(2023-09-11) Create note, with thoughts on ISA and CPU pinout

2023-09-11.md

@@ -1,2 +1,75 @@
 # 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)