[Coco] Mod10 Suggestions

[William Astle]

On 2017-02-18 12:43 AM, msmcdoug wrote:
> Actually I'm surprised noone has suggested bcd arithmetic on the result to eliminate divide by 10 loop

BCD would certainly give a predictable overall cycle count. It would 
require a significantly different approach, though. The only register 
you can use for BCD arithmetic is A and DAA is only useful after ADDA or 
ADCA.

I had thought about using BCD but had initially dismissed it due to 
possible complexity. However, upon reflection, the extra cycles to use 
BCD would probably be less than the average cycle time of the modulus 
loop combined or checking for digit overflow during the loop.

I think you could use code that looks something like the following which 
is based off Mr. Mikrut's most recent posted code. (warning: mailer 
code™ follows so it may have errors)

	ORG $1200
CCD	RMB 16
RESULT	RMB 1
START	LEAX CCD+16,PCR
	CLRA
	LDB #8
LOOP	PSHS A
	LDA ,-X
	LSLA
	CMPA #10
	BLO LOOP2
	SUBA #9
LOOP2	ADDA ,S+
	DAA
	ADDA ,-X
	DAA
	DECB
	BNE LOOP
	ANDA #$0F
	STA RESULT,PCR
ENDPGM	RTS

I'm using the stack for a temporary storage location instead of 
something PCR relative for code size reasons. You could use the "RESULT 
variable for the temporary to eliminate stack usage. That would probably 
be slightly faster at the expense of two more code bytes. This is one of 
those size/speed trade-offs.

DAA has to be used after every addition and only applies to A. Using BCD 
means we can eliminate the mod 10 loop and just mask off the upper digit 
(BCD stores two decimal digits in a byte). That gives a constant time 
for the "mod 10" result and also only takes 2 bytes (and 2 cycles).

I have also eliminated the STATUS variable and just store the result. 
You can test RESULT for non-zero trivially so there's no need for a 
separate STATUS value.

By my calculation, this version is 32 bytes, requires 1 byte of stack 
space, 17 bytes of data space, and runs in a maximum of 351 cycles (and 
a minimum of 336 cycles if none of the doubled digits goes above 9). For 
this analysis, I've assumed 8 bit offsets for the PCR references. 16 bit 
offsets in PCR mode are quite a bit more expensive (4 extra cycles and 1 
extra byte).