[Coco] 6309 Division instructions

[jdaggett at gate.net]

Tim 

from the 6309 documentation I h ave the DIVD does a 16 bit by 8 
signed division. The contents of W is divided by memory byte and 
the quotient is stored in W and the modulo (remainder) is in D. 

This yields +32,767/-32,768 divided by +127/-128 ranges. 


Initial thoughts on how to get an overflow condition would mean the 
modulo would overflow or the divisor is "one"?

james

On 1 Aug 2005 at 20:15, tim lindner wrote:

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> I was testing the division instructions against my 6309 core (in MESS)

> and found something unusual.

> 

> My Burke & Burke 6309 documents say that result of DIVD is a signed

> 8-bit value in register B and an unsigned remainder in register A.

> 

> Further more it says that if the quotient overflows, the value of A

> and B will be unchanged and the V condition code will be set.

> 

> But this is not exactly the behiavior I am seeing on real hardware.

> 

> What I am seeing is a sort of two stage overflow. If the quotient

> doesn't fit in an signed 8 bit container but would fit in a unsigned 8

> bit container, then the correct absolute value is wirrten to B and the

> V condition code is set.

> 

> If the value overflows an unsgined container, the registers A and B

> set to the absolute value the the orginal numerator and the V

> condition code is set.

> 

> I have not seen this behiavor described anywhere and I would be

> interested in other peoples thoughts on it.

> 

> -- 

> tim lindner

> tlindner at ix.netcom.com                                   Bright

> 

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