Pasm - set a specific bit

Agent420

Probably the Dumb Question Of The Day™


Say I have a register that contains the decimal number of a bit I want to set (0-31)...· What is the most efficient method to accomplish this?

I need to create a 1 bit mask·for later use.· X contains the bit # to set.· In this example, I reset the mask to 1 because I do not believe I will encounter a non-zero value.

                        mov     mask,#1                 '+ 4 reset pixel mask
                        mov     temp,x                  '+ 4 copy x to work   
                        and     temp,#31                '+ 4 calc pixel bit #
                        shl     mask,temp               '+ 4 set pixel mask

I'm guessing that if I needed to address a potential 0 value, I'd test and use the carry flag in a rotate, but I have limited clock cycles for this application.

Look good, or is there a better method?

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lonesock

I haven't tried it, but does ROL automatically use only the bottom 5 bits? If so the and $31 would be automatic.

mov mask,#1
rol mask,x

I haven't tried it, though.

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lonesock
Piranha are people too.

Phil Pilgrim (PhiPi)

You don't need the and, and so you can dispense with temp. The shl only uses the lower five bits of the source.

-Phil

Agent420

I should have prefaced that my x value could well be > 31, but I do another shift elsewhere to calculate that value to be used as an offset.

Nonetheless, I did not realize the shift and rotate operations were limited to the lower 5 bits.· Thanks - that will give me a few more cycles ;-)

Thanks!

:read_x                 rdlong  x, idx                  '+ 8 read x coord
                        mov     mask,#1                 '+ 4 reset pixel mask
                        shl     mask,x                  '+ 4 set pixel mask
                        shr     x,#5                    '+ 4 divide x by 32 = offset
:xoff                   mov     xoff,x                  '+ 4 store offset
:mpix                   mov     mpix,mask               '+ 4 store pixel

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Post Edited (Agent420) : 9/10/2009 7:12:12 PM GMT