6502 CMP instruction doesn't compare as expected

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4
down vote

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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.

edited 4 mins ago

ThorbjÃ¸rn Ravn Andersen

1657

asked 5 hours ago

user115898

232

New contributor

2

It would help if you could also say which assembler and machine you're trying to code for. (regardless I suspect the answer that's been given is correct)
â€“Â PeterI
5 hours ago

Which 6502-based machine has screen (colour?) memory at $0200?
â€“Â berendi
5 hours ago

This is just a simple assembler/ide that I picked up from Github user skilldrick, Its not really a full machine.
â€“Â user115898
5 hours ago

Unless Y is declared as a label somewhere (which is probably not a good label name to use, but for the sake of argument), that should not even assemble. Sounds like it's not a very mature assembler. Update: It's definitely not. Tried LDA X, STA Y at skilldrick.github.io/easy6502 and that assembles without errors (to an address operand of 0xFF 0xFF, as does any unknown labels).Â¯_(Ã£ÂƒÂ„)_/Â¯
â€“Â Cumbayah
2 hours ago

add a commentÂ |Â

up vote
4
down vote

favorite

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

edited 4 mins ago

ThorbjÃ¸rn Ravn Andersen

1657

asked 5 hours ago

user115898

232

New contributor

2

It would help if you could also say which assembler and machine you're trying to code for. (regardless I suspect the answer that's been given is correct)
â€“Â PeterI
5 hours ago

Which 6502-based machine has screen (colour?) memory at $0200?
â€“Â berendi
5 hours ago

This is just a simple assembler/ide that I picked up from Github user skilldrick, Its not really a full machine.
â€“Â user115898
5 hours ago

Unless Y is declared as a label somewhere (which is probably not a good label name to use, but for the sake of argument), that should not even assemble. Sounds like it's not a very mature assembler. Update: It's definitely not. Tried LDA X, STA Y at skilldrick.github.io/easy6502 and that assembles without errors (to an address operand of 0xFF 0xFF, as does any unknown labels).Â¯_(Ã£ÂƒÂ„)_/Â¯
â€“Â Cumbayah
2 hours ago

add a commentÂ |Â

up vote
4
down vote

favorite

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

edited 4 mins ago

ThorbjÃ¸rn Ravn Andersen

1657

asked 5 hours ago

user115898

232

New contributor

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

assembly 6502

edited 4 mins ago

ThorbjÃ¸rn Ravn Andersen

1657

asked 5 hours ago

user115898

232

New contributor

edited 4 mins ago

ThorbjÃ¸rn Ravn Andersen

1657

asked 5 hours ago

user115898

232

New contributor

edited 4 mins ago

ThorbjÃ¸rn Ravn Andersen

1657

edited 4 mins ago

ThorbjÃ¸rn Ravn Andersen

1657

edited 4 mins ago

ThorbjÃ¸rn Ravn Andersen

1657

asked 5 hours ago

user115898

232

New contributor

asked 5 hours ago

user115898

232

asked 5 hours ago

user115898

232

New contributor

user115898 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

2

It would help if you could also say which assembler and machine you're trying to code for. (regardless I suspect the answer that's been given is correct)
â€“Â PeterI
5 hours ago

Which 6502-based machine has screen (colour?) memory at $0200?
â€“Â berendi
5 hours ago

This is just a simple assembler/ide that I picked up from Github user skilldrick, Its not really a full machine.
â€“Â user115898
5 hours ago

Unless Y is declared as a label somewhere (which is probably not a good label name to use, but for the sake of argument), that should not even assemble. Sounds like it's not a very mature assembler. Update: It's definitely not. Tried LDA X, STA Y at skilldrick.github.io/easy6502 and that assembles without errors (to an address operand of 0xFF 0xFF, as does any unknown labels).Â¯_(Ã£ÂƒÂ„)_/Â¯
â€“Â Cumbayah
2 hours ago

add a commentÂ |Â

2

It would help if you could also say which assembler and machine you're trying to code for. (regardless I suspect the answer that's been given is correct)
â€“Â PeterI
5 hours ago

Which 6502-based machine has screen (colour?) memory at $0200?
â€“Â berendi
5 hours ago

This is just a simple assembler/ide that I picked up from Github user skilldrick, Its not really a full machine.
â€“Â user115898
5 hours ago

Unless Y is declared as a label somewhere (which is probably not a good label name to use, but for the sake of argument), that should not even assemble. Sounds like it's not a very mature assembler. Update: It's definitely not. Tried LDA X, STA Y at skilldrick.github.io/easy6502 and that assembles without errors (to an address operand of 0xFF 0xFF, as does any unknown labels).Â¯_(Ã£ÂƒÂ„)_/Â¯
â€“Â Cumbayah
2 hours ago

It would help if you could also say which assembler and machine you're trying to code for. (regardless I suspect the answer that's been given is correct)
â€“Â PeterI
5 hours ago

Which 6502-based machine has screen (colour?) memory at $0200?
â€“Â berendi
5 hours ago

This is just a simple assembler/ide that I picked up from Github user skilldrick, Its not really a full machine.
â€“Â user115898
5 hours ago

Unless Y is declared as a label somewhere (which is probably not a good label name to use, but for the sake of argument), that should not even assemble. Sounds like it's not a very mature assembler. Update: It's definitely not. Tried LDA X, STA Y at skilldrick.github.io/easy6502 and that assembles without errors (to an address operand of 0xFF 0xFF, as does any unknown labels).Â¯_(Ã£ÂƒÂ„)_/Â¯
â€“Â Cumbayah
2 hours ago

add a commentÂ |Â

2 Answers
2

active

oldest

votes

up vote
8
down vote

accepted

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.

edited 5 hours ago

answered 5 hours ago

Tommy

12.8k13264

How would I do that with CPY, from what I know about CPY you have to have a memory address or immediate as a source. I never gave any memory address the label Y but I think I get what you are saying here. So I guess the question I should be asking here is how I would compare the accumulator register to the Y register? If you would be willing to post the answer to this question it would be really helpful!
â€“Â user115898
5 hours ago

1

I've suffered a major memory failure here; somehow CPY had mutated into an implied operation in my head. You're right, it's not, it's exactly like CMP but for Y. So I don't think you can directly compare the two registers.
â€“Â Tommy
5 hours ago

add a commentÂ |Â

up vote
3
down vote

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 :)))

edited 3 hours ago

answered 4 hours ago

Raffzahn

37.8k485151

add a commentÂ |Â

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2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

up vote
8
down vote

accepted

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.

edited 5 hours ago

answered 5 hours ago

Tommy

12.8k13264

How would I do that with CPY, from what I know about CPY you have to have a memory address or immediate as a source. I never gave any memory address the label Y but I think I get what you are saying here. So I guess the question I should be asking here is how I would compare the accumulator register to the Y register? If you would be willing to post the answer to this question it would be really helpful!
â€“Â user115898
5 hours ago

1

I've suffered a major memory failure here; somehow CPY had mutated into an implied operation in my head. You're right, it's not, it's exactly like CMP but for Y. So I don't think you can directly compare the two registers.
â€“Â Tommy
5 hours ago

add a commentÂ |Â

up vote
8
down vote

accepted

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.

edited 5 hours ago

answered 5 hours ago

Tommy

12.8k13264

How would I do that with CPY, from what I know about CPY you have to have a memory address or immediate as a source. I never gave any memory address the label Y but I think I get what you are saying here. So I guess the question I should be asking here is how I would compare the accumulator register to the Y register? If you would be willing to post the answer to this question it would be really helpful!
â€“Â user115898
5 hours ago

1

I've suffered a major memory failure here; somehow CPY had mutated into an implied operation in my head. You're right, it's not, it's exactly like CMP but for Y. So I don't think you can directly compare the two registers.
â€“Â Tommy
5 hours ago

add a commentÂ |Â

up vote
8
down vote

accepted

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.

edited 5 hours ago

answered 5 hours ago

Tommy

12.8k13264

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.

edited 5 hours ago

answered 5 hours ago

Tommy

12.8k13264

edited 5 hours ago

answered 5 hours ago

Tommy

12.8k13264

answered 5 hours ago

Tommy

12.8k13264

answered 5 hours ago

Tommy

12.8k13264

How would I do that with CPY, from what I know about CPY you have to have a memory address or immediate as a source. I never gave any memory address the label Y but I think I get what you are saying here. So I guess the question I should be asking here is how I would compare the accumulator register to the Y register? If you would be willing to post the answer to this question it would be really helpful!
â€“Â user115898
5 hours ago

1

I've suffered a major memory failure here; somehow CPY had mutated into an implied operation in my head. You're right, it's not, it's exactly like CMP but for Y. So I don't think you can directly compare the two registers.
â€“Â Tommy
5 hours ago

add a commentÂ |Â

How would I do that with CPY, from what I know about CPY you have to have a memory address or immediate as a source. I never gave any memory address the label Y but I think I get what you are saying here. So I guess the question I should be asking here is how I would compare the accumulator register to the Y register? If you would be willing to post the answer to this question it would be really helpful!
â€“Â user115898
5 hours ago

1

I've suffered a major memory failure here; somehow CPY had mutated into an implied operation in my head. You're right, it's not, it's exactly like CMP but for Y. So I don't think you can directly compare the two registers.
â€“Â Tommy
5 hours ago

How would I do that with CPY, from what I know about CPY you have to have a memory address or immediate as a source. I never gave any memory address the label Y but I think I get what you are saying here. So I guess the question I should be asking here is how I would compare the accumulator register to the Y register? If you would be willing to post the answer to this question it would be really helpful!
â€“Â user115898
5 hours ago

I've suffered a major memory failure here; somehow CPY had mutated into an implied operation in my head. You're right, it's not, it's exactly like CMP but for Y. So I don't think you can directly compare the two registers.
â€“Â Tommy
5 hours ago

add a commentÂ |Â

up vote
3
down vote

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)

 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.

edited 3 hours ago

answered 4 hours ago

Raffzahn

37.8k485151

add a commentÂ |Â

up vote
3
down vote

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)

 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.

edited 3 hours ago

answered 4 hours ago

Raffzahn

37.8k485151

add a commentÂ |Â

up vote
3
down vote

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)

 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.

edited 3 hours ago

answered 4 hours ago

Raffzahn

37.8k485151

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)

 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.

edited 3 hours ago

answered 4 hours ago

Raffzahn

37.8k485151

edited 3 hours ago

answered 4 hours ago

Raffzahn

37.8k485151

answered 4 hours ago

Raffzahn

37.8k485151

answered 4 hours ago

Raffzahn

37.8k485151

add a commentÂ |Â

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