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cardiograph-computer/2023-08-24--dev-notes.md

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Dev notes — 2023-08-24

CPU start-up

Thinking about booting up... how would the "pointer to display memory" etc. get initialized?

They could be in ROM, but then they're not re-locatable.

Maybe there's a start-up routine in ROM that sets them up?

cf. C64 bank switching

The C64 uses I/O lines on the CPU (+ others) to signal to the PLA that it's time to switch ROM banks in or out.

Maybe I need something like that?

A simplified approach might be to put the pointer-to-display and ...

Current memory map:

  • 00-19 - display (5x5)
  • 1A - pointer to display memory
  • 1B - keypad: value of latest key pressed
  • 1C - reserved for future use (bank switching flag)
  • 1D - initial IP
  • 1D-FF - free

Thinking about changing it for better bank-switching:

  • 00-19 - display (5x5)
  • 20 - initial IP
  • - free
  • FC - I/O controller - pointer to display memory
  • FD - I/O controller - reserved for future use (bank switching flag)
  • FE - I/O controller - keypad: value of latest key pressed
  • FF - ROM - pointer to initial IP

Ah so actually the issue is that the CPU needs to be paired with an I/O controller, and that needs to know where to look to find these pointers/where to put the keypad info.

And I think that can just be hand-waved away for now?

Here's the plan

But I got started on this because I was trying to work out how FF gets loaded with the initial IP, and I think that's still a question.

(And maybe some way to switch ROM in and out would be good for the secret-advanced-mode, since it would be great to have a ROM pre-loaded with a set of convenient utility routines.)

I think maybe we just leave the IP hardcoded for now; say that there's a 1-byte ROM at $FF.

  • 00-19 - display (5x5)

  • 1A - pointer to display memory

  • 1B - keypad: value of latest key pressed

  • 1C - reserved for future use (bank switching flag)

  • 1D - initial IP

  • 1D-FE - free

  • FF - ROM (unwriteable) - pointer to initial IP

  • store $1D at $FF

  • make CPU execute JMP $FF on startup

  • make ROM unwriteable

More step-by-step:

  • Change memory from a Uint8Array to a regular array, and make every entry { number | { type: 'ROM', value: number }}
  • Store ROM as an object in machine.config.js
  • Load ROM data into memory at CPU startup (startCPU(RAM, ROM))

(And continue to handwave away how that RAM already contains data, for now...)

TODO

  • Check that we're starting execution at $1D now
  • Programming
    • Subroutine stack
    • Conway's Life
  • ? bank switching
  • Keypad handling
  • Start-up process described above