[Coco] HD63x09 Question

[jdaggett at gate.net]

Robert

Thanks. Nice of Google to keep some old USENET postings. It was only 
recently that I finally decided to play with the 6309 and after some readings 
I was not certain and this was a godd test of my understanding. 

Well  I guess I willstay with the old unsigned 8x8 MUL opcode and use the 
sum of partial products. That is tried and true an dbattle hardened.  

james 



On 5 Jan 2005 at 11:47, Robert Gault wrote:

Date sent:      	Wed, 05 Jan 2005 11:47:26 -0500
From:           	Robert Gault <robert.gault at worldnet.att.net>
To:             	CoCoList for Color Computer Enthusiasts 
<coco at maltedmedia.com>
Subject:        	Re: [Coco] HD63x09 Question
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> You are correct in your assumptions. Be careful with the MUL and DIV
> instructions as there are very unexpected results from overflow. In
> fact, the DIV instructions seem not to work at all if an overflow
> would be the result.
> 
> It is possible to write code that uses MULD to get an unsigned answer
> but the overhead may make it not worth the effort.
> 
> *Unsigned 16by16 bit multiply
> *ENTER: numbers in regD & REGX
> *EXIT: regD&X unchanged, answer in regY&U
> 
> MUL16	PSHS	D,X
>  MULD	2,S
>  TFR	D,Y
>  TFR	W,U	regY&U now contain signed answer
>  BNE	NOTZRO
>  TSTW
>  BEQ	ZRO
> *Convert signed answer to unsigned. Only regY (MSW) will change.
> 
> *If answer is Neg, add positive multipliers
> NOTZRO	BPL	MULP2
>  PULS	D,X
>  TSTA
>  BMI	MB1
>  ADDR	D,Y
> MB1	CMPX	#$8000
>  BHS	MC1
>  ADDR	X,Y
> MC1	RTS
> 
> *If answer is Pos, add negative multipliers
> MULP2	PULS	D,X
>  TSTA
>  BPL	MB2
>  ADDR	D,Y
> MB2	CMPX	#$8000
>  BLO	MC2
>  ADDR	X,Y
> MC2	RTS