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