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Bithacks: Difference between revisions
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; signed division by two, n times | ; signed division by two, n times | ||
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 | |||
</pre> | </pre> | ||
Revision as of 07:48, 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.
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