 if error.
 */
unsigned int timer_reset_irq()
{
    return sys_call(SYSCALL_TIMER_WRITE,
            TIMER_RESETIRQ,
            0, 0, 0);
}

/*
 * timer_get_time()
 *
 * This function returns the current timing value of a timer.
 * - Returns 0 if success, > 0 if error.
 */
unsigned int timer_get_time(unsigned int *time)
{
    return sys_call(SYSCALL_TIMER_READ,
            TIMER_VALUE,
            (unsigned int)time,
            0, 0);
}

/////  GCD (Greatest Common Divider) device related system calls

#define GCD_OPA     0
#define GCD_OPB     1
#define GCD_START   2
#define GCD_STATUS  3

//////////////////////////////////////////////////////////////////////////////////
//      gcd_set_opa()
// This function sets the operand A in the GCD coprocessor.
// - Returns 0 if success, > 0 if error.
//////////////////////////////////////////////////////////////////////////////////
unsigned int gcd_set_opa(unsigned int val)
{
    return sys_call(SYSCALL_GCD_WRITE,
                    GCD_OPA,
                    val,
                    0, 0);
}
//////////////////////////////////////////////////////////////////////////////////
//      gcd_set_opb()
// This function sets operand B in the GCD coprocessor.
// - Returns 0 if success, > 0 if error.
//////////////////////////////////////////////////////////////////////////////////
unsigned int gcd_set_opb(unsigned int val)
{
    return sys_call(SYSCALL_GCD_WRITE,
                    GCD_OPB,
                    val,
                    0, 0);
}
//////////////////////////////////////////////////////////////////////////////////
//      gcd_start()
// This function starts the computation in the GCD coprocessor.
// - Returns 0 if success, > 0 if error.
//////////////////////////////////////////////////////////////////////////////////
unsigned int gcd_start()
{
    return sys_call(SYSCALL_GCD_WRITE,
                    GCD_START,
                    0, 0, 0);
}
//////////////////////////////////////////////////////////////////////////////////
//      gcd_get_status()
// This function gets the status fromn the GCD coprocessor.
// - The value is 0 when the coprocessor is idle (computation completed).
//////////////////////////////////////////////////////////////////////////////////
unsigned int gcd_get_status(unsigned int *val)
{
    return sys_call(SYSCALL_GCD_READ,
            GCD_STATUS,
            (unsigned int)val,
            0, 0);
}
//////////////////////////////////////////////////////////////////////////////////
//      gcd_get_result()
// This function gets the result of the computation from the GCD coprocessor.
//////////////////////////////////////////////////////////////////////////////////
unsigned int gcd_get_result(unsigned int *val)
{
    return sys_call(SYSCALL_GCD_READ,
            GCD_OPA,
            (unsigned int)val,
            0, 0);
}

///// Block device related system calls  /////

//////////////////////////////////////////////////////////////////////////////////
//      ioc_write()
// Transfer data from a memory buffer to a file on the block_device.
//     lba    : Logical Block Address (first block index)
//     buffer : base address of the memory buffer
//     count  : number of blocks to be transfered
// - Returns 0 if success, > 0 if error (e.g. memory buffer not in user space).
//////////////////////////////////////////////////////////////////////////////////
unsigned int ioc_write(unsigned int lba, void *buffer, unsigned int count)
{
    return sys_call(SYSCALL_IOC_WRITE,
            lba,
            (unsigned int)buffer,
            count,
            0);
}
//////////////////////////////////////////////////////////////////////////////////
//      ioc_read()
// Transfer data from a file on the block_device to a memory buffer.
//     lba    : Logical Block Address (first block index)
//     buffer : base address of the memory buffer
//     count  : number of blocks to be transfered
// - Returns 0 if success, > 0 if error (e.g. memory buffer not in user space).
//////////////////////////////////////////////////////////////////////////////////
unsigned int ioc_read(unsigned int lba, void *buffer, unsigned int count)
{
    return sys_call(SYSCALL_IOC_READ,
            lba,
            (unsigned int)buffer,
            count,
            0);
}
//////////////////////////////////////////////////////////////////////////////////
//      ioc_completed()
// This blocking function returns 0 when the I/O transfer is
// successfully completed, and returns 1 if an address error has been detected.
//////////////////////////////////////////////////////////////////////////////////
unsigned int ioc_completed()
{
    return sys_call(SYSCALL_IOC_COMPLETED,
            0, 0, 0, 0);
}

/////  Frame buffer device related system calls  /////

//////////////////////////////////////////////////////////////////////////////////
//      fb_sync_write()
// This blocking function use a memory copy strategy to transfer data from a
// user buffer to the frame buffer device in kernel space.
//     offset : offset (in bytes) in the frame buffer
//     buffer : base address of the memory buffer
//     length : number of bytes to be transfered
// - Returns 0 if success, > 0 if error (e.g. memory buffer not in user space).
//////////////////////////////////////////////////////////////////////////////////
unsigned int fb_sync_write(unsigned int offset, void *buffer, unsigned int length)
{
    return sys_call(SYSCALL_FB_SYNC_WRITE,
            offset,
            (unsigned int)buffer,
            length,
            0);
}
//////////////////////////////////////////////////////////////////////////////////
//      fb_sync_read()
// This blocking function use a memory copy strategy to transfer data from the
// frame buffer device in kernel space to an user buffer.
//     offset : offset (in bytes) in the frame buffer
//     buffer : base address of the user buffer
//     length : number of bytes to be transfered
// - Returns 0 if success, > 0 if error (e.g. memory buffer not in user space).
//////////////////////////////////////////////////////////////////////////////////
unsigned int fb_sync_read(unsigned int offset, void *buffer, unsigned int length)
{
    return sys_call(SYSCALL_FB_SYNC_READ,
            offset,
            (unsigned int)buffer,
            length,
            0);
}
//////////////////////////////////////////////////////////////////////////////////
//      fb_write()
// This non-blocking function use the DMA coprocessor to transfer data from a
// user buffer to the frame buffer device in kernel space.
// - offset : offset (in bytes) in the frame buffer
// - buffer : base address of the user buffer
// - length : number of bytes to be transfered
// The transfer completion is signaled by an IRQ, and must be tested by the
// fb_completed() function.
// - Returns 0 if success, > 0 if error (e.g. memory buffer not in user space).
//////////////////////////////////////////////////////////////////////////////////
unsigned int fb_write(unsigned int offset, void *buffer, unsigned int length)
{
    return sys_call(SYSCALL_FB_WRITE,
            offset,
            (unsigned int)buffer,
            length,
            0);
}
//////////////////////////////////////////////////////////////////////////////////
//       fb_read()
// This non-blocking function use the DMA coprocessor to transfer data from the
// frame buffer device in kernel space to an user buffer.
// - offset : offset (in bytes) in the frame buffer
// - buffer : base address of the memory buffer
// - length : number of bytes to be transfered
// The transfer completion is signaled by an IRQ, and must be tested by the
// fb_completed() function.
// - Returns 0 if success, > 0 if error (e.g. memory buffer not in user space).
//////////////////////////////////////////////////////////////////////////////////
unsigned int fb_read( unsigned int offset, 
                      void *buffer, 
                      unsigned int length )
{
    return sys_call(SYSCALL_FB_READ,
                    offset,
                    (unsigned int)buffer,
                    length,
                    0);
}
//////////////////////////////////////////////////////////////////////////////////
//      fb_completed()
// This blocking function returns when the transfer is completed.
// - Returns 0 if success, > 0 if error.
//////////////////////////////////////////////////////////////////////////////////
unsigned int fb_completed()
{
    return sys_call(SYSCALL_FB_COMPLETED,
                    0, 0, 0, 0);
}

///// Platform or mapping related system calls /////

//////////////////////////////////////////////////////////////////////////////////
//      mwmr_base()
//      TODO!
// This function returns in argument buffer the virtual base address 
// of a MWMR communication channel, identified by the two arguments
// vspace_name and channel_name.
// As the GIET does not support dynamic allocation, the MWMR channel
// must be declared in the mapping_info data structure to be initialised
// in the boot phase.
// - Returns the address if success,  0 if error ( channel not defined )
//////////////////////////////////////////////////////////////////////////////////
unsigned int vobj_get_vbase( char* vspace_name, char* vobj_name,
                        unsigned int vobj_type, unsigned int* vobj_buffer)
{
    return sys_call(SYSCALL_VOBJ_GET_VBASE, 
                    (unsigned int)vspace_name,
                    (unsigned int)vobj_name,
                    (unsigned int)vobj_type,
                    (unsigned int)vobj_buffer);
}
////////////////////////////////////////////////////////////////////////////////////
//      proc_number()
// This function returns in the buffer argument the number of processors 
// in the cluster specified by the cluster_id argument.
// - Returns 0 if success, > 0 if error ( cluster index too large )
////////////////////////////////////////////////////////////////////////////////////
unsigned int proc_number( unsigned int cluster_id,
                          unsigned int* buffer )
{
    return sys_call(SYSCALL_PROC_NUMBER, 
                    cluster_id,
                    (unsigned int)buffer,
                    0, 0);
}

/////  Miscellaneous system calls /////

//////////////////////////////////////////////////////////////////////////////////
// exit()
// This function stops execution of the calling task with a TTY message, 
// and enter an infinite loop.
// The task is blocked, but it still consume processor cycles ...
//////////////////////////////////////////////////////////////////////////////////
void exit()
{
    unsigned int proc_index = procid();
    sys_call(SYSCALL_EXIT, 
             proc_index, 
             0, 0, 0);
}
///////////////////////////////////////////////////////////////////////////////////
//      rand()
// This function returns a pseudo-random value derived from the processor cycle
// count. This value is comprised between 0 & 65535.
///////////////////////////////////////////////////////////////////////////////////
unsigned int rand()
{
    unsigned int x = sys_call(SYSCALL_PROCTIME, 
                              0, 0, 0, 0);
    if((x & 0xF) > 7)
        return (x*x & 0xFFFF);
    else
        return (x*x*x & 0xFFFF);
}
//////////////////////////////////////////////////////////////////////////////////
//      ctx_switch()
// The user task calling this function is descheduled and
// the processor is allocated to another task.
//////////////////////////////////////////////////////////////////////////////////
unsigned int ctx_switch()
{
    return sys_call(SYSCALL_CTX_SWITCH, 
                    0, 0, 0, 0);
}