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01-03.md
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------------------------ --------------------------------- --------------------
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[Previous](01-02.html) [Table of Contents](index.html) [Next](01-04.html)
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------------------------ --------------------------------- --------------------
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* * * * *
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Listing
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Borland
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Microsoft
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Borland
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Microsoft
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Assembly
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Optimization\
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Ratio
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* * * * *
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(no opt)
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(no opt)
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(opt)
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(opt)
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1
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166.9
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166.8
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167.0
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165.8
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155.1
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1.08
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4
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13.5
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13.6
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13.5
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13.5
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...
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1.01
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5
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4.7
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5.5
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3.8
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3.4
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2.7
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2.04
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Ratio best\
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designed\
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to worst\
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designed
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35.51
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30.33
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43.95
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48.76
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57.44
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**Note:** The execution times (in seconds) for this chapter’s listings
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were timed when the compiled listings were run on the WordPerfect 4.2
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thesaurus file TH.WP (362,293 bytes in size), as compiled in the small
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model with Borland and Microsoft compilers with optimization on (opt)
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and off (no opt). All times were measured with Paradigm Systems’ TIMER
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program on a 10 MHz 1-wait-state AT clone with a 28-ms hard disk, with
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disk caching turned off.
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* * * * *
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Table 1.1 Execution Times for WordPerfect Checksum.
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* * * * *
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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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#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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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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**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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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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Well, then, how are we going to improve our design? Before we can do
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that, we have to understand what’s wrong with the current design.
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#### Know the Territory {#Heading9}
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Just why is Listing 1.1 so slow? In a word: overhead. The C library
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implements the **read()** function by calling DOS to read the desired
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number of bytes. (I figured this out by watching the code execute with a
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debugger, but you can buy library source code from both Microsoft and
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Borland.) That means that Listing 1.1 (and Listing 1.3 as well) executes
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one DOS function per byte processed—and DOS functions, especially this
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one, come with a lot of overhead.
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For starters, DOS functions are invoked with interrupts, and interrupts
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are among the slowest instructions of the x86 family CPUs. Then, DOS has
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to set up internally and branch to the desired function, expending more
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cycles in the process. Finally, DOS has to search its own buffers to see
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if the desired byte has already been read, read it from the disk if not,
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store the byte in the specified location, and return. All of that takes
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a *long* time—far, far longer than the rest of the main loop in Listing
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1.1. In short, Listing 1.1 spends virtually all of its time executing
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**read(),** and most of that time is spent somewhere down in DOS.
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You can verify this for yourself by watching the code with a debugger or
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using a code profiler, but take my word for it: There’s a great deal of
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overhead to DOS calls, and that’s what’s draining the life out of
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Listing 1.1.
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How can we speed up Listing 1.1? It should be clear that we must somehow
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avoid invoking DOS for every byte in the file, and that means reading
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more than one byte at a time, then buffering the data and parceling it
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out for examination one byte at a time. By gosh, that’s a description of
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C’s stream I/O feature, whereby C reads files in chunks and buffers the
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bytes internally, doling them out to the application as needed by
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reading them from memory rather than calling DOS. Let’s try using stream
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I/O and see what happens.
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Listing 1.4 is similar to Listing 1.1, but uses **fopen()** and
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**getc()** (rather than **open()** and **read()**) to access the file
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being checksummed. The results confirm our theories splendidly, and
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validate our new design. As shown in Table 1.1, Listing 1.4 runs more
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than an order of magnitude faster than even the assembly version of
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Listing 1.1, *even though Listing 1.1 and Listing 1.4 look almost the
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same*. To the casual observer, **read()** and **getc()** would seem
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slightly different but pretty much interchangeable, and yet in this
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application the performance difference between the two is about the same
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as that between a 4.77 MHz PC and a 16 MHz 386.
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------------------------ --------------------------------- --------------------
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[Previous](01-02.html) [Table of Contents](index.html) [Next](01-04.html)
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------------------------ --------------------------------- --------------------
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* * * * *
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Graphics Programming Black Book © 2001 Michael Abrash
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