如何从 FASM 执行 DOS 的 COMMAND.COM 命令？

Question

是否有任何 DOS 函数可以执行包含 command.com 行的字符串？我需要类似 C 的 system()，但用于 DOS 和 FASM。

Answer 1

我以前使用过这样的代码来执行 DOS shell 命令。它适用于 nasm，但您可以根据自己的目的对其进行调整。此代码专门将我们自己的命令行尾部作为 DOS 命令执行，但如果您想执行不同的命令，您可以修补一些其他命令行尾部。

section .data
comspec db      "COMSPEC="      ; for finding COMSPEC in the env block
comlen  equ     $-comspec
command db      "COMMAND.COM",0

        ; EXEC parameter block
execpar dw      0               ; environment for child (use ours)
        dw      80h, 0h         ; command tail (use ours)
        dw      5Ch, 0h         ; first FCB (use ours)
        dw      6Ch, 0h         ; second FCB (use ours)

section .text
        ; execute DOS command
doexec:
        mov     bx, execpar     ; EXEC parameter block
        mov     [bx+4], cs      ; fix up segment for command tail
        mov     [bx+8], cs      ; fix up segment for first FCB
        mov     [bx+12], cs     ; fix up segment for second FCB

        call    fndcom          ; write pointer to COMSPEC value to DS:SI
        mov     dx, si
        mov     ax, 4B00h       ; LOAD AND EXECUTE PROGRAM
        int     21h
        jnc     .ok             ; error occured?

        push    ax              ; remember error code
        call    fndcom          ; find COMSPEC value anew
        pop     ax              ; restore error code
        call    perror          ; print error message

.ok:    push    cs              ; restore ds
        pop     ds

        ret

        ; find COMSPEC in the environment block
        ; and load pointer to it to DS:SI
        ; preserves bx
fndcom: mov     ds, [2Ch]       ; environment block
        xor     si, si          ; beginning of env block

        ; loop invariant: si points to the beginning of
        ; a string in the environment
.loop1: cmp     byte [si], 0    ; end of environment reached?
        je      .nope

        mov     di, comspec     ; find "COMSPEC="
        mov     cx, comlen      ;
        repe    cmpsb           ; compare strings
        je      .found          ; if found, we are done

        dec     si              ; go back to mismatched character
.loop2: lodsb                   ; search si for end of string
        test    al, al          ; end of string reached?
        jne     .loop2          ; nope

        jmp     .loop1          ; check next string in environment

        ; COMSPEC unset
.nope:  push    cs
        pop     ds              ; restore ds
        mov     si, command     ; "COMMAND.COM"
        ret

        ; COMSPEC found
.found: ret

基本思路是通过在环境块中搜索名为COMSPEC的变量来查找命令解释器的名称。如果找到 none，我们默认为 COMMAND.COM。然后，我们构建一个exec parameter block with useful details for the program we would like to execute, including the command line. This is where you need to put the shell command to be executed (in the format of a command line tail). Finally, we invoke DOS function 4b00: execute program来执行命令解释器，运行我们的命令。

如何从 FASM 执行 DOS 的 COMMAND.COM 命令？

How to execute DOS' COMMAND.COM command from FASM?

assembly

dos

x86-16

fasm