# Dev notes — 2023-08-15

## Goals for today

- [x] Review planned changes to simulator
  - [x] 'opcodes' and 'operands'
  - [x] fix $00 contains $20 bug

- [x] Review planned changes to the system
  - [x] CHP, CFC -> FHP, FTG -- dev note 2023-08-07
  - [/] bank-switching flag in 0 page
      - added notes below, but decided to leave implementation for another day
  - [x] ? 5x5 display

- [/] Implement any changes necessary for writing a program?
- [-] Write a program
  - [-] LOGO turtle on 5x5?

## Overflow flag

Ken Shirriff, [The 6502 overflow flag explained mathematically](https://www.righto.com/2012/12/the-6502-overflow-flag-explained.html):

> A common definition of overflow is `V = C6 xor C7`. That is, overflow happens if the carry into bit 7 is different from the carry out.

## Bank switching

### Planned memory map

- `00-0F` - display (4x4)
- `10-1F` - keypad? (details TBD)
- `20   ` - pointer to display memory
- `21   ` - pointer to keypad memory
- `22   ` - pointer to memory bank
- `23-2F` - reserved for future use / variable storage
- `30   ` - initial value for IP
- `30-80` - free
- `80-FF` - free, can be bank-switched

## Looping using an interval timer

    const loop = setInterval(async () => {
      step = step + 1;
      // Temporary limit as a lazy way to halt infinite loops:
      if (CYCLE_LIMIT && (step > CYCLE_LIMIT)) {
        console.log('SIMULATION HALTING - reached cycle limit');
        clearInterval(loop);
      }
      if (!CPU.running) clearInterval(loop);
      if (CPU.IP >= CPU.memory.length) clearInterval(loop);
      stepCPU();
      await logCPUState(debug);
    }, frameRate);