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Next let’s look at some simple byte variable manipulation. Here is the first sample Action program:
And the assembly code
This code starts at memory location $24B3 which is where the global variable I is stored. Next we have a jump instruction that gets us to the MAIN procedure. Like last time this is a little inefficient but not a big problem at the start of the program.
Next we load the immediate value 1 into I. I assume they use the Y register for this so that A can be reserved for math operations which will allow for some optimizations under certain conditions. I have another example that I will post at some point that shows this. In this situation by using Y they actually miss out on the opportunity for an optimization. If A had been used the LDA would not have been necessary. Finally a normal 6502, 8-bit add sequence is executed and the result stored by in I.
Now let’s look at almost the same program, but it will add one to I instead of two:
As you can see the code is quite a bit smaller. The compiler recognized that I=I+1 is an increment so it uses the INC instruction on the memory location where I is stored resulting in a savings of three instructions.
You may notice that this example is at a different memory location then the first one. I am not actually sure why this happened. I have been compiling these examples using an emulator and those two examples were compiled at different times so it’s possible that I had the emulator configured differently each time.
BYTE I PROC MAIN() I=1 I=I+2 RETURN
And the assembly code
24B3: .byte $00 24B4: JMP $24B7 24B7: LDY #$01 24B9: STY $24B3 24BC: CLC 24BD: LDA $24B3 24C0: ADC #$02 24C2: STA $24B3 24C5: RTS
This code starts at memory location $24B3 which is where the global variable I is stored. Next we have a jump instruction that gets us to the MAIN procedure. Like last time this is a little inefficient but not a big problem at the start of the program.
Next we load the immediate value 1 into I. I assume they use the Y register for this so that A can be reserved for math operations which will allow for some optimizations under certain conditions. I have another example that I will post at some point that shows this. In this situation by using Y they actually miss out on the opportunity for an optimization. If A had been used the LDA would not have been necessary. Finally a normal 6502, 8-bit add sequence is executed and the result stored by in I.
Now let’s look at almost the same program, but it will add one to I instead of two:
BYTE I PROC MAIN() I=1 I=I+1 RETURN
0E58: .byte #$00 0E59: JMP $0E5C 0E5C: LDY #$01 0E5E: STY $0E58 0E61: INC $0E58 0E64: RTS
As you can see the code is quite a bit smaller. The compiler recognized that I=I+1 is an increment so it uses the INC instruction on the memory location where I is stored resulting in a savings of three instructions.
You may notice that this example is at a different memory location then the first one. I am not actually sure why this happened. I have been compiling these examples using an emulator and those two examples were compiled at different times so it’s possible that I had the emulator configured differently each time.
