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6502 CMP instruction doesn't compare correctly
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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.
assembly 6502
edited 2 hours ago
George Phillips
3,3931421
asked 2 hours ago
user115898
132
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user115898 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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up vote
2
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
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.
assembly 6502
edited 2 hours ago
George Phillips
3,3931421
asked 2 hours ago
user115898
132
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)
– PeterI
2 hours ago
Which 6502-based machine has screen (colour?) memory at$0200?
– berendi
2 hours ago
This is just a simple assembler/ide that I picked up from Github user skilldrick, Its not really a full machine.
– user115898
2 hours ago
add a comment |Â
up vote
2
down vote
favorite
up vote
2
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
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.
assembly 6502
edited 2 hours ago
George Phillips
3,3931421
asked 2 hours ago
user115898
132
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.
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.
assembly 6502
assembly 6502
edited 2 hours ago
George Phillips
3,3931421
asked 2 hours ago
user115898
132
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.
edited 2 hours ago
George Phillips
3,3931421
asked 2 hours ago
user115898
132
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.
edited 2 hours ago
George Phillips
3,3931421
edited 2 hours ago
George Phillips
3,3931421
edited 2 hours ago
George Phillips
3,3931421
3,3931421
asked 2 hours ago
user115898
132
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.
asked 2 hours ago
user115898
132
asked 2 hours ago
user115898
132
132
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.
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.
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)
– PeterI
2 hours ago
Which 6502-based machine has screen (colour?) memory at$0200?
– berendi
2 hours ago
This is just a simple assembler/ide that I picked up from Github user skilldrick, Its not really a full machine.
– user115898
2 hours ago
add a comment |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)
– PeterI
2 hours ago
Which 6502-based machine has screen (colour?) memory at$0200?
– berendi
2 hours ago
This is just a simple assembler/ide that I picked up from Github user skilldrick, Its not really a full machine.
– user115898
2 hours ago
2
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)
– PeterI
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)
– PeterI
2 hours ago
Which 6502-based machine has screen (colour?) memory at
$0200?– berendi
2 hours ago
Which 6502-based machine has screen (colour?) memory at
$0200?– berendi
2 hours ago
This is just a simple assembler/ide that I picked up from Github user skilldrick, Its not really a full machine.
– user115898
2 hours ago
This is just a simple assembler/ide that I picked up from Github user skilldrick, Its not really a full machine.
– user115898
2 hours ago
add a comment |Â
2 Answers
2
active
oldest
votes
up vote
3
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.
answered 2 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!
– user115898
2 hours ago
1
I've suffered a major memory failure here; somehowCPYhad mutated into an implied operation in my head. You're right, it's not, it's exactly likeCMPbut forY. So I don't think you can directly compare the two registers.
– Tommy
2 hours ago
add a comment |Â
up vote
1
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 :)))
answered 1 hour ago
Raffzahn
37.8k485151
add a comment |Â
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
3
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.
answered 2 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!
– user115898
2 hours ago
1
I've suffered a major memory failure here; somehowCPYhad mutated into an implied operation in my head. You're right, it's not, it's exactly likeCMPbut forY. So I don't think you can directly compare the two registers.
– Tommy
2 hours ago
add a comment |Â
up vote
3
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.
answered 2 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!
– user115898
2 hours ago
1
I've suffered a major memory failure here; somehowCPYhad mutated into an implied operation in my head. You're right, it's not, it's exactly likeCMPbut forY. So I don't think you can directly compare the two registers.
– Tommy
2 hours ago
add a comment |Â
up vote
3
down vote
accepted
up vote
3
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.
answered 2 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.
answered 2 hours ago
Tommy
12.8k13264
edited 2 hours ago
answered 2 hours ago
Tommy
12.8k13264
answered 2 hours ago
Tommy
12.8k13264
answered 2 hours ago
Tommy
12.8k13264
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!
– user115898
2 hours ago
1
I've suffered a major memory failure here; somehowCPYhad mutated into an implied operation in my head. You're right, it's not, it's exactly likeCMPbut forY. So I don't think you can directly compare the two registers.
– Tommy
2 hours ago
add a comment |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!
– user115898
2 hours ago
1
I've suffered a major memory failure here; somehowCPYhad mutated into an implied operation in my head. You're right, it's not, it's exactly likeCMPbut forY. So I don't think you can directly compare the two registers.
– Tommy
2 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!
– user115898
2 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!
– user115898
2 hours ago
1
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.– Tommy
2 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.– Tommy
2 hours ago
add a comment |Â
up vote
1
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 :)))
answered 1 hour ago
Raffzahn
37.8k485151
add a comment |Â
up vote
1
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 :)))
answered 1 hour ago
Raffzahn
37.8k485151
add a comment |Â
up vote
1
down vote
up vote
1
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 :)))
answered 1 hour 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)
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 :)))
answered 1 hour ago
Raffzahn
37.8k485151
edited 34 mins ago
answered 1 hour ago
Raffzahn
37.8k485151
answered 1 hour ago
Raffzahn
37.8k485151
answered 1 hour ago
Raffzahn
37.8k485151
37.8k485151
add a comment |Â
add a comment |Â
user115898 is a new contributor. Be nice, and check out our Code of Conduct.
user115898 is a new contributor. Be nice, and check out our Code of Conduct.
user115898 is a new contributor. Be nice, and check out our Code of Conduct.
user115898 is a new contributor. Be nice, and check out our Code of Conduct.
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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)
– PeterI
2 hours ago
Which 6502-based machine has screen (colour?) memory at
$0200?– berendi
2 hours ago
This is just a simple assembler/ide that I picked up from Github user skilldrick, Its not really a full machine.
– user115898
2 hours ago