Use backtick code blocks for syntax highlighting
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James Gregory
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16-03.md
183
16-03.md
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@ -12,97 +12,98 @@ pages: 303-305
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**LISTING 16.4 L16-4.ASM**
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; Assembly subroutine for Listing 16.2. Scans through Buffer, of
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; length BufferLength, counting words and updating WordCount as
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; appropriate, using a lookup table-based approach. BufferLength
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; must be > 0. *CharFlag and *WordCount should equal 0 on the
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; first call. Tested with TASM.
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; C near-callable as:
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; void ScanBuffer(char *Buffer, unsigned int BufferLength,
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; char *CharFlag, unsigned long *WordCount);
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parms struc
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dw 2 dup(?) ;pushed return address & BP
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Buffer dw ? ;buffer to scan
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BufferLength dw ? ;length of buffer to scan
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CharFlag dw ? ;pointer to flag for state of last
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;char processed on entry (0 on
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;initial call). Updated on exit
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WordCount dw ? ;pointer to 32-bit count of words
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; found (0 on initial call)
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parms ends
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.model small
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.data
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; Table of char/not statuses for byte values 0-255 (128-255 are
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; duplicates of 0-127 to effectively mask off bit 7, which some
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; word processors set as an internal flag).
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CharStatusTable label byte
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REPT 2
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db 39 dup(0)
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db 1 ;apostrophe
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db 8 dup(0)
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db 10 dup(1) ;0-9
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db 7 dup(0)
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db 26 dup(1) ;A-Z
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db 6 dup(0)
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db 26 dup(1) ;a-z
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db 5 dup(0)
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ENDM
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.code
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public _ScanBuffer
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_ScanBuffer proc near
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push bp ;preserve caller's stack frame
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mov bp,sp ;set up local stack frame
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push si ;preserve caller's register vars
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push di
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mov si,[bp+Buffer] ;point to buffer to scan
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mov bx,[bp+WordCount]
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mov di,[bx] ;get current 32-bit word count
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mov dx,[bx+2]
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mov bx,[bp+CharFlag]
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mov al,[bx] ;get current CharFlag
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mov cx,[bp+BufferLength] ;get # of bytes to scan
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mov bx,offset CharStatusTable
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ScanLoop:
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and al,al ;ZF=0 if last byte was a char,
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; ZF=1 if not
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lodsb ;get the next byte
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;***doesn't change flags***
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xlat ;look up its char/not status
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;***doesn't change flags***
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jz ScanLoopBottom ;don't count a word if last byte was
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; not a character
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and al,al ;last byte was a character; is the
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; current byte a character?
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jz CountWord ;no, so count a word
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ScanLoopBottom:
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dec cx ;count down buffer length
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jnz ScanLoop
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Done:
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mov si,[bp+CharFlag]
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mov [si],al ;set new CharFlag
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mov bx,[bp+WordCount]
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mov [bx],di ;set new word count
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mov [bx+2],dx
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pop di ;restore caller's register vars
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pop si
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pop bp ;restore caller's stack frame
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ret
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align 2
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CountWord:
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add di,1 ;increment the word count
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adc dx,0
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dec cx ;count down buffer length
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jnz ScanLoop
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jmp Done
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_ScanBuffer endp
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end
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```nasm
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; Assembly subroutine for Listing 16.2. Scans through Buffer, of
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; length BufferLength, counting words and updating WordCount as
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; appropriate, using a lookup table-based approach. BufferLength
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; must be > 0. *CharFlag and *WordCount should equal 0 on the
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; first call. Tested with TASM.
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; C near-callable as:
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; void ScanBuffer(char *Buffer, unsigned int BufferLength,
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; char *CharFlag, unsigned long *WordCount);
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parms struc
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dw 2 dup(?) ;pushed return address & BP
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Buffer dw ? ;buffer to scan
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BufferLength dw ? ;length of buffer to scan
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CharFlag dw ? ;pointer to flag for state of last
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;char processed on entry (0 on
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;initial call). Updated on exit
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WordCount dw ? ;pointer to 32-bit count of words
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; found (0 on initial call)
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parms ends
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.model small
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.data
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; Table of char/not statuses for byte values 0-255 (128-255 are
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; duplicates of 0-127 to effectively mask off bit 7, which some
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; word processors set as an internal flag).
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CharStatusTable label byte
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REPT 2
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db 39 dup(0)
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db 1 ;apostrophe
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db 8 dup(0)
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db 10 dup(1) ;0-9
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db 7 dup(0)
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db 26 dup(1) ;A-Z
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db 6 dup(0)
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db 26 dup(1) ;a-z
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db 5 dup(0)
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ENDM
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.code
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public _ScanBuffer
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_ScanBuffer proc near
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push bp ;preserve caller's stack frame
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mov bp,sp ;set up local stack frame
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push si ;preserve caller's register vars
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push di
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mov si,[bp+Buffer] ;point to buffer to scan
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mov bx,[bp+WordCount]
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mov di,[bx] ;get current 32-bit word count
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mov dx,[bx+2]
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mov bx,[bp+CharFlag]
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mov al,[bx] ;get current CharFlag
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mov cx,[bp+BufferLength] ;get # of bytes to scan
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mov bx,offset CharStatusTable
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ScanLoop:
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and al,al ;ZF=0 if last byte was a char,
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; ZF=1 if not
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lodsb ;get the next byte
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;***doesn't change flags***
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xlat ;look up its char/not status
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;***doesn't change flags***
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jz ScanLoopBottom ;don't count a word if last byte was
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; not a character
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and al,al ;last byte was a character; is the
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; current byte a character?
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jz CountWord ;no, so count a word
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ScanLoopBottom:
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dec cx ;count down buffer length
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jnz ScanLoop
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Done:
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mov si,[bp+CharFlag]
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mov [si],al ;set new CharFlag
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mov bx,[bp+WordCount]
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mov [bx],di ;set new word count
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mov [bx+2],dx
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pop di ;restore caller's register vars
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pop si
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pop bp ;restore caller's stack frame
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ret
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align 2
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CountWord:
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add di,1 ;increment the word count
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adc dx,0
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dec cx ;count down buffer length
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jnz ScanLoop
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jmp Done
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_ScanBuffer endp
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end
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```
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Listing 16.4 features several interesting tricks. First, it uses
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**LODSB** and **XLAT** in succession, a very neat way to get a
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