# 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](https://www.c64-wiki.com/wiki/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