6502 assembly optimisations
This page is about optimisations that are possible in assembly language, or various things one programmer has to keep in mind to make his code as optimal as possible.
There is two major kind of optimisations: Optimisation for speed (code executes in fewer cycles) and optimisation for size (the code takes fewer bytes).
There is also some other kinds of optimisations, such as constant-executing-time optimisation (code execute in a constant number of cycle no matter what it has to do), or RAM usage optimisation (use as few variables as possible). Because those optimisations have more to do with the algorithm than with its implementation in assembly, only speed and size optimisations will be discussed in this article.
Optimise both speed and size of the code
Avoid a jsr + rts chain
A tail call occurs when a subroutine finishes by calling another subroutine. This can be optimised into a JMP instruction:
<source lang="6502tasm"> MySubroutine
lda Foo sta Bar jsr SomeRandomRoutine rts
</source>
becomes :
<source lang="6502tasm"> MySubroutine
lda Foo sta Bar jmp SomeRandomRoutine
</source>
Savings : 9 cycles, 1 byte
Split word tables in high and low components
This optimisation is not human friendly, makes the source code much bigger, but still makes the compiled[1] size smaller and faster:
<source lang="6502tasm"> Example
lda FooBar asl A tax lda PointerTable,X sta Temp lda PointerTable+1,X sta Temp+1 ....
PointerTable
.dw Pointer1, Pointer2, ....
</source>
Becomes :
<source lang="6502tasm"> Example
ldx FooBar lda PointerTableL,X sta Temp lda PointerTableH,X sta Temp+1 ....
PointerTableL
.byt <Pointer1, <Pointer2, ....
PointerTableH
.byt >Pointer1, >Pointer2, ....
</source>
Some assemblers may have a way to implement a macro to automatically make the table coded like this (Unofficial MagicKit Assembler is one such program).
Savings : 2 bytes, 4 cycles
Use Jump tables with RTS instruction instead of JMP indirect instruction
The so-called RTS Trick is a method of implementing jump tables by pushing a subroutine's entry point to the stack.
<source lang="6502tasm">
- This
ldx JumpEntry lda PointerTableL,X sta Temp lda PointerTableH,X sta Temp+1 jmp [Temp]
- becomes this
ldx JumpEntry lda PointerTableH,X pha lda PointerTableL,X pha rts
</source>
Note that PointerTable entries must point to one byte before the intended target when the RTS trick is used, because RTS will add 1 to the offset.
Savings : 4 bytes, 1 cycle.
To squeeze slightly more, it's possible to combine this with the tail call optimization:
<source lang="6502tasm">
- This
jsr SomeOtherFunction ; MUST NOT modify JumpEntry ldx JumpEntry lda PointerTableH,X pha lda PointerTableL,X pha rts
- Becomes this
ldx JumpEntry lda PointerTableH,X pha lda PointerTableL,X pha jmp SomeOtherFunction
</source> Here, the CPU runs SomeOtherFunction, then returns to the function from the jump table, then returns to what called this dispatcher.
Use a macro instead of a subroutine which is only called once
What is the point to call a subroutine if you only call it at a single place? It would be more optimal to just insert the code where the subroutine is called. However this makes the code less structured and harder to understand. Inline expansion of a subroutine into another subroutine can be done with a macro. One drawback is that if the subroutine is called in a loop, it may require some JMPing to work around the 128-byte limit on branch length.
How macros are used depends on the assembler so no code examples will be placed here to avoid further confusion.
In C, the static inline keyword acts as a hint to expand a function as a macro.
Savings : 4 bytes, 12 cycles.
Arithmetic shift right
Compact way to divide a variable by 2 but keep its sign:
<source lang="6502tasm">
cmp #$80 ror A
</source>
Easily test 2 upper bits of a variable
<source lang="6502tasm">
lda FooBar asl A ;C = b7, N = b6
</source>
Alternative:
<source lang="6502tasm">
bit Foobar ;N = b7, V = b6, regardless of the value of A.
</source>
This can be e.g. used to poll the sprite-0-hit flag in $2002.
Negating a value without temporaries
<source lang="6502tasm">
eor #$FF clc adc #1
</source>
Avoiding the need for CLC/SEC with ADC/SBC
If you are using ADC #imm, and you know your carry is already cleared,
you do not need to do CLC. However, if you know that carry is set
(for example, your code is located in a branch that is only ever entered with a BCS instruction),
you can still avoid using CLC by just doing ADC #(value-1).
The PLOT subroutine in the Apple II Monitor uses this.
Similarly for SBC #imm: If you know know that carry is clear, you can still avoid using SEC by just doing SBC #(value+1).
Test bits in decreasing order
<source lang="6502tasm">
lda foobar
bmi bit7_set
cmp #$40 ; we know that bit 7 wasn't set
bcs bit6_set
cmp #$20
bcs bit5_set
; and so on
</source>
Or if you do not need to preserve the value of A:
<source lang="6502tasm">
lda foobar
asl
bcs bit7_set
asl
bcs bit6_set
asl
bcs bit5_set
; and so on
</source>
This saves one byte per comparison, but 2 cycles more are used because of the extra ASL.
Test bits in increasing order
<source lang="6502tasm">
lda foobar
lsr
bcs bit0_set
lsr
bcs bit1_set
lsr
bcs bit2_set
; and so on
</source>
Note: This does not preserve the value of A.
Test bits without destroying the accumulator
The AND instruction can be used to test bits, but this destroy the value in the accumulator. The BIT can do this but it has no immediate adressing mode. A way to do it is to look for an opcode that has the bits you want to test, and use bit $xxxx on this opcode.
<source lang="6502tasm"> Example
lda foobar and #$30 beq bits_clear lda foobar ....
bits_clear
lda foobar .....
</source>
becomes :
<source lang="6502tasm"> Example
lda foobar bit _bmi_instruction ;equivalent to and #$30 but preserves A beq bits_clear ....
bits_clear
.....
anywhere_in_the_code
....
_bmi_instruction ;The BMI opcode = $30
bmi somewhere
</source>
Savings : 2 cycles, 3 bytes
Use opposite rotate instead of a great number of shifts
To retrieve the 3 highest bits of a value in the low positions, you might be tempted to do 5 LSRs in a row. However, if you do not need the 5 top bits to be cleared, this is more efficient: <source lang="6502tasm">
lda value ; got: 76543210 c rol ; got: 6543210c 7 rol ; got: 543210c7 6 rol ; got: 43210c76 5 rol ; got: 3210c765 4 ; Only care about these ^^^
</source>
It works the same for replacing 5 ASLs with 4 RORs.
To replace 6 ASLs you can use 3 RORs: <source lang="6502tasm">
lda value ; got: 76453210 c ror ; got: c7654321 0 ror ; got: 0c765432 1 ror ; got: 10c76543 2 and #$C0 ; got: 10------
</source>
Optimise speed at the expense of size
Those optimisations will make code faster to execute, but use more ROM. Therefore, it is useful in NMI routines and other things that need to run fast.
Use identity look-up table instead of temp variable
<source lang="6502tasm"> Example
ldx Foo lda Bar stx Temp clc adc Temp ;A = Foo + Bar
</source>
becomes :
<source lang="6502tasm"> Example
ldx Foo lda Bar clc adc Identity,X ;A = Foo + Bar
Identity
.byt $00, $01, $02, $03, .....
</source>
If your program is very large, it is possible that this way uses slightly less ROM; also, it might save one byte of RAM in some circumstances.
Savings : 2 cycles
Use look-up table to shift left 4 times
Provided that the high nibble is already cleared, you can shift left by 4 by making a multiplication look-up table. <source lang="6502tasm"> Example:
lda rownum asl A asl A asl A asl A rts
</source>
becomes
<source lang="6502tasm"> Example:
ldx rownum lda times_sixteen,x rts
times_sixteen:
.byt $00, $10, $20, $30, $40, $50, $60, $70 .byt $80, $90, $A0, $B0, $C0, $D0, $E0, $F0
</source>
In very large programs, this might save some ROM space. However, it will use up the X register, so it might not be ideal.
Savings: 4 cycles
Optimise code size at the expense of cycles
Those optimisations will produce code that is smaller but takes more cycles to execute. Therefore, it can be used in the parts of the program that do not have to be fast.
Use the stack instead of a temp variable
<source lang="6502tasm"> Example
lda Foo sta Temp lda Bar .... .... lda Temp ;Restores Foo .....
</source>
becomes:
<source lang="6502tasm"> Example
lda Foo pha lda Bar .... .... pla ;Restores Foo .....
</source>
Savings : 2 bytes.
Use an "intelligent" argument system
Each time a routine needs multiple bytes of arguments (>3) it's hard to code it without wasting a lot of bytes.
<source lang="6502tasm"> Example
lda Argument1 sta Temp lda Argument2 ldx Argument3 ldy Argument4 jsr RoutineWhichNeeds4Args .....
</source>
Becomes something like:
<source lang="6502tasm"> Example
jsr PassArguments .dw RoutineWhichNeeds4Args .db Argument1, Argument2, Argument3, Argument4 .db $00 ....
PassArguments
pla tay pla pha ; put the high byte back sta pointer+1 ldx #$00 beq SKIP
LOOP
sta parameters,x inx
SKIP
iny ; pointing one short first pass here fixes that lda (pointer),y bne LOOP iny lda (pointer),y beq LOOP
dey ; fix the return address guess we can't return to a
; break
tya
pha
jmp (parameters)
</source>
Syscalls in Apple ProDOS[2] and FDS BIOS work this way.
Savings : Complicated to estimate - only saves bytes if the trick is used fairly often across the program, in order to compensate for the size of the PassArguments routine.
Using relative branch instruction instead of absolute
If the state of one of the processor flags is already known at this point and the branch target is not too far away, then you can use a condition branch instruction.
Savings : 1 byte.
See also
Notes
- ↑ Pedants may complain that "compile" is incorrect terminology for "translate a program written in assembly language into object code". But it is the most familiar term meaning "translate a program, no matter the language, into object code", and the same issues apply to code generators within a compiler that targets the 6502 as to programs written in 6502 assembly language.
- ↑ ProDOS 8 Technical Reference Manual
