(notes) 2023-08-24 - Create note with brainstorming re: a more accurate start-up process (and memory/peripheral setup) + a todo list

notes/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](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