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help me bug fix 32bit multiplication


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Hi !

 

I have here a 32bit multiplication routine which is simply adapted from this code from the Codebase64.

 

http://codebase64.org/doku.php?id=base:16bit_multiplication_32-bit_product

 

The following code was adapted by FJC.

Num1	.ds 4
Num2	.ds 4
Num3	.ds 8

​.proc MulLong ; 32 bit multiplication: Num1 by Num2, result in Num3
	lda #0
	sta Num3+4
	sta Num3+5
	sta Num3+6
	sta Num3+7
	ldx #32 ; loop for each bit
multloop
	lsr Num1+3 ; divide multiplier by 2
	lsr Num1+2
	lsr Num1+1
	ror Num1
	bcc rotate
	lda Num3+4 ; get upper half of product and add multiplicand
	clc
	adc Num2
	sta Num3+4
	lda Num3+5
	adc Num2+1
	sta Num3+5
	lda Num3+6
	adc Num2+2
	sta Num3+6
	lda Num3+7
	adc Num2+3
rotate
	ror ; rotate partial product 
	sta Num3+7
	ror Num3+6
	ror Num3+5
	ror Num3+4
	ror Num3+3
	ror Num3+2
	ror Num3+1
	ror Num3
	dex
	bne multloop
	lda Num3
	ldx Num3+1
	rts
.endp

But I'm afraid it has a bug which I can't really fix. The following shows the dilemma: $4 * $16b62 = $5ad88
	mda #$16b62 Num1
	mda #$4 Num2
	jsr MulLong
	mda Num3 Temp	; save result

	jmp *

It's producing: "$1AD88" rather than "$5AD88".

 

However this one works: $16b62 * $4 = $5ad88

	mda #$4 Num1
	mda #$16b62 Num2
	jsr MulLong
	mda Num3 Temp	; save result

FJC: It seems that some upper bits of multiplier/multiplicand aren't handled properly (so result is different depending on which has set or cleared upper bits).

 

Any ideas?! Any help is appreciated!!

 

greetings,

\thomas

Edited by twh/f2
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