# 2023-09-11 — Dev notes

## ISA

### To do

- Remove the END instruction, and use `$00` for NOP

### Goals

I want to keep a basic "bare minimum" set at `$00`-`$0F`, since that serves the educational goal.

That leaves the high byte free for adding more complex/convenience features.

### Possible additions

#### Things that are hard to do otherwise

- bitwise operations
  - rotate left
  - rotate right

#### Conveniences

- multiplication
- division
- add with carry
- sub with borrow
- comparisons
  - less than
  - greater than

### Layout idea

Maybe try to roughly map additional features onto the same areas as the basic ones:

| x | Mnm | Mnm |
|---|-----|-----|
| 0 | NOP |           |
| 1 | STO | ROL       |
| 2 | STO | ROR       |
| 3 | LDA | ARL       |
| 4 | LDA | ARR       |
| 5 | ADD | MUL       |
| 6 | ADD | MUL       |
| 7 | SUB | DIV       |
| 8 | SUB | DIV       |
| 9 | HOP | HLT hop < |
| A | HOP | HLT hop < |
| B | JMP | HGT hop > |
| C | JMP | HGT hop > |
| D | FTG |           |
| E | FHP |           |
| F |     |           |

### Extended system

- Additional 'B' accumulator?? (cf. motorola 6800)

### Earlier notes

- [2023-08-23](2023-08-23.md#isa---instruction-set-layout-notes)
- [2023-09-04](2023-09-04.md#isa)
- [2023-09-07](2023-09-07.md#isa)

## Pinout

- [ ] TODO: review highlights in Petzold book

### Earlier notes

- [2023-09-07: 1802 note](2023-09-07.md#1802-note)
- [2023-09-07: pinout brainstorm ](2023-09-07.md#cpu-pinout-brainstorm)
- [2023-08-30: 1802](2023-08-30.md#1802)
- [2023-09-04: screen memory as DMA area](2023-09-04.md#screen-memory-as-dma-area-general-purpose)