Misc/abrash-black-book
Watch
1
0
Fork
You've already forked abrash-black-book
 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

Use backtick code blocks for syntax highlighting

Browse source
This commit is contained in:
James Gregory 2013-12-31 11:38:48 +11:00
commit f658bfbb40
203 changed files with 26546 additions and 25801 deletions
Download patch file Download diff file Expand all files Collapse all files

100
01-06.md
View file

@ -12,55 +12,57 @@ pages: 018-019
**LISTING 1.7 L1-7.ASM**
; Assembler subroutine to perform a 16-bit checksum on a block of
; bytes 1 to 64K in size. Adds checksum for block into passed-in
; checksum.
;
; Call as:
; void ChecksumChunk(unsigned char *Buffer,
; unsigned int BufferLength, unsigned int *Checksum);
;
; where:
; Buffer = pointer to start of block of bytes to checksum
; BufferLength = # of bytes to checksum (0 means 64K, not 0)
; Checksum = pointer to unsigned int variable checksum is
;stored in
;
; Parameter structure:
;
Parms struc
dw ? ;pushed BP
dw ? ;return address
Buffer dw ?
BufferLength dw ?
Checksum dw ?
Parmsends
;
.model small
.code
public _ChecksumChunk
_ChecksumChunkprocnear
push bp
mov bp,sp
push si ;save C's register variable
;
cld ;make LODSB increment SI
mov si,[bp+Buffer] ;point to buffer
mov cx,[bp+BufferLength] ;get buffer length
mov bx,[bp+Checksum] ;point to checksum variable
mov dx,[bx] ;get the current checksum
sub ah,ah ;so AX will be a 16-bit value after LODSB
ChecksumLoop:
lodsb ;get the next byte
add dx,ax ;add it into the checksum total
loop ChecksumLoop ;continue for all bytes in block
mov [bx],dx ;save the new checksum
;
pop si ;restore C's register variable
pop bp
ret
_ChecksumChunkendp
end
```nasm
; Assembler subroutine to perform a 16-bit checksum on a block of
; bytes 1 to 64K in size. Adds checksum for block into passed-in
; checksum.
;
; Call as:
; void ChecksumChunk(unsigned char *Buffer,
; unsigned int BufferLength, unsigned int *Checksum);
;
; where:
; Buffer = pointer to start of block of bytes to checksum
; BufferLength = # of bytes to checksum (0 means 64K, not 0)
; Checksum = pointer to unsigned int variable checksum is
;stored in
;
; Parameter structure:
;
Parms struc
dw ? ;pushed BP
dw ? ;return address
Buffer dw ?
BufferLength dw ?
Checksum dw ?
Parmsends
;
.model small
.code
public _ChecksumChunk
_ChecksumChunkprocnear
push bp
mov bp,sp
push si ;save C's register variable
;
cld ;make LODSB increment SI
mov si,[bp+Buffer] ;point to buffer
mov cx,[bp+BufferLength] ;get buffer length
mov bx,[bp+Checksum] ;point to checksum variable
mov dx,[bx] ;get the current checksum
sub ah,ah ;so AX will be a 16-bit value after LODSB
ChecksumLoop:
lodsb ;get the next byte
add dx,ax ;add it into the checksum total
loop ChecksumLoop ;continue for all bytes in block
mov [bx],dx ;save the new checksum
;
pop si ;restore C's register variable
pop bp
ret
_ChecksumChunkendp
end
```
Note that in Table 1.1, optimization makes little difference except in
the case of Listing 1.5, where the design has been refined considerably.