Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.naic.edu/~phil/hardware/vertex/sharemegsvertex/lcu/pcr/c600/source/STARTUP/CHKSUM.ASM Дата изменения: Tue Mar 6 22:20:55 2001 Дата индексирования: Thu Jan 15 16:12:07 2009 Кодировка: Поисковые слова: жмхптеугеогйс

page ,132
title chksum - _nullcheck routine for C
;***
;chksum.asm - _nullcheck routine for C
;
; Copyright (c) 1985-1990, Microsoft Corporation. All rights reserved.
;
;Purpose:
; This routine is used to check for assignment through a null pointer.
; Memory at DGROUP:0 is checked for destructive assignments. This
; routine is not particularly effective in compact and large models.
; A stub may be provoded for this routine without affecting the
; behavior of a correctly written C program.
;
;*******************************************************************************

?DF= 1 ; this is special for c startup
include version.inc
.xlist
include cmacros.inc
include rterr.inc
.list

createSeg _TEXT, code, word, public, CODE, <>

createSeg NULL, null, para, public, BEGDATA,DGROUP
createSeg _DATA, data, word, public, DATA, DGROUP

createSeg HDR, nhdr, byte, public, MSG, DGROUP
createSeg MSG, nmsg, byte, public, MSG, DGROUP
createSeg PAD, npad, byte, common, MSG, DGROUP
createSeg EPAD, nepad, byte, common, MSG, DGROUP

defGrp DGROUP ; define DGROUP

codeOFFSET equ offset _TEXT:
dataOFFSET equ offset DGROUP:


sBegin null
assumes ds,data

BIAS= 55h

chkpt db 8 dup(0) ; for null pointer assignment
CHKSUM= 18h ; has to be correct or error
db 'MS Run-Time Library - Copyright (c) 1990, Microsoft Corp'
chkb db CHKSUM ; checksum byte
db 0 ; leaves al = 0
chkln= $ - chkpt ; length to checksum

public __anullsize ; label at end of null segment
labelB _anullsize

sEnd null

sBegin nmsg
assumes ds,data

_RTERR _RT_NULLPTR, 'null pointer assignment', _RT_STANDARD

sEnd

sBegin npad
assumes ds,data
dw -1
; no padding for now;
; MAX padding would be
; db 20 dup(0)
sEnd

externP _NMSG_WRITE ; pascal calling
externP _FF_MSGBANNER ; pascal calling

sBegin code
assumes cs,code
assumes ds,data


page
;***
;_nullcheck - check-sum of start of DGROUP segment to detect null-ptr-assignmt.
;
;Purpose:
; _nullcheck cumulatively xor's all the bytes from ds:0 through 1 past end
; of copyright string, finally xor'ing an arbitrary non-zero constant.
; This is used to check if a null pointer has been written to.
;
; This version can be called as many times as the user wants.
; The function returns zero if the checksum is OK.
;
; Note that this checksum only detects (DS:0) null pointer assignments
; but not (0:0) null pointer assignments.
;
;Entry:
; Assumes DS points to the beginning of DGROUP.
;
;Exit:
; Returns : AX = 0 if no error; AX = 1 if error.
;
;Uses:
; BX,CX,DX,ES are destroyed.
;
;Exceptions:
; If _nullcheck check-sum fails, an error message is written
; to standard error, and the routine returns an error code (AX = 1).
;
;*******************************************************************************

cProc _nullcheck,,<>
cBegin nogen ; no arguments - so no frame


push si

xor si,si ; start at DS:0
mov cx,chkln
xor ah,ah
cld

chkloop: ; loop to 1 past end of copyrt. string
lodsb
xor ah,al ; accumulate xor total in AH
loop chkloop

xor ah,BIAS ; XOR out the initial BIAS
jz setzero

callcrt _FF_MSGBANNER ; (1) "\r\n" to stderr
; (2) FORTRAN $DEBUG file/line
; if _Fline is set (not in C)
; (3) "run-time error" banner
mov ax,_RT_NULLPTR ; null pointer assignment message no.
push ax
callcrt _NMSG_WRITE ; write message out
mov ax,1 ; indicate error occurred

; ax = 0 if the checksum is OK

setzero:
pop si



ret

cEnd nogen


sEnd code

end