Microsoft Programmer's Library 1.3

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Assembly Source File | 1988-10-01 | 10.0 KB | 418 lines

name shell page 55,132 title SHELL.ASM -- simple MS-DOS shell ; ; SHELL.ASM Simple extendable command interpreter ; for MS-DOS version 2.0 and later ; ; Copyright 1988 by Ray Duncan ; ; Build: C>MASM SHELL; ; C>LINK SHELL; ; ; Usage: C>SHELL; ; stdin equ 0 ; standard input handle stdout equ 1 ; standard output handle stderr equ 2 ; standard error handle cr equ 0dh ; ASCII carriage return lf equ 0ah ; ASCII line feed blank equ 20h ; ASCII blank code escape equ 01bh ; ASCII escape code _TEXT segment word public 'CODE' assume cs:_TEXT,ds:_DATA,ss:STACK shell proc far ; at entry DS = ES = PSP mov ax,_DATA ; make our data segment mov ds,ax ; addressable mov ax,es:[002ch] ; get environment segment mov env_seg,ax ; from PSP and save it ; release unneeded memory... ; ES already = PSP segment mov bx,100h ; BX = paragraphs needed mov ah,4ah ; Function 4AH = resize block int 21h ; transfer to MS-DOS jnc shell1 ; jump if resize OK mov dx,offset msg1 ; resize failed, display mov cx,msg1_length ; error message and exit jmp shell4 shell1: call get_comspec ; get COMMAND.COM filespec jnc shell2 ; jump if it was found mov dx,offset msg3 ; COMSPEC not found in mov cx,msg3_length ; environment, display error jmp shell4 ; message and exit shell2: mov dx,offset shell3 ; set Ctrl-C vector (Int 23H) mov ax,cs ; for this program's handler mov ds,ax ; DS:DX = handler address mov ax,2523H ; Fxn 25H = set vector int 21h ; transfer to MS-DOS mov ax,_DATA ; make our data segment mov ds,ax ; addressable again mov es,ax shell3: ; main interpreter loop call get_cmd ; get a command from user call intrinsic ; check if intrinsic function jnc shell3 ; yes, it was processed call extrinsic ; no, pass it to COMMAND.COM jmp shell3 ; then get another command shell4: ; come here if error detected ; DS:DX = message address ; CX = message length mov bx,stderr ; BX = standard error handle mov ah,40h ; Fxn 40H = write int 21h ; transfer to MS-DOS mov ax,4c01h ; Fxn 4CH = terminate with ; return code = 1 int 21h ; transfer to MS-DOS shell endp intrinsic proc near ; Decode user entry against ; the table "COMMANDS" ; If match, run the routine, ; and return Carry = False ; If no match, Carry = True ; return Carry = True. mov si,offset commands ; DS:SI = command table intr1: cmp byte ptr [si],0 ; end of table? je intr7 ; jump, end of table found mov di,offset inp_buf ; no, let DI = addr of user input intr2: cmp byte ptr [di],blank ; scan off any leading blanks jne intr3 inc di ; found blank, go past it jmp intr2 intr3: mov al,[si] ; next character from table or al,al ; end of string? jz intr4 ; jump, entire string matched cmp al,[di] ; compare to input character jnz intr6 ; jump, found mismatch inc si ; advance string pointers inc di jmp intr3 intr4: cmp byte ptr [di],cr ; make sure user's entry je intr5 ; is the same length... cmp byte ptr [di],blank ; next character in entry jne intr6 ; must be blank or Return intr5: call word ptr [si+1] ; run the command routine clc ; return Carry Flag = False ret ; as success flag intr6: lodsb ; look for end of this or al,al ; command string (null byte) jnz intr6 ; not end yet, loop add si,2 ; skip over routine address jmp intr1 ; try to match next command intr7: stc ; command not matched, exit ret ; with Carry = True intrinsic endp extrinsic proc near ; process extrinsic command ; by passing it to ; COMMAND.COM with a ; " /C " command tail. mov al,cr ; find length of command mov cx,cmd_tail_length ; by scanning for carriage mov di,offset cmd_tail+1 ; return cld repnz scasb mov ax,di ; calculate command tail sub ax,offset cmd_tail+2 ; length without carriage mov cmd_tail,al ; return, and store it ; set command tail address mov word ptr par_cmd,offset cmd_tail call exec ; and run COMMAND.COM ret extrinsic endp get_cmd proc near ; prompt user, get command ; display the shell prompt mov dx,offset prompt ; DS:DX = message address mov cx,prompt_length ; CX = message length mov bx,stdout ; BX = standard output handle mov ah,40h ; Fxn 40H = write int 21h ; transfer to MS-DOS ; get entry from user mov dx,offset inp_buf ; DS:DX = input buffer mov cx,inp_buf_length ; CX = max length to read mov bx,stdin ; BX = standard input handle mov ah,3fh ; Fxn 3FH = read int 21h ; transfer to MS-DOS mov si,offset inp_buf ; fold lower case characters mov cx,inp_buf_length ; in entry to upper case gcmd1: cmp byte ptr [si],'a' ; check if 'a-z' jb gcmd2 ; jump, not in range cmp byte ptr [si],'z' ; check if 'a-z' ja gcmd2 ; jump, not in range sub byte ptr [si],'a'-'A' ; convert to upper case gcmd2: inc si ; advance through entry loop gcmd1 ret ; back to caller get_cmd endp get_comspec proc near ; get location of COMMAND.COM ; from environment "COMSPEC=" ; Returns Carry = False ; if COMSPEC found. ; Returns Carry=True ; if no COMSPEC. mov si,offset com_var ; DS:SI = string to match... call get_env ; search Environment Block jc gcsp2 ; jump if COMSPEC not found ; ES:DI points past "=" mov si,offset com_spec ; DS:SI = local buffer gcsp1: mov al,es:[di] ; copy COMSPEC variable mov [si],al ; to local buffer inc si inc di or al,al ; null char? (turns off Carry) jnz gcsp1 ; no, get next character gcsp2: ret ; back to caller get_comspec endp get_env proc near ; search environment ; Call DS:SI = "NAME=" ; uses contents of "ENV_SEG" ; Returns Carry=False and ES:DI ; pointing to parameter if found, ; Returns Carry=True if no match mov es,env_seg ; get environment segment xor di,di ; initialize env. offset genv1: mov bx,si ; initialize pointer to name cmp byte ptr es:[di],0 ; end of environment? jne genv2 ; jump, end not found stc ; no match, return Carry set ret genv2: mov al,[bx] ; get character from name or al,al ; end of name? (turns off Carry) jz genv3 ; yes, name matched cmp al,es:[di] ; compare to environment jne genv4 ; jump if match failed inc bx ; advance environment inc di ; and name pointers jmp genv2 genv3: ; match found, Carry = clear, ret ; ES:DI = variable genv4: xor al,al ; scan forward in environment mov cx,-1 ; for zero byte cld repnz scasb jmp genv1 ; go compare next string get_env endp exec proc near ; call MS-DOS EXEC function ; to run COMMAND.COM. mov stkseg,ss ; save stack pointer mov stkptr,sp ; now run COMMAND.COM mov dx,offset com_spec ; DS:DX = filename mov bx,offset par_blk ; ES:BX = parameter block mov ax,4b00h ; Fxn 4BH = EXEC ; Subf. 0 = load and execute int 21h ; transfer to MS-DOS mov ax,_DATA ; make data segment mov ds,ax ; addressable again mov es,ax cli ; (for bug in some 8088s) mov ss,stkseg ; restore stack pointer mov sp,stkptr sti ; (for bug in some 8088s) jnc exec1 ; jump if no errors ; display error message mov dx,offset msg2 ; DS:DX = message address mov cx,msg2_length ; CX = message length mov bx,stderr ; BX = standard error handle mov ah,40h ; Fxn 40H = write int 21h ; transfer to MS-DOS exec1: ret ; back to caller exec endp cls_cmd proc near ; intrinsic CLS command mov dx,offset cls_str ; send the ANSI escape mov cx,cls_str_length ; sequence to clear mov bx,stdout ; the screen mov ah,40h int 21h ret cls_cmd endp dos_cmd proc near ; intrinsic DOS command ; set null command tail mov word ptr par_cmd,offset nultail call exec ; and run COMMAND.COM ret dos_cmd endp exit_cmd proc near ; intrinsic EXIT Command mov ax,4c00h ; call MS-DOS terminate int 21h ; function with ; return code of zero exit_cmd endp _TEXT ends STACK segment para stack 'STACK' ; declare stack segment dw 64 dup (?) STACK ends _DATA segment word public 'DATA' commands equ $ ; "intrinsic" commands table ; each entry is ASCIIZ string ; followed by the offset ; of the procedure to be ; executed for that command. db 'CLS',0 dw cls_cmd db 'DOS',0 dw dos_cmd db 'EXIT',0 dw exit_cmd db 0 ; end of table com_var db 'COMSPEC=',0 ; environment variable ; COMMAND.COM filespec com_spec db 80 dup (0) ; from environment COMSPEC= nultail db 0,cr ; null command tail for ; invoking COMMAND.COM ; as another shell cmd_tail db 0,' /C ' ; command tail for invoking ; COMMAND.COM as a transient inp_buf db 80 dup (0) ; command line from Standard Input inp_buf_length equ $-inp_buf cmd_tail_length equ $-cmd_tail-1 prompt db cr,lf,'sh: ' ; SHELL's user prompt prompt_length equ $-prompt env_seg dw 0 ; segment of Environment Block msg1 db cr,lf db 'Unable to release memory.' db cr,lf msg1_length equ $-msg1 msg2 db cr,lf db 'EXEC of COMMAND.COM failed.' db cr,lf msg2_length equ $-msg2 msg3 db cr,lf db 'No COMSPEC variable in environment.' db cr,lf msg3_length equ $-msg3 cls_str db escape,'[2J' ; ANSI escape sequence cls_str_length equ $-cls_str ; to clear the screen ; EXEC parameter block par_blk dw 0 ; environment segment par_cmd dd cmd_tail ; command line dd fcb1 ; file control block #1 dd fcb2 ; file control block #2 fcb1 db 0 ; file control block #1 db 11 dup (' ') db 25 dup (0) fcb2 db 0 ; file control block #2 db 11 dup (' ') db 25 dup (0) stkseg dw 0 ; original SS contents stkptr dw 0 ; original SP contents _DATA ends end shell