[Coco] Mod10 Suggestions
William Astle
lost at l-w.ca
Sat Feb 18 00:52:26 EST 2017
On 2017-02-17 07:48 PM, Brett Gordon wrote:
> tfr a,b no aax ;(
> abx accum even byte
A couple of points:
Since you know that A is going to be in the range of 0...9, you don't
need to use abx. You can simply use "leax a,x". Which also has the
benefit of being 1 byte shorter and 4 cycles faster.
Also, since the maximum possible sum from the algorithm is 216, you
should be able to do the mod calculation on B only. The only difference
would be that you'd be looking for a carry instead of negative after the
subtractions. That will be a real saving considering that that mod
calculation loop could run 21 times. (saves 1 cycle for each subtraction).
Here's how I arrived at 216: 8 of the digits are added as is and 8 of
them are doubled and added. Since no digit can be higher than 9, the
maximum value is 24*9 which is 216.
If doing position independent code (PIC), you can't use the "CMPY
#num+16" method of ending the loop. There is no reasonable way simulate
that. You would need to use a counter of some kind in that case. The
most efficient way to do that is a single byte counter somewhere,
possibly on the stack.
If you do decide to use absolute addresses for the data area, then I
would recommend using U for the pointer instead of Y just to save a byte
in the setup.
In case you're wondering, the typical priority order for using the index
registers is:
X
U
Y
Don't let the "user stack pointer" name for "U" make you think you
shouldn't use U as an index register. There is nothing magical about U
that will cause it to suddenly be used as a stack without your direct
intervention as the programmer.
The reason for the listed ordering is as follows. All operations on X
have a single byte opcode. LEAU, LDU and STU have single byte opcodes.
All operations on Y except LEAY have double byte opcodes. There is no
difference in size or speed when using any of the four pointer registers
(S is also a pointer register) in an indexed operand.
X or Y can both be used as 16 bit counters because LEAX and LEAY will
set Z. (LEAU and LEAS do not.) X would be preferred there in most cases
but if you have tension between index registers and counters, it comes
down to judgement.
X also has the ABX operation which can be useful so that can affect how
you allocate registers, too.
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