# Paper computer simulator experiment

## ISA

0 END
1 STO lit# ; store ... mem[lit#] <- A
2 STO addr ; store ... mem[mem[addr]] <- A
3 LDA lit# ; load  ... A <- lit#
4 LDA addr ; load  ... A <- mem[addr]
5 ADD lit# ; add   ... A <- A + lit#     ... and un/set carry flag
6 ADD addr ; add   ... A <- A + mem[addr] ... and un/set carry flag
7 SUB lit# ; sub   ... A <- A - lit#     ... and un/set carry flag
8 SUB addr ; sub   ... A <- A - mem[addr] ... and un/set carry flag
9 HOP lit# ; skip next instruction if A == lit# ... when true: IP <- PC + 2
A HOP addr ; skip next instruction if A == addr ... when true: IP <- PC + 2
B JMP lit# ; ... IP <- lit#
C JMP addr ; ... IP <- addr
~~D CAT ---- ; Carry Flag Toggle ... CF <- !CF~~
D CCF ———— ; Clear Carry Flag ... CF = 0
E CHP ---- ; skip next instruction if Carry Flag is set ... when true: IP <- PC + 2
F

### Nice features that didn't fit

- hop IF< and hop IF>
- MUL and DIV
- rotates and shifts

## Registers

- A -- accumulator
- IP -- instruction pointer (aka program counter)

## Flags

- CF -- carry flag

## Memory-mapped peripherals

- 4x4 display
- hex keypad (details TBD)
- ? bank selector (for bank-switching)

### Maybe someday

- timer (for a version in software/electronic-hardware)

## Tentative memory map

00-0F - display
10-1F - keypad? (details TBD)
20    - initial value for IP ?
21-FF - free