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Assembly Source File | 1985-07-08 | 11.1 KB | 343 lines

page 60,132 title COMZAP.ASM ;................... ;Author: Edward Nisley ;Entered from July 1985 issue of PC Tech Journal ;................... ;Program to force DOS 2.0 and 2.1 to reload COMMAND.COM ; using the file name specified by SET COMSPEC= ;Particularly useful with Vdisks and non-bootable ; fixed disks. ; ;The DOS 2.0 and 2.1 SET COMSPEC= command does not work as ; documented. DOS will always attempt to reload COMMAND.COM ; from the root directory of the disk drive used during ; system initialization (the boot disk). ; ;This utility finds the string A:\COMMAND.COM in DOS ; and replaces it with the file name set by SET COMSPEC= ; ;How it's done: ; the BIOS memory-size interrupt determines the ; range of RAM searched for the string. ;The string A:\COMMAND.COM always starts at an ; offset address xxx9, with a binary zero byte at ; offset xxx8 just preceding it. These observations ; are used to distinguish the true DOS file name from ; the string elsewhere (as in the program.COM file ; a VDISK or the program as loaded in RAM) ;[Or from same in the keyboard buffer under DEBUG.] ;Offset 2C in the PSP contains the segment address of ; the environment area passed to this program. The ; COMSPEC= string is located within the envrionment. ;The COMSPEC= string is copied to the DOS string area ; and displayed on the screen as it is being copied. ;If the COMSPEC= string is too long, your PC is dead! ;The string is truncated to fit the largest space ; available, which means that it cannot be used to ; reload the command interprteer. Power-off time! ; ;Assumptions: ;-DOS 2.0 or DOS 2.1 ;-Power-on boot drive is "A" ;-You have enough RAM in your system that the ; string is not in the last segment. The last segment ; is not checked, except in 64K systems. ;-The SET COMSPEC string is not too long. The maximum ; length is one less than the number defined in cspecmax. ; ;Exit: DOS function 4CH is used to pass a return code ; code 0=success! ; code 1=DOS file name string wasn't found ; code 2=COMSPEC string wasn't found ; code 3=COMSPEC string was too long, truncated ; page ;........................... ;hocus pocus to start up the assembler ; and get the addresses correct for a .COM file comseg segment 'codeseg' assume cs:comseg,ds:comseg,ss:comseg,es:comseg comzap proc org 100H start: jmp around ;......................... ;The file name we are trying to find. The leading ; 0FFH byte ensures that this file name isn't preceded ; by a binary zero. This prevents incorrect matching ; when Murphy puts it at the correct offset. db 0FFH bootID db 'A' ;the normal boot disk CCstr db ':\COMMAND.COM' ;last part of string CCstr_l equ $-CCstr ;string length ;......................... ;The COMSPEC= string header in the environment area specstr db 'COMSPEC=' specstr_l equ $-specstr ;string length ;......................... ;Various & sundry constants FN_hexit dw 0009H ;file name offset hexit env_seg_at equ 002CH ;address of environment segment prt_chr equ 02H ;DOS single-character output prt_str equ 09H ;DOS print-string function quit equ 4CH ;DOS termination function mem_sz equ 12H ;BIOS memory-size interrupt dosint equ 21H ;DOS function interrupt cr equ 0DH ;useful characters lf equ 0Ah term equ '$' ;the prt_str terminator ;..................... ;A macro to simplify the DOS interface DOScall macro DOS_fn mov ah,DOS_fn int dosint endm page ;....................... ;Variables FN_at label dword ;seg:offset address of FN_off dw 0 ;DOS file name string FN_seg dw 0 cspec_at label dword ;seg:offset of COMSPEC= cspec_off dw 0 ;string in enfironment cspec_seg dw 0 cspecmax equ 39 ;longest string allowed ; includes trailing zero last_seg dw 0 ;last segment to check ;....................... ;Messages Msg0 db 'COMZAP: a DOS fixit utility',cr,lf db 'Revised 4 April 85',cr,lf db cr,lf db 'Searching for A:\COMMAND.COM...' CrLf db cr,lf,term Msg1 db ' >> not found.',cr,lf db ' >> Remember that COMZAP.COM can' db cr,lf db ' >> be run only once per cold boot' db cr,lf,term Msg2 db cr,lf db 'The DOS command interpreter will be' db 'reloaded from',cr,lf db ' ',term Msg3 db 'Searching for COMSPEC= string...' db cr,lf,term Msg4 db ' >> not found.',cr,lf db ' >> Did you use SET COMSPEC=filespec' db cr,lf db ' >> before running COMZAP.COM?' db cr,lf,term Msg5 db cr,lf db ' >> WARNING: COMSPEC string too long,' db cr,lf db ' >> DOS will not reload the interpreter' db cr,lf,term page ;........................ ;Start of the program code ; ;Show some identification around: cld ;set increment mode mov dx,offset Msg0 ;show ID message DOScall prt_str ;......................... ;Get storage size and convert to segment address. ;Because the last segment may not be full, we will not ; test it for the string. ;Because we are testing in units of 64K (full segments) ; only the high-order hexit of the segment address ; is useful in comparisons. ;64K PC's (are there any left?) are special-cases. int mem_sz ;storage in 1K units mov cl,6 ;shift MSB to bit 15 shl ax,cl and ax,0F000H ;isolate 64K seg hexit jz oneseg ;handle 64K machines sub ax,01000H ;set last segment oneseg: mov last_seg,ax ;........................ ;Search for the starting "A" in "A:\COMMAND.COM" ;The inner loop uses CMP to check the 4096 offset ; addresses ending in the hexit 9 in each segment. ;The outer loop verifies inner-loop hits and ticks ; the segment addresses at the end of segments mov al,bootID ;the test byte mov bx,00000H ;first segment address mov es,bx ;into segment register assume es:nothing ;tell the assembler CClp0: mov si,FN_hexit ;get the offset address sub si,0010h ;fix for pre-increment mov cx,1000H ;64K/16 = # of 9's CClp1: add si,0010H ;tick offset pointer cmp al,es:[si] ;test the byte loopne CClp1 ;loop if not equal ;..................... ;Inner loop ended: ; if "not equal", the entire segment has been examined ; without finding a qualifying "A", so go to next seg jne nextseg ;...................... ;We've found an "A" at an offset xxx9 ; if it's preceded by a binary zero, look closer... cmp byte ptr es:[si-1],00H je testmore ;hmm... looking good! page ;....................... ;The "A" doesn't pass muster, so continue with the ; next byte in the segment. ;If CX is zero, all bytes have been tested, ; so we must step to the next segment cmp cx,0 jne CClp1 jmp nextseg ;...................... ;An "A" and a binary zero have been located!!! ;Time to compare the strings. ;Save the current state, just in case it's a miss testmore: push si ;save current offest push es ; ... segment push cx ; ... count push ax ; ... test byte ;................. ;Set the segment address to the paragraph just before ; the string to avoid overrunning the segment boundary ;This is complicated a bit by the restricted nature of ; the 8088's "general" registers mov bx,si ;get current offset mov cl,4 ;convert to paragraphs shr bx,cl mov cx,es ;add the segment add bx,cx mov es,bx ;set in in seg reg mov FN_seg,bx ;save segment ;..................... ;Set up the offset addresses to the known string and ; the candidate we just found ;The candidate offset is always FN_hexit by definition mov di,FN_hexit ;known candidate offset mov FN_off,di ;save offset inc di ;step over "A" mov si,offset CCstr ;known string mov cx,CCstr_l ; ... its length ;.................... ;After all that, this is the string com0parison. ;Registers are restored before the branch simply to ; avoid a double branch. ;The CMPS operands are coded for human readability. ;Failure returns us to the inner test loop above. ;Success passes us to the COMSPEC= string eaarch. repe cmps byte ptr ds:[si],es:[di] pop ax ;restore saved regs pop cx pop es pop si jne CClp1 ;no good, continue jmp gotfile ;swell, break out page ;..................... ;outer loop ending... ;Didn't find the string in this segment ;Trick the segment until we've completed the search. ;Remember that the RAM in the last segment will not ; be examined because the segment may not be filled ; with RAM... ;A tige more code would make the search complete. nextseg: mov bx,es ;tick the segment pointer add bx,1000H mov es,bx cmp bx,last_seg ;done? jbe CClp0 ;nope, back to loops mov dx,offset Msg1 ;present error message DOScall prt_str mov al,1 ;return code DOScall quit page ;...................... ;Tah-DAH! Found the DOS file specifier in RAM. ;Now we have to look for the COMSPEC string. ;Each string in the environment is terminated with a ; binary zero. The end of the environment area is ; marked by two binary zeros. ;Note how the "general" pointer registers aren't... gotfile: mov dx,offset Msg3 ;say where we are DOScall prt_str mov si,env_seg_at ;points to env seg ptr mov es,[si] ;pick up segment addr mov cspec_seg,es ;save seg addr findspec: mov di,cspec_off ;get offs addr cmp byte ptr es:[di],00h ;at end? je nospec ;sigh, give up mov si,offset specstr ;test string mov cx,specstr_l repe cmps byte ptr ds:[si],es:[di] je gotspec ;kapow! mov si,di ;use right "general" reg junkit: lods byte ptr es:[si] ;scan for end cmp al,00H jne junkit mov cspec_off,si ;set up pointer jmp findspec ;and try again ;.......................... ;Didn't find a COMSPEC= string in the environment. ;Return to DOS with an error code. nospec: mov dx,offset Msg4 ;show a sign DOScall prt_str mov al,2 ;set return code DOScall quit page ;...................... ;Found the COMSPEC= string in the enfironment area ;ES:DI now points to the start of the file spec string ;Need to get source in DS:SI and target in ES:DI ;Transfer the COMSPEC= string to the DOS file string ; and display it as it is transferred. ;The trailing binary zero is transferred as well, to ; overwrite any previous file name you may have ; manually inserted into the DOS file string while ; debugging this program... ;The string will be truncated to prevent stamping on ; other DOS data following the file name area. gotspec: mov cspec_off,di ;save offset mov dx,offset Msg2 ;say what's happening DOScall prt_str mov cx,cspecmax ;max length allowed les di,FN_at ;DOS string address lds si,cspec_at ;COMSPEC string addr xfer: lods byte ptr ds:[si] ;COMSPEC char stos byte ptr es:[di] ;to DOS char cmp al,00H ;hit the end? je alldone ;yup, quit mov dl,al ;nope, show it DOScall prt_chr loop xfer ;repeat for count ;if fall through, error! mov ax,cs ;restore DS mov ds,ax mov dx,offset Msg5 ;tell them about truncation DOScall prt_str mov al,3 ;set return code DOScall quit ;.................. ;Done with transfer and display, return to DOS alldone: mov ax,cs ;restore DS mov ds,ax mov dx,offset crlf ;insert some space DOScall prt_str mov al,0 ;good return code DOScall quit ;and exit page ;.................... ;hocus pocus to turn off the assembler ; and set up .COM starting address comzap endp comseg ends end start