# Paper computer simulator experiment

## TODO

- [ ] Document current state of affairs (how to run it etc)

Computer:
- [ ] Implement carry flag on SUB
- [ ] Move screen memory down,
      and add a pointer to screen mem at $00
      to make the screen relocatable

Misc:
- [ ] Write some more complex test programs
    - [ ] Display (hex) numbers
- [ ] Do a proper binary version... (or lose the typed arrays?)

## 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 and Flags

- `A` - accumulator
- `IP` - instruction pointer (aka program counter)
- `CF` - carry flag

## Memory map / Peripherals

- `00-0F` - 4x4 display
- `10-1F` - keypad? (details TBD)
- `20   `    - initial value for IP
- `20-FF` - free

### Maybe someday

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

## Assembly language


    ADD $01         ; comments follow a `;`

    ADD $FF         ; this is direct addressing
    ADD ($CC)       ; this is indirect addressing

    END             ; END, CFC, and CHP don't require arguments
                    ; (a default value of 0 will be used as their operand)

    @subroutine     ; create a label
        ADD $01     ; (it must be on the line before the code it names)
        ADD $02

    JMP @subroutine ; use a label as an argument
                    ; the label will be replaced with
                    ; the address of the label

- Hexadecimal numbers are preceded by a `$`
- Whitespace is ignored