# Paper computer simulator experiment

## TODO

- [ ] Implement carry flag on SUB
- [ ] Write some more complex test programs
- [ ] Consider adding a way to visualize screen memory
- [ ] Do a proper binary version...

## Instruction set

    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#   ; hop   ... skip next instruction if A == lit# ... when true: IP <- PC + 2
    A   HOP addr   ; hop   ... skip next instruction if A == addr ... when true: IP <- PC + 2
    B   JMP lit#   ; jump  ... IP <- lit#
    C   JMP addr   ; jump  ... IP <- addr
    D   CCF ————   ; clear Carry Flag ... CF = 0
    E   CHP ————   ; carry hop ... 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