163 lines
6.8 KiB
Markdown
163 lines
6.8 KiB
Markdown
---
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title: Michael Abrash's Graphics Programming Black Book, Special Edition
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author: Michael Abrash
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date: '1997-07-01'
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isbn: '1576101746'
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publisher: The Coriolis Group
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category: 'Web and Software Development: Game Development,Web and Software Development:
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Graphics and Multimedia Development'
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chapter: '07'
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pages: 143-145
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---
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**LISTING 7.2 L7-2.ASM**
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```nasm
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; Program to illustrate searching through a buffer of a specified
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; length until a specified zero byte is encountered.
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; A loop unrolled four times and terminated with LOOP is used.
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.model small
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.stack 100h
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.data
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; Sample string to search through.
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SampleStringlabelbyte
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db ‘This is a sample string of a long enough length '
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db ‘so that raw searching speed can outweigh any '
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db ‘extra set-up time that may be required.',0
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SAMPLE_STRING_LENGTH equ $-SampleString
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; User prompt.
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Prompt db ‘Enter character to search for:$'
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; Result status messages.
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ByteFoundMsg db 0dh,0ah
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db ‘Specified byte found.',0dh,0ah,‘$'
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ZeroByteFoundMsg db 0dh,0ah
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db ‘Zero byte encountered.', 0dh, 0ah, ‘$'
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NoByteFoundMsg db 0dh,0ah
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db ‘Buffer exhausted with no match.', 0dh, 0ah, ‘$'
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; Table of initial, possibly partial loop entry points for
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; SearchMaxLength.
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SearchMaxLengthEntryTable labelword
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dw SearchMaxLengthEntry4
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dw SearchMaxLengthEntry1
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dw SearchMaxLengthEntry2
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dw SearchMaxLengthEntry3
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.code
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Start proc near
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mov ax,@data ;point to standard data segment
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mov ds,ax
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mov dx,offset Prompt
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mov ah,9 ;DOS print string function
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int 21h ;prompt the user
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mov ah,1 ;DOS get key function
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int 21h ;get the key to search for
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mov ah,al ;put character to search for in AH
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mov cx,SAMPLE_STRING_LENGTH ;# of bytes to search
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mov si,offset SampleString ;point to buffer to search
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call SearchMaxLength ;search the buffer
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mov dx,offset ByteFoundMsg ;assume we found the byte
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jc PrintStatus ;we did find the byte
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;we didn't find the byte, figure out
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;whether we found a zero byte or
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;ran out of buffer
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mov dx,offset NoByteFoundMsg
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;assume we didn't find a zero byte
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jcxz PrintStatus ;we didn't find a zero byte
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mov dx,offset ZeroByteFoundMsg ;we found a zero byte
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PrintStatus:
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mov ah,9 ;DOS print string function
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int 21h ;report status
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mov ah,4ch ;return to DOS
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int 21h
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Startendp
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; Function to search a buffer of a specified length until either a
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; specified byte or a zero byte is encountered.
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; Input:
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; AH = character to search for
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; CX = maximum length to be searched (must be > 0)
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; DS:SI = pointer to buffer to be searched
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; Output:
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; CX = 0 if and only if we ran out of bytes without finding
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; either the desired byte or a zero byte
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; DS:SI = pointer to searched-for byte if found, otherwise byte
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; after zero byte if found, otherwise byte after last
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; byte checked if neither searched-for byte nor zero
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; byte is found
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; Carry Flag = set if searched-for byte found, reset otherwise
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SearchMaxLength proc near
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cld
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mov bx,cx
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add cx,3 ;calculate the maximum # of passes
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shr cx,1 ;through the loop, which is
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shr cx,1 ;unrolled 4 times
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and bx,3 ;calculate the index into the entry
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;point table for the first,
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;possibly partial loop
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shl bx,1 ;prepare for a word-sized look-up
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jmp SearchMaxLengthEntryTable[bx]
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;branch into the unrolled loop to do
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;the first, possibly partial loop
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SearchMaxLengthLoop:
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SearchMaxLengthEntry4:
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lodsb ;get the next byte
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cmp al,ah ;is this the byte we want?
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jz ByteFound ;yes, we're done with success
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and al,al ;is this the terminating 0 byte?
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jz ByteNotFound ;yes, we're done with failure
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SearchMaxLengthEntry3:
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lodsb ;get the next byte
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cmp al,ah ;is this the byte we want?
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jz ByteFound ;yes, we're done with success
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and al,al ;is this the terminating 0 byte?
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jz ByteNotFound ;yes, we're done with failure
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SearchMaxLengthEntry2:
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lodsb ;get the next byte
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cmp al,ah ;is this the byte we want?
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jz ByteFound ;yes, we're done with success
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and al,al ;is this the terminating 0 byte?
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jz ByteNotFound ;yes, we're done with failure
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SearchMaxLengthEntry1:
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lodsb ;get the next byte
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cmp al,ah ;is this the byte we want?
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jz ByteFound ;yes, we're done with success
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and al,al ;is this the terminating 0 byte?
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jz ByteNotFound ;yes, we're done with failure
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loop SearchMaxLengthLoop ;it's neither, so check the next
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; four bytes, if any
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ByteNotFound:
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clc ;return "not found" status
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ret
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ByteFound:
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dec si ;point back to the location at which
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; we found the searched-for byte
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stc ;return "found" status
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ret
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SearchMaxLengthendp
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end Start
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```
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How much difference? Listing 7.2 runs in 121 µs—40 percent faster than
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Listing 7.1, even though Listing 7.2 still uses `LOOP` rather than
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`DEC CX/JNZ`. (The loop in Listing 7.2 could be unrolled further, too;
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it's just a question of how much more memory you want to trade for
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ever-decreasing performance benefits.) That's typical of local
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optimization; it won't often yield the order-of-magnitude improvements
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that algorithmic improvements can produce, but it can get you a critical
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50 percent or 100 percent improvement when you've exhausted all other
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avenues.
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> 
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> The point is simply this: You can gain far more by stepping back a bit
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> and thinking of the fastest overall way for the CPU to perform a task
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> than you can by saving a cycle here or there using different
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> instructions. Try to think at the level of sequences of instructions
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> rather than individual instructions, and learn to treat x86 instructions
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> as building blocks with unique characteristics rather than as
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> instructions dedicated to specific tasks.
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