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Does the NES handle unlisted processor instructions differently from the Famicom?
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As you may be aware, developers used unregistered machine codes in their games, as discussed in this thread. My question isn't why they would do this, but instead if these instructions yielded a difforent result on the NES than it did on the Famicom. Keep in mind that these codes were tested to work on the Famicom, but Nintendo probably didn't care to make sure they had the same behavior on the NES, which was, in a way, a remake of the Famicom for American consumers.
nes nintendo undocumented-opcodes famicom
asked 5 hours ago
Jack Kasbrack
12911
add a comment |Â
up vote
2
down vote
favorite
As you may be aware, developers used unregistered machine codes in their games, as discussed in this thread. My question isn't why they would do this, but instead if these instructions yielded a difforent result on the NES than it did on the Famicom. Keep in mind that these codes were tested to work on the Famicom, but Nintendo probably didn't care to make sure they had the same behavior on the NES, which was, in a way, a remake of the Famicom for American consumers.
nes nintendo undocumented-opcodes famicom
asked 5 hours ago
Jack Kasbrack
12911
add a comment |Â
up vote
2
down vote
favorite
up vote
2
down vote
favorite
As you may be aware, developers used unregistered machine codes in their games, as discussed in this thread. My question isn't why they would do this, but instead if these instructions yielded a difforent result on the NES than it did on the Famicom. Keep in mind that these codes were tested to work on the Famicom, but Nintendo probably didn't care to make sure they had the same behavior on the NES, which was, in a way, a remake of the Famicom for American consumers.
nes nintendo undocumented-opcodes famicom
asked 5 hours ago
Jack Kasbrack
12911
As you may be aware, developers used unregistered machine codes in their games, as discussed in this thread. My question isn't why they would do this, but instead if these instructions yielded a difforent result on the NES than it did on the Famicom. Keep in mind that these codes were tested to work on the Famicom, but Nintendo probably didn't care to make sure they had the same behavior on the NES, which was, in a way, a remake of the Famicom for American consumers.
nes nintendo undocumented-opcodes famicom
nes nintendo undocumented-opcodes famicom
asked 5 hours ago
Jack Kasbrack
12911
asked 5 hours ago
Jack Kasbrack
12911
asked 5 hours ago
Jack Kasbrack
12911
asked 5 hours ago
Jack Kasbrack
12911
asked 5 hours ago
Jack Kasbrack
12911
12911
add a comment |Â
add a comment |Â
2 Answers
2
active
oldest
votes
up vote
4
down vote
accepted
To my knowledge, the NES and the Famicom had the exact same CPU (a Ricoh 2A03). So, it's unlikely there were any differences in behavior between the two with regards to undocumented opcodes. As the Ricoh had a 6502 core, you could also expect it has the same undocumented opcodes as the 6502 (not your question, though) - And this document seems to confirm this.
Other than that, there were some peripheral differences like the cartridge and controller ports, but these would not relate to your question.
To my knowledge, there is no NES game that wouldn't run on the Famicom or vice versa (except the ones that are incompatible with the changed slot design)
answered 4 hours ago
tofro
13.5k32877
According to the creator of (I think) FCEUX, the CPU is different in regard to clock cycles, and sometimes the CPU has to add clock cycles to keep it in sync. I didn't write that in my question, because I'm not 100% sure if that's the case, but clock cycles would seem to be the biggest culprit in terms of version differences.
– Jack Kasbrack
4 hours ago
You are correct, however, that all documented 6502 instructions were kept in tact, with an almost identical CPU.
– Jack Kasbrack
3 hours ago
Well, clock behavior and undocumented instructions are two different things after all.
– tofro
3 hours ago
yes, but they are related in this case. As I said, use of undocumented instructions may be correlated with clock cycle differences, but I do not know for sure.
– Jack Kasbrack
3 hours ago
1
@JackKasbrack "the CPU is different in regard to clock cycles, and sometimes the CPU has to add clock cycles to keep it in sync" That's not something I've heard before; can you provide a source for that?
– NobodyNada
3 hours ago
 |Â
show 3 more comments
up vote
2
down vote
While the Famicom was different from the NES, a lot of its internal chips, including the CPU, were the same. in other words, there would be no reason why they would act differently. If you were to recreate the CPU yourself, on the other hand, and didn't keep undocumented opcodes in mind, you may come up with a different result.
answered 19 mins ago
Jonathan O'Brady
314
New contributor
Jonathan O'Brady is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
1
You answer is a wee bit similar to the accepted one.
– Jack Kasbrack
18 mins ago
1
I can't comment yet, so I added this answer for clarification's sake.
– Jonathan O'Brady
17 mins ago
Fair enough. try not to do that too often, though.
– Jack Kasbrack
16 mins ago
add a comment |Â
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
4
down vote
accepted
To my knowledge, the NES and the Famicom had the exact same CPU (a Ricoh 2A03). So, it's unlikely there were any differences in behavior between the two with regards to undocumented opcodes. As the Ricoh had a 6502 core, you could also expect it has the same undocumented opcodes as the 6502 (not your question, though) - And this document seems to confirm this.
Other than that, there were some peripheral differences like the cartridge and controller ports, but these would not relate to your question.
To my knowledge, there is no NES game that wouldn't run on the Famicom or vice versa (except the ones that are incompatible with the changed slot design)
answered 4 hours ago
tofro
13.5k32877
According to the creator of (I think) FCEUX, the CPU is different in regard to clock cycles, and sometimes the CPU has to add clock cycles to keep it in sync. I didn't write that in my question, because I'm not 100% sure if that's the case, but clock cycles would seem to be the biggest culprit in terms of version differences.
– Jack Kasbrack
4 hours ago
You are correct, however, that all documented 6502 instructions were kept in tact, with an almost identical CPU.
– Jack Kasbrack
3 hours ago
Well, clock behavior and undocumented instructions are two different things after all.
– tofro
3 hours ago
yes, but they are related in this case. As I said, use of undocumented instructions may be correlated with clock cycle differences, but I do not know for sure.
– Jack Kasbrack
3 hours ago
1
@JackKasbrack "the CPU is different in regard to clock cycles, and sometimes the CPU has to add clock cycles to keep it in sync" That's not something I've heard before; can you provide a source for that?
– NobodyNada
3 hours ago
 |Â
show 3 more comments
up vote
4
down vote
accepted
To my knowledge, the NES and the Famicom had the exact same CPU (a Ricoh 2A03). So, it's unlikely there were any differences in behavior between the two with regards to undocumented opcodes. As the Ricoh had a 6502 core, you could also expect it has the same undocumented opcodes as the 6502 (not your question, though) - And this document seems to confirm this.
Other than that, there were some peripheral differences like the cartridge and controller ports, but these would not relate to your question.
To my knowledge, there is no NES game that wouldn't run on the Famicom or vice versa (except the ones that are incompatible with the changed slot design)
answered 4 hours ago
tofro
13.5k32877
According to the creator of (I think) FCEUX, the CPU is different in regard to clock cycles, and sometimes the CPU has to add clock cycles to keep it in sync. I didn't write that in my question, because I'm not 100% sure if that's the case, but clock cycles would seem to be the biggest culprit in terms of version differences.
– Jack Kasbrack
4 hours ago
You are correct, however, that all documented 6502 instructions were kept in tact, with an almost identical CPU.
– Jack Kasbrack
3 hours ago
Well, clock behavior and undocumented instructions are two different things after all.
– tofro
3 hours ago
yes, but they are related in this case. As I said, use of undocumented instructions may be correlated with clock cycle differences, but I do not know for sure.
– Jack Kasbrack
3 hours ago
1
@JackKasbrack "the CPU is different in regard to clock cycles, and sometimes the CPU has to add clock cycles to keep it in sync" That's not something I've heard before; can you provide a source for that?
– NobodyNada
3 hours ago
 |Â
show 3 more comments
up vote
4
down vote
accepted
up vote
4
down vote
accepted
To my knowledge, the NES and the Famicom had the exact same CPU (a Ricoh 2A03). So, it's unlikely there were any differences in behavior between the two with regards to undocumented opcodes. As the Ricoh had a 6502 core, you could also expect it has the same undocumented opcodes as the 6502 (not your question, though) - And this document seems to confirm this.
Other than that, there were some peripheral differences like the cartridge and controller ports, but these would not relate to your question.
To my knowledge, there is no NES game that wouldn't run on the Famicom or vice versa (except the ones that are incompatible with the changed slot design)
answered 4 hours ago
tofro
13.5k32877
To my knowledge, the NES and the Famicom had the exact same CPU (a Ricoh 2A03). So, it's unlikely there were any differences in behavior between the two with regards to undocumented opcodes. As the Ricoh had a 6502 core, you could also expect it has the same undocumented opcodes as the 6502 (not your question, though) - And this document seems to confirm this.
Other than that, there were some peripheral differences like the cartridge and controller ports, but these would not relate to your question.
To my knowledge, there is no NES game that wouldn't run on the Famicom or vice versa (except the ones that are incompatible with the changed slot design)
answered 4 hours ago
tofro
13.5k32877
edited 3 hours ago
answered 4 hours ago
tofro
13.5k32877
answered 4 hours ago
tofro
13.5k32877
answered 4 hours ago
tofro
13.5k32877
13.5k32877
According to the creator of (I think) FCEUX, the CPU is different in regard to clock cycles, and sometimes the CPU has to add clock cycles to keep it in sync. I didn't write that in my question, because I'm not 100% sure if that's the case, but clock cycles would seem to be the biggest culprit in terms of version differences.
– Jack Kasbrack
4 hours ago
You are correct, however, that all documented 6502 instructions were kept in tact, with an almost identical CPU.
– Jack Kasbrack
3 hours ago
Well, clock behavior and undocumented instructions are two different things after all.
– tofro
3 hours ago
yes, but they are related in this case. As I said, use of undocumented instructions may be correlated with clock cycle differences, but I do not know for sure.
– Jack Kasbrack
3 hours ago
1
@JackKasbrack "the CPU is different in regard to clock cycles, and sometimes the CPU has to add clock cycles to keep it in sync" That's not something I've heard before; can you provide a source for that?
– NobodyNada
3 hours ago
 |Â
show 3 more comments
According to the creator of (I think) FCEUX, the CPU is different in regard to clock cycles, and sometimes the CPU has to add clock cycles to keep it in sync. I didn't write that in my question, because I'm not 100% sure if that's the case, but clock cycles would seem to be the biggest culprit in terms of version differences.
– Jack Kasbrack
4 hours ago
You are correct, however, that all documented 6502 instructions were kept in tact, with an almost identical CPU.
– Jack Kasbrack
3 hours ago
Well, clock behavior and undocumented instructions are two different things after all.
– tofro
3 hours ago
yes, but they are related in this case. As I said, use of undocumented instructions may be correlated with clock cycle differences, but I do not know for sure.
– Jack Kasbrack
3 hours ago
1
@JackKasbrack "the CPU is different in regard to clock cycles, and sometimes the CPU has to add clock cycles to keep it in sync" That's not something I've heard before; can you provide a source for that?
– NobodyNada
3 hours ago
According to the creator of (I think) FCEUX, the CPU is different in regard to clock cycles, and sometimes the CPU has to add clock cycles to keep it in sync. I didn't write that in my question, because I'm not 100% sure if that's the case, but clock cycles would seem to be the biggest culprit in terms of version differences.
– Jack Kasbrack
4 hours ago
According to the creator of (I think) FCEUX, the CPU is different in regard to clock cycles, and sometimes the CPU has to add clock cycles to keep it in sync. I didn't write that in my question, because I'm not 100% sure if that's the case, but clock cycles would seem to be the biggest culprit in terms of version differences.
– Jack Kasbrack
4 hours ago
You are correct, however, that all documented 6502 instructions were kept in tact, with an almost identical CPU.
– Jack Kasbrack
3 hours ago
You are correct, however, that all documented 6502 instructions were kept in tact, with an almost identical CPU.
– Jack Kasbrack
3 hours ago
Well, clock behavior and undocumented instructions are two different things after all.
– tofro
3 hours ago
Well, clock behavior and undocumented instructions are two different things after all.
– tofro
3 hours ago
yes, but they are related in this case. As I said, use of undocumented instructions may be correlated with clock cycle differences, but I do not know for sure.
– Jack Kasbrack
3 hours ago
yes, but they are related in this case. As I said, use of undocumented instructions may be correlated with clock cycle differences, but I do not know for sure.
– Jack Kasbrack
3 hours ago
1
1
@JackKasbrack "the CPU is different in regard to clock cycles, and sometimes the CPU has to add clock cycles to keep it in sync" That's not something I've heard before; can you provide a source for that?
– NobodyNada
3 hours ago
@JackKasbrack "the CPU is different in regard to clock cycles, and sometimes the CPU has to add clock cycles to keep it in sync" That's not something I've heard before; can you provide a source for that?
– NobodyNada
3 hours ago
 |Â
show 3 more comments
up vote
2
down vote
While the Famicom was different from the NES, a lot of its internal chips, including the CPU, were the same. in other words, there would be no reason why they would act differently. If you were to recreate the CPU yourself, on the other hand, and didn't keep undocumented opcodes in mind, you may come up with a different result.
answered 19 mins ago
Jonathan O'Brady
314
New contributor
Jonathan O'Brady is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
1
You answer is a wee bit similar to the accepted one.
– Jack Kasbrack
18 mins ago
1
I can't comment yet, so I added this answer for clarification's sake.
– Jonathan O'Brady
17 mins ago
Fair enough. try not to do that too often, though.
– Jack Kasbrack
16 mins ago
add a comment |Â
up vote
2
down vote
While the Famicom was different from the NES, a lot of its internal chips, including the CPU, were the same. in other words, there would be no reason why they would act differently. If you were to recreate the CPU yourself, on the other hand, and didn't keep undocumented opcodes in mind, you may come up with a different result.
answered 19 mins ago
Jonathan O'Brady
314
New contributor
Jonathan O'Brady is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
1
You answer is a wee bit similar to the accepted one.
– Jack Kasbrack
18 mins ago
1
I can't comment yet, so I added this answer for clarification's sake.
– Jonathan O'Brady
17 mins ago
Fair enough. try not to do that too often, though.
– Jack Kasbrack
16 mins ago
add a comment |Â
up vote
2
down vote
up vote
2
down vote
While the Famicom was different from the NES, a lot of its internal chips, including the CPU, were the same. in other words, there would be no reason why they would act differently. If you were to recreate the CPU yourself, on the other hand, and didn't keep undocumented opcodes in mind, you may come up with a different result.
answered 19 mins ago
Jonathan O'Brady
314
New contributor
Jonathan O'Brady is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
While the Famicom was different from the NES, a lot of its internal chips, including the CPU, were the same. in other words, there would be no reason why they would act differently. If you were to recreate the CPU yourself, on the other hand, and didn't keep undocumented opcodes in mind, you may come up with a different result.
answered 19 mins ago
Jonathan O'Brady
314
New contributor
Jonathan O'Brady is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited 13 mins ago
answered 19 mins ago
Jonathan O'Brady
314
New contributor
Jonathan O'Brady is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
answered 19 mins ago
Jonathan O'Brady
314
answered 19 mins ago
Jonathan O'Brady
314
314
New contributor
Jonathan O'Brady is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
Jonathan O'Brady is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
Jonathan O'Brady is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
1
You answer is a wee bit similar to the accepted one.
– Jack Kasbrack
18 mins ago
1
I can't comment yet, so I added this answer for clarification's sake.
– Jonathan O'Brady
17 mins ago
Fair enough. try not to do that too often, though.
– Jack Kasbrack
16 mins ago
add a comment |Â
1
You answer is a wee bit similar to the accepted one.
– Jack Kasbrack
18 mins ago
1
I can't comment yet, so I added this answer for clarification's sake.
– Jonathan O'Brady
17 mins ago
Fair enough. try not to do that too often, though.
– Jack Kasbrack
16 mins ago
1
1
You answer is a wee bit similar to the accepted one.
– Jack Kasbrack
18 mins ago
You answer is a wee bit similar to the accepted one.
– Jack Kasbrack
18 mins ago
1
1
I can't comment yet, so I added this answer for clarification's sake.
– Jonathan O'Brady
17 mins ago
I can't comment yet, so I added this answer for clarification's sake.
– Jonathan O'Brady
17 mins ago
Fair enough. try not to do that too often, though.
– Jack Kasbrack
16 mins ago
Fair enough. try not to do that too often, though.
– Jack Kasbrack
16 mins ago
add a comment |Â
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