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Assembly Source File | 1986-01-01 | 9.9 KB | 330 lines

name shell page 55,132 title 'SHELL.ASM -- simple MS-DOS shell' ; ; SHELL.ASM a simple user-extendable command interpreter ; for MS-DOS 2.X or 3.X ; ; Copyright (C) 1985 by Ray Duncan ; ; To assemble and link this program into the executable SHELL.EXE: ; ; C>MASM SHELL; ; C>LINK SHELL; ; stdin equ 0 ;Standard Input Device stdout equ 1 ;Standard Output Device stderr equ 2 ;Standard Error Device cr equ 0dh ;ASCII carriage return lf equ 0ah ;ASCII line feed blank equ 20h ;ASCII blank code esc equ 01bh ;ASCII escape code cseg segment para public 'CODE' assume cs:cseg,ds:data,ss:stack shell proc far ;at entry DS = ES = PSP mov ax,data ;let DS point to our data segment mov ds,ax mov ax,es:[002ch] ;get segment of Environment Block mov env_seg,ax ;from PSP and save it. ;now release unneeded memory... mov bx,100h ;ES=segment PSP, BX=paragraphs needed mov ah,4ah ;Function 4AH = modify memory block int 21h ;transfer to DOS jnc shell1 ;jump if request successful mov dx,offset msg1 ;otherwise display error message mov cx,msg1_length ;and exit jmp shell4 shell1: call get_comspec ;get file spec for COMMAND.COM jnc shell2 ;jump if it was found ok mov dx,offset msg3 ;COMSPEC variable not found in mov cx,msg3_length ;Environment block, print error jmp shell4 ;message and exit shell2: mov dx,offset shell3 ;set Control-C vector (Int 23H) mov ax,cs ;so that shell can keep control mov ds,ax ;DS:DX = addr of Control-C handler mov ax,2523H ;Function 25H = Set Interrupt int 21h mov ax,data ;make DS and ES point to our mov ds,ax ;data segment again mov es,ax shell3: ;main loop of command interpreter 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, with ;DS:DX=message addr, CX=length mov bx,stderr ;print error message on mov ah,40h ;Standard Error Device int 21h mov ax,4c01h ;exit to DOS with return code = 1 int 21h ;indicating an error condition shell endp ;these variables must be in Code Seg stk_seg dw 0 ;original SS contents stk_ptr dw 0 ;original SP contents intrinsic proc near ;Decode a user command against ;the table "COMMANDS". If match ;found, run the subroutine, and ;return Carry = False. If no match, ;return Carry = True. mov si,offset commands ;let SI = start of command table intr1: cmp byte ptr [si],0 ;is command table exhausted? 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 jmp intr2 intr3: mov al,[si] ;get next command char or al,al ;check if end of string jz intr4 ;jump, entire string matched cmp al,[di] ;compare to next input char jnz intr6 ;jump, found mismatch inc si ;go to next char in strings inc di jmp intr3 intr4: cmp byte ptr [di],cr ;make sure user's command je intr5 ;is the same length, next char cmp byte ptr [di],blank ;must be blank or Return jne intr6 intr5: call word ptr [si+1] ;run the command routine clc ;then return Carry Flag = False ret ;as success flag intr6: lodsb ;look for end of this command string or al,al jnz intr6 ;not end yet, loop add si,2 ;skip over command routine offset jmp intr1 ;try to match next command intr7: stc ;no match on command, exit ret ;with Carry = True intrinsic endp extrinsic proc near ;process an extrinsic command ;by passing it to COMMAND.COM ;with a " /C " command tail. mov al,cr ;find length of the command mov cx,cmd_tail_length ;by scanning for Carriage Return mov di,offset cmd_tail+1 cld repnz scasb mov ax,di ;calc length of command tail, sub ax,offset cmd_tail+2 ;not including the Carriage Return ;store length of synthesized mov cmd_tail,al ;command tail for EXEC function mov par_cmd,offset cmd_tail ;address of command tail call exec ;call the EXEC function to pass ret ;command line to COMMAND.COM extrinsic endp get_cmd proc near ;Prompt user and get a command. mov dx,offset prompt ;display the shell prompt mov cx,prompt_length ;on the Standard Output Device mov bx,stdout mov ah,40h ;Function 40H=Write file or device int 21h mov dx,offset inp_buf ;get a line from the Standard mov cx,inp_buf_length ;Input Device and place in our mov bx,stdin ;command line buffer mov ah,3fh ;Function 3FH=Read file or device int 21h mov si,offset inp_buf ;fold all lower case characters mov cx,inp_buf_length ;in the command line to upper case gcmd1: cmp byte ptr [si],'a' jb gcmd2 cmp byte ptr [si],'z' ja gcmd2 sub byte ptr [si],'a'-'A' gcmd2: inc si loop gcmd1 ret ;back to caller get_cmd endp get_comspec proc near ;Get file specification of COMMAND.COM ;from Environment "COMSPEC=" variable. ;Returns Carry=False if COMSPEC found. ;Returns Carry=True if no COMSPEC. mov si,offset com_var ;let DS:SI = string to match... call get_env ;go search Environment Block ;if environment variable not found, jc gcsp2 ;return Carry=True as failure code ;if var found, ES:DI points past "=" mov si,offset com_spec ;copy Env variable to our data segment gcsp1: mov al,es:[di] ;transfer null-terminated string mov [si],al inc si inc di or al,al ;null found yet? (turns off Carry) jnz gcsp1 ;no, get next character ;success, return Carry Flag=False gcsp2: ret get_comspec endp get_env proc near ;Search Environment Block ;Call DS:SI = "NAME=" to match ;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 segment of Environment Block xor di,di ;initialize offset to Env Block genv1: mov bx,si ;initialize pointer to pattern cmp byte ptr es:[di],0 ;end of Environment block? jne genv2 ;jump if end not found stc ;return Carry = True as failure flag ret genv2: mov al,[bx] ;get character from pattern or al,al ;end of pattern? (turns off Carry) jz genv3 ;yes, entire match succeeded cmp al,es:[di] ;compare to char in Environment Block jne genv4 ;jump if match failed inc bx inc di jmp genv2 genv3: ;All matched, return ES:DI pointing ret ;to parameter, Carry Flag = False genv4: xor al,al ;scan forward in Environment Block 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. push ds ;save data segments push es mov cs:stk_seg,ss ;save copy of SS:SP for use mov cs:stk_ptr,sp ;after return from overlay mov dx,offset com_spec ;now load and execute COMMAND.COM mov bx,offset par_blk mov ah,4bh ;function 4BH = EXEC mov al,0 ;subfunction 0 = load and execute int 21h mov ss,cs:stk_seg ;restore stack segment mov sp,cs:stk_ptr ;and stack pointer pop es ;restore data segments pop ds jnc exec1 ;jump if no errors mov dx,offset msg2 ;EXEC failed, print error mov cx,msg2_length ;message mov bx,stderr mov ah,40h int 21h exec1: ret ;back to caller exec endp cls_cmd proc near ;intrinsic CLS Command ; = clear the screen mov dx,offset cls_str ;send the ANSI control sequence mov cx,cls_str_length ;to clear the screen mov bx,stdout mov ah,40h int 21h ret cls_cmd endp dos_cmd proc near ;intrinsic DOS Command ; = run COMMAND.COM mov par_cmd,offset nultail ;set command tail to null string call exec ;now EXEC the COMMAND.COM ret dos_cmd endp exit_cmd proc near ;intrinsic EXIT Command ; = leave this shell mov ax,4c00h ;call DOS terminate function int 21h ;with return code of zero exit_cmd endp cseg ends stack segment para stack 'STACK' ;declare stack segment dw 64 dup (?) stack ends data segment para public 'DATA' ;declare data segment commands equ $ ;table of "intrinsic" commands ;each entry is a null-terminated ;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 ;table terminated with null string com_var db 'COMSPEC=',0 ;Environment Block variable to match ;filespec of COMMAND.COM moved com_spec db 80 dup (0) ;here from the Environment Block nultail db 0,cr ;a "Null" command tail for invoking ;COMMAND.COM as another shell cmd_tail db 0,' /C ' ;command tail invoking COMMAND.COM ;as a transient command processor 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: ' ;the shell's prompt to the user prompt_length equ $-prompt env_seg dw 0 ;segment of Environment Block msg1 db cr,lf db 'Unable to de-allocate 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 esc,'[2J' ;this is the ANSI standard control cls_str_length equ $-cls_str ;sequence to clear the screen par_blk equ $ ;Parameter Block for EXEC call dw 0 ;segment address, environment block par_cmd dw offset cmd_tail ;address of command line dw seg cmd_tail dd -1 ;address of default FCB #1 dd -1 ;address of default FCB #2 data ends end shell