115 lines
2.6 KiB
NASM
115 lines
2.6 KiB
NASM
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title chksum -- _nullcheck routine for C
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;--------------------------------------------------------------------------
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;
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; Microsoft C Compiler Runtime for MS-DOS
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;
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; (C)Copyright Microsoft Corporation, 1984, 1985, 1986
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;
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;--------------------------------------------------------------------------
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;
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; Purpose
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;
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; This routine is used to check for assignment through a null pointer.
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; Memory at DGROUP:0 is checked for destructive assignments. This
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; routine is not particularly effective in compact and large models.
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; A stub may be provoded for this routine without affecting the
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; behavior of a correctly written C program.
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;----------------------------------------------------------------------------
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?DF= 1 ; this is special for c startup
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include version.inc
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.xlist
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include cmacros.inc
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.list
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createSeg _TEXT, code, byte, public, CODE, <>
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createSeg NULL, null, para, public, BEGDATA,DGROUP
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createSeg _DATA, data, word, public, DATA, DGROUP
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createSeg HDR, nhdr, byte, public, MSG, DGROUP
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createSeg MSG, nmsg, byte, public, MSG, DGROUP
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createSeg PAD, npad, byte, common, MSG, DGROUP
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createSeg EPAD, nepad, byte, common, MSG, DGROUP
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defGrp DGROUP ; define DGROUP
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codeOFFSET equ offset _TEXT:
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dataOFFSET equ offset DGROUP:
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sBegin null
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assumes ds,data
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CHKSUM= 1Fh ; has to be correct or error
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BIAS= 55h
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chkpt db 8 dup(0) ; for null pointer assignment
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db 'C Library - (C)Copyright Microsoft Corp 1986'
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chkb db CHKSUM ; checksum byte
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db 0 ; leaves al = 0
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chkln= $ - chkpt ; length to checksum
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sEnd null
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sBegin nmsg
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assumes ds,data
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dw 1
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db 13,10,'error 2001: Null pointer assignment',13,10,0
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sEnd
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externP _NMSG_WRITE ; pascal calling
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sBegin code
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assumes cs,code
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assumes ds,data
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; _nullcheck
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;
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; _chksum cumulatively xor's all the bytes from ds:0 through 1 past end
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; of copyright string, finally xor'ing an arbitrary non-zero constant.
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; This is used to check if a null pointer has been written to.
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;
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; This version can be called as many times as the user wants.
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; The function returns zero if the checksum is OK.
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cProc _nullcheck,<PUBLIC>,<>
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cBegin nogen ; no arguments - so no frame
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push si
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xor si,si ; start at DS:0
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mov cx,chkln
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xor ah,ah
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cld
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chkloop: ; loop to 1 past end of copyrt. string
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lodsb
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xor ah,al ; accumulate xor total in AH
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loop chkloop
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xor ah,BIAS ; XOR out the initial BIAS
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jz setzero
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mov ax,1 ; Null pointer assignment
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push ax
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call _NMSG_WRITE ; write message out
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; ax = 0 if the checksum is OK
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setzero:
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pop si
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ret
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cEnd nogen
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sEnd code
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end
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