4 ms·

Nope, the 65816 address space is flat. If you read the docs and look at the opcodes I see how it can give the impression of being segmented (with all the "bank"

by jjss 15y ago

Nope, the 65816 address space is flat. If you read the docs and look at the opcodes I see how it can give the impression of being segmented (with all the "bank" terminology), but it's a full 24-bit address space with no restrictions.

rbanffy 15y ago
Sort of, at least. IIRC, program code could not execute across bank boundaries, although you could do long jumps to any 24-bit address.
- dfox 15y ago
  Which is probably the case also for this hack. It's probably possible to devise logic that would remove this limitation, but I assume that would be mentioned in the article.
  - rbanffy 15y ago
    You can always check if all address lines transitioned from 1 to 0 on an instruction fetch and increment whatever you use for PBR...
    - dfox 15y ago
      It's conceptually simple, but there are two problems: 1) you have to detect near jumps that jump back inside one bank. (at least from instruction ending on 0xffff to instruction starting on 0x0000, on the other hand you can plausibly ignore this as it is very unlikely case) 2) more importantly, I understand that this coprocessor contraption does not interact in any way with lower 16 bits of address bus. You would need to actually snoop on address bus and detect the wraparound and as this thing is built from MSI logic, detecting transition 1 -> 0 on all bits of address bus - while conceptually trivial - would require significant amount of hardware. You can detect 1 -> 0 transition only on 15th bit of address, but then you really need to detect jumps in microcode and disable this logic in case of jump.
  - 0x12 15y ago
    Read the article, he's thought of that. There is a trick with two registers in there that allows you to pivot from one bank to another. More explanation in the appendix on the instruction set. The opcodes are 13, 23, eb and fb: http://www.laughtonelectronics.com/arcana/BrideOfSon%20KK%20Instruction%20List.html http://www.laughtonelectronics.com/arcana/BrideOfSon%20KK%20... Bottom of the list.
    - dfox 15y ago
      It allows you to jump between banks, but not to overflow from one bank to another without explicit jump on the boundary.

by jjss 15y ago

rbanffy 15y ago

Sort of, at least. IIRC, program code could not execute across bank boundaries, although you could do long jumps to any 24-bit address.

dfox 15y ago
Which is probably the case also for this hack. It's probably possible to devise logic that would remove this limitation, but I assume that would be mentioned in the article.
- rbanffy 15y ago
  You can always check if all address lines transitioned from 1 to 0 on an instruction fetch and increment whatever you use for PBR...
  - dfox 15y ago
    It's conceptually simple, but there are two problems: 1) you have to detect near jumps that jump back inside one bank. (at least from instruction ending on 0xffff to instruction starting on 0x0000, on the other hand you can plausibly ignore this as it is very unlikely case) 2) more importantly, I understand that this coprocessor contraption does not interact in any way with lower 16 bits of address bus. You would need to actually snoop on address bus and detect the wraparound and as this thing is built from MSI logic, detecting transition 1 -> 0 on all bits of address bus - while conceptually trivial - would require significant amount of hardware. You can detect 1 -> 0 transition only on 15th bit of address, but then you really need to detect jumps in microcode and disable this logic in case of jump.
- 0x12 15y ago
  Read the article, he's thought of that. There is a trick with two registers in there that allows you to pivot from one bank to another. More explanation in the appendix on the instruction set. The opcodes are 13, 23, eb and fb: http://www.laughtonelectronics.com/arcana/BrideOfSon%20KK%20Instruction%20List.html http://www.laughtonelectronics.com/arcana/BrideOfSon%20KK%20... Bottom of the list.
  - dfox 15y ago
    It allows you to jump between banks, but not to overflow from one bank to another without explicit jump on the boundary.