cardiograph-computer/notes/2023-08-15--dev-notes.md

Dev notes — 2023-08-15

Goals for today

• Review planned changes to simulator

  • 'opcodes' and 'operands'
  • fix $00 contains $20 bug

• Review planned changes to the system

  • 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
  • ? 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:

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);