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Bithacks
From SnesLab
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!