/*
LD386.C -- undocumented DOS call from 386|DOS-Extender

Watcom C/386: wcl386 ld386
386|DOS-Extender: run386 ld386
*/

#include <stdlib.h>
#include <stdio.h>
#include <dos.h>

#ifndef __WATCOMC__
#error This program requires Watcom C/386
#endif

typedef struct {
    unsigned short intno, ds, es, fs, gs;
    unsigned eax, edx;
    } RMODE_PARAM_BLOCK;

main()
{
    RMODE_PARAM_BLOCK rpb;
    union REGS r;
    struct SREGS s;
    char far *doslist;
    unsigned lastdrv;
    
    /* load real-mode param block for INT 21h AH=52h */
    memset(&rpb, 0, sizeof(RMODE_PARAM_BLOCK));  /* zero it out */
    rpb.intno = 0x21;
    rpb.eax = 0x5200;
    
    /* call 386|DOS-Extender service to "Issue Real Mode Interrupt,
        Registers Specified" (INT 21h AX=2511h). */
    r.x.eax = 0x2511;
    r.x.edx = &rpb;
    segread(&s);
    int386x(0x21, &r, &r, &s);
    
    /* use 386|DOS-Extender selector 34h (writeable data segment that
        maps the entire first megabyte of memory used by MS-DOS) */
    doslist = MK_FP(0x34, (rpb.es << 4) + r.x.ebx);
    
    /* we now have protected-mode ptr to DOS List Of Lists -- 
        use normally */
    lastdrv = doslist[_osmajor==3 && _osminor==0 ? 0x1B : 0x21];
    fputs("LASTDRIVE=", stdout);
    putchar('A' - 1 + lastdrv);
    putchar('\n');
    return lastdrv;
}