Subversion Repositories SvarDOS

/* DR-DOS specific boot processing: check for F5/F8 boot key presses and reset

 * the wild pointer to DR-DOS kernel (CONFIG.SYS) environment because it is not

 * allocated memory hence will be overwritten soon.

 * details: https://github.com/SvarDOS/edrdos/issues/83

 * this function returns 0, FLAG_SKIP_AUTOEXEC or FLAG_STEPBYSTEP */

static unsigned char drdos_init(void) {

  unsigned short kernenvseg = 0;

  unsigned char far *e;

  unsigned short far *scancode;

  /* If I am init then query kernel's private data via INT 21,4458 (DR-DOS).

   * On success (CF not set) ES:BX contains a pointer to the private data.

   * Segment of kernel's environment is at offset 12h. This environment may

   * be terminated by a 1Ah code followed by a boot key scan code to record

   * an F5 or F8 key press during boot time. */

  _asm {

    push ax

    push bx

    push es

    mov ax, 0x4458       /* DR-DOS 5+ get ptr to internal variable table */

    int 0x21             /* ES:BX contains the ptr to the var table */

    jc FAIL              /* not DR-DOS */

    add bx, 0x12

    mov ax, es:[bx]      /* read the segment of the kernel environment */

    mov kernenvseg, ax   /* save the kern env segment for later */

    xor ax, ax

    mov es:[bx], ax      /* reset the pointer to kernel env as done by DR COMMAND.COM */

    FAIL:

    pop es

    pop bx

    pop ax

  }

  if (kernenvseg == 0) return(0); /* either not DR-DOS, or kern env was read already or something failed */

  e = MK_FP(kernenvseg, 0);

  /* move forward until the DRDOS' environment 1Ah terminator is found */

  while (*e != 0x1A) e++;

  e++;

  /* next I have the boot key press scancode: either 0x0000, 0x3F00 or 0x4200

   * 0x3F00 means "F5 was pressed" while 0x4200 is for F8 */

  scancode = (void far *)e;

  if (*scancode == 0x3F00) return(FLAG_SKIP_AUTOEXEC);

  if (*scancode == 0x4200) return(FLAG_STEPBYSTEP);

/*

  printf("kernel env seg is at %04X and starts with bytes 0x%02X 0x%02X 0x%02X 0x%02X\r\n", kernenvseg, e[0], e[1], e[2], e[3]);

  {

    int i;

    printf("=== KERNEL ENV BEGINS ===\r\n");

    for (i = 0; i < 100; i++) {

      printf("%c", e[i]);

    }

    printf("\r\n=== KERNEL ENV ENDS, DUMP FOLLOWS ===\r\n");

    for (i = 0; i < 260; i++) {

      if ((i > 0) && ((i % 26) == 0)) printf("\r\n");

      printf("%02X ", e[i]);

    }

    printf("\r\n=== DUMP ENDS ===\r\n");

  }

*/

  return(0);

}

    /* DR-DOS specific: if I am the init shell (zeroed env seg) then detect F5/F8 now

     * This must be done BEFORE rmod_install() because DR-DOS's boot environment

     * is located at an unallocated memory location that is likely to be overwritten

     * by rmod_install(). */

    cfg.flags |= drdos_init();