Use backtick code blocks for syntax highlighting
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James Gregory
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203 changed files with 26546 additions and 25801 deletions
188
01-03.md
188
01-03.md
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@ -30,105 +30,109 @@ disk caching turned off.
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**LISTING 1.2 L1-2.C**
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/*
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* Program to calculate the 16-bit checksum of the stream of bytes
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* from the specified file. Obtains the bytes one at a time in
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* assembler, via direct calls to DOS.
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*/
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```c
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/*
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* Program to calculate the 16-bit checksum of the stream of bytes
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* from the specified file. Obtains the bytes one at a time in
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* assembler, via direct calls to DOS.
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*/
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#include <stdio.h>
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#include <fcntl.h>
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#include <stdio.h>
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#include <fcntl.h>
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main(int argc, char *argv[]) {
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int Handle;
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unsigned char Byte;
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unsigned int Checksum;
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int ReadLength;
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main(int argc, char *argv[]) {
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int Handle;
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unsigned char Byte;
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unsigned int Checksum;
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int ReadLength;
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if ( argc != 2 ) {
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printf("usage: checksum filename\n");
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exit(1);
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}
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if ( (Handle = open(argv[1], O_RDONLY | O_BINARY)) == -1 ) {
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printf("Can't open file: %s\n", argv[1]);
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exit(1);
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}
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if ( !ChecksumFile(Handle, &Checksum) ) {
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printf("Error reading file %s\n", argv[1]);
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exit(1);
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}
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if ( argc != 2 ) {
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printf("usage: checksum filename\n");
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exit(1);
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}
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if ( (Handle = open(argv[1], O_RDONLY | O_BINARY)) == -1 ) {
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printf("Can't open file: %s\n", argv[1]);
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exit(1);
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}
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if ( !ChecksumFile(Handle, &Checksum) ) {
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printf("Error reading file %s\n", argv[1]);
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exit(1);
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}
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/* Report the result */
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printf("The checksum is: %u\n", Checksum);
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exit(0);
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}
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/* Report the result */
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printf("The checksum is: %u\n", Checksum);
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exit(0);
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}
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```
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**LISTING 1.3 L1-3.ASM**
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; Assembler subroutine to perform a 16-bit checksum on the file
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; opened on the passed-in handle. Stores the result in the
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; passed-in checksum variable. Returns 1 for success, 0 for error.
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;
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; Call as:
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; int ChecksumFile(unsigned int Handle, unsigned int *Checksum);
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;
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; where:
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; Handle = handle # under which file to checksum is open
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; Checksum = pointer to unsigned int variable checksum is
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; to be stored in
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;
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; Parameter structure:
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;
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Parms struc
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dw ? ;pushed BP
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dw ? ;return address
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Handle dw ?
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Checksum dw ?
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Parms ends
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;
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.model small
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.data
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TempWord label word
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TempByte db ? ;each byte read by DOS will be stored here
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db 0 ;high byte of TempWord is always 0
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;for 16-bit adds
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;
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.code
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public _ChecksumFile
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_ChecksumFile proc near
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push bp
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mov bp,sp
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push si ;save C's register variable
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;
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mov bx,[bp+Handle] ;get file handle
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sub si,si ;zero the checksum ;accumulator
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mov cx,1 ;request one byte on each ;read
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mov dx,offset TempByte ;point DX to the byte in
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;which DOS should store
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;each byte read
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ChecksumLoop:
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mov ah,3fh ;DOS read file function #
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int 21h ;read the byte
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jcErrorEnd;an error occurred
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and ax,ax ;any bytes read?
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jz Success ;no-end of file reached-we're done
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add si,[TempWord] ;add the byte into the
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;checksum total
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jmpChecksumLoop
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ErrorEnd:
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sub ax,ax ;error
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jmp short Done
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Success:
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mov bx,[bp+Checksum] ;point to the checksum variable
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mov [bx],si ;save the new checksum
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mov ax,1 ;success
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;
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Done:
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pop si ;restore C's register variable
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pop bp
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ret
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_ChecksumFileendp
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end
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```nasm
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; Assembler subroutine to perform a 16-bit checksum on the file
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; opened on the passed-in handle. Stores the result in the
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; passed-in checksum variable. Returns 1 for success, 0 for error.
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;
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; Call as:
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; int ChecksumFile(unsigned int Handle, unsigned int *Checksum);
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;
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; where:
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; Handle = handle # under which file to checksum is open
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; Checksum = pointer to unsigned int variable checksum is
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; to be stored in
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;
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; Parameter structure:
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;
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Parms struc
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dw ? ;pushed BP
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dw ? ;return address
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Handle dw ?
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Checksum dw ?
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Parms ends
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;
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.model small
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.data
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TempWord label word
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TempByte db ? ;each byte read by DOS will be stored here
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db 0 ;high byte of TempWord is always 0
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;for 16-bit adds
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;
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.code
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public _ChecksumFile
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_ChecksumFile proc near
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push bp
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mov bp,sp
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push si ;save C's register variable
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;
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mov bx,[bp+Handle] ;get file handle
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sub si,si ;zero the checksum ;accumulator
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mov cx,1 ;request one byte on each ;read
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mov dx,offset TempByte ;point DX to the byte in
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;which DOS should store
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;each byte read
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ChecksumLoop:
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mov ah,3fh ;DOS read file function #
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int 21h ;read the byte
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jcErrorEnd;an error occurred
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and ax,ax ;any bytes read?
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jz Success ;no-end of file reached-we're done
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add si,[TempWord] ;add the byte into the
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;checksum total
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jmpChecksumLoop
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ErrorEnd:
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sub ax,ax ;error
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jmp short Done
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Success:
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mov bx,[bp+Checksum] ;point to the checksum variable
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mov [bx],si ;save the new checksum
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mov ax,1 ;success
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;
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Done:
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pop si ;restore C's register variable
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pop bp
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ret
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_ChecksumFileendp
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end
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```
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The lesson is clear: Optimization makes code faster, but without proper
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design, optimization just creates fast slow code.
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