81 lines
2.7 KiB
Markdown
81 lines
2.7 KiB
Markdown
# Dev notes — 2023-08-24
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## CPU start-up
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Thinking about booting up...
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how would the "pointer to display memory" etc. get initialized?
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They could be in ROM, but then they're not re-locatable.
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Maybe there's a start-up routine in ROM that sets them up?
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[cf. C64 bank switching](https://www.c64-wiki.com/wiki/Bank_Switching)
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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.
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Maybe I need something like that?
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~~A simplified approach might be to put the pointer-to-display and ...~~
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Current memory map:
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- `00-19` - display (5x5)
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- `1A ` - pointer to display memory
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- `1B ` - keypad: value of latest key pressed
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- `1C ` - reserved for future use (bank switching flag)
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- `1D ` - initial IP
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- `1D-FF` - free
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Thinking about changing it for better bank-switching:
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- `00-19` - display (5x5)
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- `20 ` - initial IP
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- ` ` - free
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- `FC ` - I/O controller - pointer to display memory
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- `FD ` - I/O controller - reserved for future use (bank switching flag)
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- `FE ` - I/O controller - keypad: value of latest key pressed
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- `FF ` - ROM - pointer to initial IP
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Ah so actually the issue is that the CPU needs to be paired with an I/O controller,
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and that needs to know where to look to find these pointers/where to put the keypad info.
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And I think that can just be hand-waved away for now?
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### Here's the plan
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**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.**
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***(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.)***
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I think maybe we just leave the IP hardcoded for now; say that there's a 1-byte ROM at $FF.
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- `00-19` - display (5x5)
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- `1A ` - pointer to display memory
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- `1B ` - keypad: value of latest key pressed
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- `1C ` - reserved for future use (bank switching flag)
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- `1D ` - initial IP
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- `1D-FE` - free
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- `FF ` - ROM (unwriteable) - pointer to initial IP
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- [ ] store `$1D` at `$FF`
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- [ ] make CPU execute `JMP $FF` on startup
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- [ ] make ROM unwriteable
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More step-by-step:
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- Change memory from a Uint8Array to a regular array,
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and make every entry { number | { type: 'ROM', value: number }}
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- Store ROM as an object in machine.config.js
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- Load ROM data into memory at CPU startup (`startCPU(RAM, ROM)`)
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(And continue to handwave away how that RAM already contains data, for now...)
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## TODO
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- [ ] Check that we're starting execution at $1D now
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- [ ] Programming
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- [ ] Subroutine stack
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- [ ] Conway's Life
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- [ ] ? bank switching
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- [ ] Keypad handling
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- [ ] Start-up process described above |