115 lines
2.6 KiB
NASM
115 lines
2.6 KiB
NASM
title chksum -- _nullcheck routine for C
|
|
|
|
;--------------------------------------------------------------------------
|
|
;
|
|
; Microsoft C Compiler Runtime for MS-DOS
|
|
;
|
|
; (C)Copyright Microsoft Corporation, 1984, 1985, 1986
|
|
;
|
|
;--------------------------------------------------------------------------
|
|
;
|
|
; 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
|
|
.list
|
|
|
|
createSeg _TEXT, code, byte, 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
|
|
|
|
CHKSUM= 1Fh ; has to be correct or error
|
|
|
|
BIAS= 55h
|
|
|
|
chkpt db 8 dup(0) ; for null pointer assignment
|
|
db 'C Library - (C)Copyright Microsoft Corp 1986'
|
|
chkb db CHKSUM ; checksum byte
|
|
db 0 ; leaves al = 0
|
|
chkln= $ - chkpt ; length to checksum
|
|
|
|
sEnd null
|
|
|
|
sBegin nmsg
|
|
assumes ds,data
|
|
|
|
dw 1
|
|
db 13,10,'error 2001: Null pointer assignment',13,10,0
|
|
|
|
sEnd
|
|
|
|
|
|
externP _NMSG_WRITE ; pascal calling
|
|
|
|
sBegin code
|
|
assumes cs,code
|
|
assumes ds,data
|
|
|
|
; _nullcheck
|
|
;
|
|
; _chksum 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.
|
|
|
|
cProc _nullcheck,<PUBLIC>,<>
|
|
|
|
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
|
|
|
|
mov ax,1 ; Null pointer assignment
|
|
push ax
|
|
call _NMSG_WRITE ; write message out
|
|
|
|
; ax = 0 if the checksum is OK
|
|
|
|
setzero:
|
|
pop si
|
|
ret
|
|
|
|
cEnd nogen
|
|
|
|
|
|
sEnd code
|
|
|
|
end
|