mjhjmtu

I am trying to code in 6502 assembly and for some reason the CMP instruction doesn't work. For example:

 CLD
 LDY #$03
 LDA #$00
 LDX #$05
 CMP Y
 BEQ Equal
 STX $0200
Equal:
 BRK

This should place a green square on the screen if I am correct but it doesn't, it skips right over the STX line because apparently the equal flag is raised. I'm new to 6502 assembly so I could just be making a mistake.

The code you've posted:

• loads the immediate value 0 into A;


• loads the immediate value 3 into Y;


• then compares the 0 in A to whatever is in memory at the address you've given the label Y.

There are no register-to-register comparisons on the 6502.

There are no register/register operations on the 6502 (except for transfer). The 6502 follows a strict accumulator/memory scheme (with a few extensions for index-register/memory)

Your example is a bit useless, as comparing two constants doesn't make sense, one needs to be a variable at least, right? Lets assume the first (#$03) is a memory location called VALUE instead. So a useful check for VALUE holding 00 would work like this:

 LDX #$05 * Prepare X
 LDA VALUE * Value to compare
 CMP #$00 * Compare with this
 BEQ Equal * Do not store X when equal
 STX $0200 * Store X
Equal:
 BRK

In a real life 6502 programm the CMP instruction can be left out at all, as loading a value already performs a test for Zero.

(Further, CLD is not needed, as decimal mode has no influence beside addition/subtractions and there is no need to use X, as storing a constant value (or moving one) can be done by using A after the BEQ - same programm length but fewer cycles when not needed. And so on :)))