# Bithacks

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.**

See also: Useful Code Snippets

## Contents

# Math Bithacks

## Signed Division By 2

*3 bytes / 4 cycles*

__inputs:__ A

__outputs:__ A

CMP #$80 ROR

note: This is a sign-preserving division so -1÷2 will stay -1 since 0 is positive

## Signed Division By 2^{n}

*6+n bytes / 6+2n cycles*

__inputs:__ A

__outputs:__ A

; signed division by two, n times macro SignedDiv_2N(n) LSR #<n> BIT.b #$80>><n> BEQ ?positive ORA.b #$FF00>><n> ; sign extension ?positive: endmacro ; -1 cycle and +n bytes, but must have N flag set before use macro SignedDiv_2N(n) BMI ?negative LSR #<n> BRA ?end ?negative: LSR #<n> ORA.b #$FF00>><n> ; sign extension ?end: endmacro

note: This is a sign-preserving division so -1÷2 will stay -1 since 0 is positive

## Absolute Value

*5 bytes / 6 cycles*

__inputs:__ A, (N Flag)

__outputs:__ A

macro abs() BPL ?plus EOR #$FF INC ?plus: ; only 3 cycles if branch taken endmacro

## Absolute Value (SEC)

*4 bytes / 4 cycles*

__inputs:__ A, (Carry Set)

__outputs:__ A

; compared to the branching version this is 1 byte smaller ; it's either 2 cycles slower/faster depending on branch taken EOR #$7F ; SEC ; the instant you add this in it becomes worse than the branching version SBC #$7F

## Magnitude/Extents Check

*~7 bytes / 12 cycles*

__inputs:__ A

__outputs:__ (none)

; asks "Is [A] on the zero-side of value [X] or the far side?" ; good for magnitude checks, smaller *AND* faster than alternatives ; NOTE: in the event that it is exactly [X] it will have that value at branch ; doesn't need to be an indexed CMP but is most useful this way ; this can be used to combine the BPL and BMI checks for both signs into one SEC : SBC Extents,x EOR Extents,x BMI .zero_side .far_side: ; do things .zero_side: ; do things Extents: db -$23, $23

## Sign Extend

*13 bytes / 18 cycles*

__inputs:__ 8bit value in $10

__outputs:__ A

REP #$20 LDA $10-1 ; load $10 into A high, and garbage in low AND #$FF00 ; discard garbage BPL + ORA #$00FF + XBA

# 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!

## Clear Low Byte of Accumulator

*1 byte / 2 cycles*

__inputs:__ (none)

__outputs:__ A

; "Trashes" A but clears low byte TDC

## Direction/Facing As Index

*4 bytes / 6 cycles*

__inputs:__ A

__outputs:__ A

; Ever wonder why facing flags are 0=right and 1=left? This is why. It's incredibly cheap. ; The input here is specifically a signed speed, or similar value. ASL ROL AND #$01

## Check N Conditions True

*n+7 bytes / 2n+7 cycles*

__inputs:__ A

__outputs:__ A

; You can test for multiple conditions being true (7 conditions true, at least 5 conditions, etc.) by simply using a counter and rounding to the next power of 2 and test if that bit is set. ; You can also test for "Less than N True", "More than N", etc. with variations. ; This is almost more a coding technique, but it's super helpful, so worth pointing out. ; It can allow you to re-arrange branches of code as independent blocks among other useful things. ; You can also use any RAM instead of A at a small cost. ; Example Test For 5 True Conditions: !Next_Highest_Power_of_2 = $08 !N_True_Target = $05 LDA #!Next_Highest_Power_of_2!-!N_True_Target-1 ; here we set up our rounding, the -1 isn't strictly necessary *most* of the time %TestSomeCondition() BCC + ; here we're going to say our test just returns carry set on true (but it could directly INC inside the code as well) INC + ; ... repeat the above 5 times for different tests N_True_Test: INC ; replace our -1 to bring us up to a full power of 2 if we had enough True AND #!Next_Highest_Power_of_2 BEQ .false .true: ; N Tests were True .false: ; Not exactly N tests were true

## Skip Dead Code

*1-2 bytes / 2-3 cycles*

__inputs:__ (none)

__outputs:__ (none)

; If you need to skip one byte of dead code (due to a hijack or whatever reason) you can use: NOP ; 1 byte, 2 cycles ; But if you need to skip just 2 bytes the most efficient is: ; NOTE: many times WDM is used as a breakpoint for debugging so only do this as a final pass to speed up your code! WDM ; 2 bytes, 2 cycles ; Finally, if you need to skip a large amount of dead code you can use BRA/JMP instead ; JMP is as fast as BRA on the SNES CPU, but will be slightly slower on SA-1, and 1 cycle slower on SPC. So BRA is recommended ; (The extra byte used for JMP in this case doesn't matter) BRA + ; 2 bytes, 3 cycles ; dead code +

## Check 3 Conditions

*2 bytes / 2 cycles*

__inputs:__ A

__outputs:__ (none)

; just the opcode as normal here (not counting the conditions), using any operand that's not immediate (#) ; it's worth noting that you can do up to 3 tests with a single opcode though! ; Just As A Reminder: the V & N flag are set by the *operand* to BIT not the result of the AND! BIT $00 BMI .bit7_set BVS .bit6_set BNE .bit5_set ; assuming #$20 is in $00 .bit7_set: .bit6_set: .bit5_set: