Running EXE

[Tolga]

Hi!

I am writing a real mode os. I have a problem. After loaded a EXE file to memory, how can i edit EXE file's header to run? COM files are easy. ???

Thanks

[KieranFoot]

How is the executable file being loaded?
Is it a Binary or an EXE file, if its an executable then why are you using the EXE format?

[Pype.Clicker]

iirc, you need to patch segments values according to the relocation entries found in the header, provide enough stack space and then jump to the entry point. Wotsit.org may contain more info.

[Dex4u]

This code would do it:

Code: Select all

;;;;;;;;;;;;;;;;;;;
;; Type checking ;;
;;;;;;;;;;;;;;;;;;;

        cli                             ; for stack adjustments

        mov     ax, ImageLoadSeg
        mov     es, ax

        cmp     word [es:0], 5A4Dh      ; "MZ" signature?
        je      RelocateEXE             ; yes, it's an EXE program

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Setup and Run COM program ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

        mov     ax, es
        sub     ax, 10h                 ; "org 100h" stuff :)
        mov     es, ax
        mov     ds, ax
        mov     ss, ax
        xor     sp, sp
        push    es
        push    word 100h
        jmp     Run

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Relocate, setup and run EXE program ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

RelocateEXE:
        mov     ds, ax

        add     ax, [ds:08h]            ; ax = image base
        mov     cx, [ds:06h]            ; cx = reloc items
        mov     bx, [ds:18h]            ; bx = reloc table pointer

        jcxz    RelocationDone

ReloCycle:
        mov     di, [ds:bx]             ; di = item ofs
        mov     dx, [ds:bx+2]           ; dx = item seg (rel)
        add     dx, ax                  ; dx = item seg (abs)

        push    ds
        mov     ds, dx                  ; ds = dx
        add     [ds:di], ax             ; fixup
        pop     ds

        add     bx, 4                   ; point to next entry
        loop    ReloCycle

RelocationDone:

        mov     bx, ax
        add     bx, [ds:0Eh]
        mov     ss, bx                  ; ss for EXE
        mov     sp, [ds:10h]            ; sp for EXE

        add     ax, [ds:16h]            ; cs
        push    ax
        push    word [ds:14h]           ; ip
Run:
        mov     dl, [cs:bsDriveNumber]  ; let program know boot drive
        sti
        retf