6 ms·

The BBC Master had an even funkier mode. The RAM bank accessed could depend on the current program counter. Imagine a video display taking 16K of RAM. This w

The BBC Master had an even funkier mode. The RAM bank accessed could depend on the current program counter. Imagine a video display taking 16K of RAM. This would be situated between addresses 0x4000 and 0x8000. This same memory range also included non-video RAM. The hardware transparently selected the video or non-video RAM depending on which code was accessing it. Specifically, if the program counter was at 0xC000 or above (i.e. code in the OS ROM was running) then accesses to the video range would go to the video RAM. But if the program counter was elsewhere (i.e. running user code or an application ROM) then accesses to the video range would go to user RAM, not video RAM. Additionally, there was a hardware register controlling this so that user code could choose to directly access the video RAM, and OS code could access the user RAM.

forinti 3y ago
I've been looking for a detailed explanation on this, because I would like to know how they read the PC. The New Advanced User's Manual describes the flag at &FE34 on the Master and B+ and I've read this thing about using the PC from a few places but I haven't found any specifics. Could you clarify? How do you know from the outside what's in the PC?
- ddingus 3y ago
  Would they not just mooch the address off the address bus?
  - morcheeba 3y ago
    Yep. The 65C02 has a SYNC output that goes high to indicate an instruction is being fetched on the current cycle. Since there is no cache, it's pretty simple to use this to determine the PC.
    - ddingus 3y ago
      Thought so. From there, it is a bit of logic to map the chip selects to make the writes and reads come and go from the intended resources.
    - forinti 3y ago
      That's it then. I was missing the sync pin. Thank you.
flohofwoe 3y ago
There were also 8-bit home computers (like the Amstrad CPC and C64) where a different memory bank would be accessed depending on whether the CPU would do a read or write access, e.g. a read would access a ROM bank, but a write would access a RAM bank at the same address (usually called 'shadow ROM').
- rjeli 3y ago
  Huh? How do you read from the ram after you write to it?
  - vidarh 3y ago
    You bank switch then. A common use would be to e.g. be able to use the ROM to load code into the RAM under the ROM and then bank switch, but also e.g. for extensions where you might want to first copy the ROM into the underlying ROM, and then patch whatever changes you wanted into before bank switching.
    - flohofwoe 3y ago
      And sometimes you also don't need to read the written data back (for instance when writing to video memory).

forinti 3y ago

I've been looking for a detailed explanation on this, because I would like to know how they read the PC. The New Advanced User's Manual describes the flag at &FE34 on the Master and B+ and I've read this thing about using the PC from a few places but I haven't found any specifics. Could you clarify? How do you know from the outside what's in the PC?

ddingus 3y ago
Would they not just mooch the address off the address bus?
- morcheeba 3y ago
  Yep. The 65C02 has a SYNC output that goes high to indicate an instruction is being fetched on the current cycle. Since there is no cache, it's pretty simple to use this to determine the PC.
  - ddingus 3y ago
    Thought so. From there, it is a bit of logic to map the chip selects to make the writes and reads come and go from the intended resources.
  - forinti 3y ago
    That's it then. I was missing the sync pin. Thank you.

flohofwoe 3y ago

There were also 8-bit home computers (like the Amstrad CPC and C64) where a different memory bank would be accessed depending on whether the CPU would do a read or write access, e.g. a read would access a ROM bank, but a write would access a RAM bank at the same address (usually called 'shadow ROM').

rjeli 3y ago
Huh? How do you read from the ram after you write to it?
- vidarh 3y ago
  You bank switch then. A common use would be to e.g. be able to use the ROM to load code into the RAM under the ROM and then bank switch, but also e.g. for extensions where you might want to first copy the ROM into the underlying ROM, and then patch whatever changes you wanted into before bank switching.
  - flohofwoe 3y ago
    And sometimes you also don't need to read the written data back (for instance when writing to video memory).