DOS ain't dead

To ZERO or NOT to ZERO (memory / segment basing) (DOSX)

posted by DOS386, 05.08.2007, 14:45

> So please post your adjustments!

; FASM example of writing 32-bit program using DPMI
; True FLAT ZERO based memory model
; requires a GOOD DPMI host installed in system

format MZ
heap 0                                  ; no additional memory
use16

        push    cs
        pop     ds
        push    cs
        pop     ax
        mov     [ss:0],ax

        mov     ax,1687h
        int     2Fh
        or      ax,ax                   ; DPMI installed?
        jnz     error
        test    bl,1                    ; 32-bit programs supported?
        jz      error
        mov     word [mode_switch],di
        mov     word [mode_switch+2],es
        mov     bx,si                   ; allocate memory for DPMI data
        mov     ah,48h
        int     21h
        jc      error
        mov     es,ax
        mov     ax,1
        call    far [mode_switch]       ; switch to protected mode
        jc      error

        mov     cx,1
        xor     ax,ax
        int     31h                     ; allocate descriptor for code
        mov     si,ax
        xor     ax,ax
        int     31h                     ; allocate descriptor for data

        mov     di,ax
        mov     dx,cs
        lar     cx,dx
        shr     cx,8
        or      cx,0C000h
        mov     bx,si
        mov     ax,9
        int     31h                     ; set code descriptor access rights
        mov     dx,ds
        lar     cx,dx
        shr     cx,8
        or      cx,0C000h
        mov     bx,di
        int     31h                     ; set data descriptor access rights

        xor     cx,cx                   ; & Our ZERO base
        xor     dx,dx                   ; &
        mov     es,cx                   ; #
        mov     fs,cx                   ; # Not used by now
        mov     gs,cx                   ; #
        mov     ax,7                    ; set descriptor base address
        int     31h
        mov     bx,si
        int     31h

        mov     cx,$FFFF
        mov     dx,$FFFF
        mov     ax,8                    ; set segment limit to 4 GB
        int     31h
        mov     bx,di
        int     31h

        mov     ds,di
        pushw   0
        pushw   si           ; Will land in CS one day
        mov     cx, [ss:0]
        add     cx, rmcodesizepar
        shl     ecx, 4       ; "paragraph" -> linear
        pushd   ecx          ; Start address in new segment
        retfd

    error:
        mov     ax,4CFFh
        int     21h

  mode_switch dd ?

;
; Padding to 16-byte integer multiply + calculations
;
rmcodeend:     ; No code, just for padding
temp1=rmcodeend mod 16
if (temp1<>0) ; then
  db (16-temp1) dup (0)
end if
rmcodesize:   ; No code, this is the aligned RM code size (starts at 0 !!!)
rmcodesizepar = rmcodesize shr 4 ; In 16-byte "parag" units

use32
org 0

           call lleipbase
lleipbase: pop edx
           lea esi,[edx+txhello-5]
lloop:
        lodsb
        test    al,al
        jz      ldone
        mov     dl,al
        mov     ah,2
        int     $21
        jmp     lloop

ldone:  ud2

txhello db 13,10,'Hello from PM, FLAT ZERO based memory model !',13,10,13,10,0

Done in above ^^^ code:

- ZEROized model
- got rid of MZ-fixup
- probably "fixed" BOCHS crash

ToDo:

- Get rid of 16-bit stack, SS:=DS
- format MZ -> format binary
- Save linear addresses of PSP/DTA/ENV

> the problem is that "near32" addresses (=offsets

IIRC FASM doesn't suffer from offsets

> in MZ are always relative to the address the binary is loaded to,
> and this address is never 0 in DOS.

Very true, but don't see the problem

> However, due to the way Intel cpus handle "call" and "jmp" opcodes it is
> possible to set the base of CS, SS, DS, ES to 0 and indeed run "non-flat"

Or FLAT ?

> code then ... if it avoids certain things. Among these "certain things" is
> access to global variables, so it won't be a true option usually.

Depends how one implements the global variables

Seems to be an issue of fixuping and linking

Question: is it possible / good idea to place a breakpoint at bottom of the stack with INT $31/$0B00 ?

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Complete thread:

• To ZERO or NOT to ZERO (memory / segment basing) - DOS386, 05.08.2007, 08:33 (DOSX)
  • To ZERO or NOT to ZERO (memory / segment basing) - Japheth, 05.08.2007, 09:22
    • To ZERO or NOT to ZERO (memory / segment basing) - DOS386, 05.08.2007, 14:45
      • To ZERO or NOT to ZERO (memory / segment basing) - Japheth, 06.08.2007, 07:52
        • To ZERO or NOT to ZERO (memory / segment basing) vs C - DOS386, 09.08.2007, 19:12
          • To ZERO or NOT to ZERO (memory / segment basing) vs C - Japheth, 10.08.2007, 07:18