Bit fill in PASM?

Drone

I have a long in a speed critical PASM program. If the value of the long is $00_00_00_00 I want it to stay that way. If the long is $00_00_00_01 I want to fill the LSByte with ones, so $00_00_00_01 maps to $00_00_00_FF. The values o of the long will only ever start with $00_00_00_00 or $00_00_00_01.

I can do this with if-else, but when if-else tests false it wastes four clock cycles. I would like to know if there's a single direct operation that can perform this. Something like a shift left that preserves the LSB. I can't seem to find a solution.

Thanks for any suggestions - David

Post Edited (Drone) : 8/18/2009 11:34:58 AM GMT

Ale

it depends how many instructions are you willing to use, some methods could be:

using a table

   movs  temp, my_long
   add   temp, #table
temp
   mov  result, 0-0

table  long 0
         long $00_00_00_ff

with shifts, assuming result has the value already:

  ror  result, #1
  asr  result, #31
  shr result, #24

another could be, if the value is in memory, this could be the shortest I can think of...

  test value, #1 wc
  muxc value, #$ff

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ericball

The only way you can do it in a single instruction is if you set a flag in a previous instruction. The easiest would be the Z flag when the value is loaded into a register. Then you could use the flag to perform a conditional load #$FF.

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Mike Green

Typically whatever instruction produced the value 1 or 0 could set the zero flag. Then you could do:

MUXNZ value,#$FF

lonesock

Drone said...
...
I can do this with if-else, but when if-else tests false it wastes four clock cycles. I would like to know if there's a single direct operation that can perform this. Something like a shift left that preserves the LSB. I can't seem to find a solution.
...

RCL will load the C flag into multiple LSBs, but of course the flag must be set first.

Jonathan

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lonesock
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Drone

Thanks for all the suggestions. Flags seem the way to go. Need to do some experiments and comparisons tonight. With your help I'm getting better with PASM, but still a long way to go

Best Regards, David