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Bithacks: Difference between revisions

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macro SignedDiv_2N(n)
macro SignedDiv_2N(n)
BMI ?negative
BMI ?negative
LSR #!n
LSR #<n>
BRA ?end
BRA ?end
?negative:
?negative:
LSR #!n
LSR #<n>
ORA #($FF<<(8-!n)) ; sign extension
ORA #($FF<<(8-<n>)) ; sign extension
?end:
?end:
endmacro
endmacro

Revision as of 08:11, 6 April 2021

Bithacks are optimization tricks that utilize information in bits and bit manipulation to accomplish their tasks. Usually they work in a slightly non-obvious way, (the most famous being the fast inverse sqrt), and bit manipulation in general is harder on the 65c816. To that end here is a collection of some useful tricks.
Note: cycle counts are intended to be a worst case measure.

Math Bithacks

Signed Division By 2 (5 bytes / 10 cycles)
inputs: A
outputs: A

STA $00
ASL $00
ROR

Signed Division By 2n (6+2n bytes / 5+2n cycles)
inputs: A, n
outputs: A

; signed division by two, n times
macro SignedDiv_2N(n)
	BMI ?negative
	LSR #<n>
	BRA ?end
?negative:
	LSR #<n>
	ORA #($FF<<(8-<n>))	; sign extension
?end:
endmacro

Misc. Tricks

As this list grows tricks here will be consolidated into their own sections. Clever optimization tricks that aren't necessarily what someone might personally call a "bithack" are okay here as well!