Use backtick code blocks for syntax highlighting
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James Gregory
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09-06.md
154
09-06.md
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@ -12,91 +12,95 @@ pages: 182-185
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**LISTING 9.5 L9-5.ASM**
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; Divides an arbitrarily long unsigned dividend by a 16-bit unsigned
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; divisor. C near-callable as:
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; unsigned int Div(unsigned int * Dividend,
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; int DividendLength, unsigned int Divisor,
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; unsigned int * Quotient);
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;
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; Returns the remainder of the division.
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;
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; Tested with TASM 2.
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```nasm
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; Divides an arbitrarily long unsigned dividend by a 16-bit unsigned
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; divisor. C near-callable as:
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; unsigned int Div(unsigned int * Dividend,
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; int DividendLength, unsigned int Divisor,
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; unsigned int * Quotient);
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;
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; Returns the remainder of the division.
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;
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; Tested with TASM 2.
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parms struc
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dw 2 dup (?) ;pushed BP & return address
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Dividend dw ? ;pointer to value to divide, stored in Intel
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; order, with lsb at lowest address, msb at
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; highest. Must be composed of an integral
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; number of words
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DividendLength dw ? ;# of bytes in Dividend. Must be a multiple
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; of 2
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Divisor dw ? ;value by which to divide. Must not be zero,
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; or a Divide By Zero interrupt will occur
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Quotient dw ? ;pointer to buffer in which to store the
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; result of the division, in Intel order.
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; The quotient returned is of the same
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; length as the dividend
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parmsends
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parms struc
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dw 2 dup (?) ;pushed BP & return address
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Dividend dw ? ;pointer to value to divide, stored in Intel
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; order, with lsb at lowest address, msb at
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; highest. Must be composed of an integral
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; number of words
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DividendLength dw ? ;# of bytes in Dividend. Must be a multiple
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; of 2
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Divisor dw ? ;value by which to divide. Must not be zero,
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; or a Divide By Zero interrupt will occur
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Quotient dw ? ;pointer to buffer in which to store the
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; result of the division, in Intel order.
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; The quotient returned is of the same
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; length as the dividend
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parmsends
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.model small
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.code
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public _Div
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_Divprocnear
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push bp ;preserve caller's stack frame
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mov bp,sp ;point to our stack frame
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push si ;preserve caller's register variables
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push di
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.model small
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.code
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public _Div
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_Divprocnear
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push bp ;preserve caller's stack frame
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mov bp,sp ;point to our stack frame
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push si ;preserve caller's register variables
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push di
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std ;we're working from msb to lsb
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mov ax,ds
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mov es,ax ;for STOS
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mov cx,[bp+DividendLength]
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sub cx,2
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mov si,[bp+Dividend]
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add si,cx ;point to the last word of the dividend
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; (the most significant word)
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mov di,[bp+Quotient]
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add di,cx ;point to the last word of the quotient
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; buffer (the most significant word)
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mov bx,[bp+Divisor]
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shr cx,1
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inc cx ;# of words to process
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sub dx,dx ;convert initial divisor word to a 32-bit
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;value for DIV
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DivLoop:
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lod sw ;get next most significant word of divisor
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div bx
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sto sw ;save this word of the quotient
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;DX contains the remainder at this point,
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; ready to prepend to the next divisor word
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loop DivLoop
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mov ax,dx ;return the remainder
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cld ;restore default Direction flag setting
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pop di ;restore caller's register variables
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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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_Divendp
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end
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std ;we're working from msb to lsb
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mov ax,ds
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mov es,ax ;for STOS
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mov cx,[bp+DividendLength]
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sub cx,2
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mov si,[bp+Dividend]
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add si,cx ;point to the last word of the dividend
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; (the most significant word)
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mov di,[bp+Quotient]
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add di,cx ;point to the last word of the quotient
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; buffer (the most significant word)
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mov bx,[bp+Divisor]
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shr cx,1
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inc cx ;# of words to process
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sub dx,dx ;convert initial divisor word to a 32-bit
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;value for DIV
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DivLoop:
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lod sw ;get next most significant word of divisor
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div bx
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sto sw ;save this word of the quotient
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;DX contains the remainder at this point,
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; ready to prepend to the next divisor word
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loop DivLoop
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mov ax,dx ;return the remainder
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cld ;restore default Direction flag setting
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pop di ;restore caller's register variables
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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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_Divendp
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end
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```
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**LISTING 9.6 L9-6.C**
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/* Sample use of Div function to perform division when the result
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doesn't fit in 16 bits */
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```c
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/* Sample use of Div function to perform division when the result
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doesn't fit in 16 bits */
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#include <stdio.h>
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#include <stdio.h>
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extern unsigned int Div(unsigned int * Dividend,
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int DividendLength, unsigned int Divisor,
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unsigned int * Quotient);
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extern unsigned int Div(unsigned int * Dividend,
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int DividendLength, unsigned int Divisor,
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unsigned int * Quotient);
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main() {
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unsigned long m, i = 0x20000001;
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unsigned int k, j = 0x10;
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main() {
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unsigned long m, i = 0x20000001;
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unsigned int k, j = 0x10;
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k = Div((unsigned int *)&i, sizeof(i), j, (unsigned int *)&m);
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printf("%lu / %u = %lu r %u\n", i, j, m, k);
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}
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k = Div((unsigned int *)&i, sizeof(i), j, (unsigned int *)&m);
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printf("%lu / %u = %lu r %u\n", i, j, m, k);
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}
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```
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#### Sweet Spot Revisited {#Heading9}
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