# Cardiograph Mark I — simulator for an imaginary computer

Cardiograph is an imaginary computer. It has three main components:

1. the CPU, *Card* (short for 'Completely Analogue Risc Machine')
2. an input-output processor, *IO*
3. a display, *Graph*

## Simulator

### Run assembler

```./assembler.js source_code.asm [output.txt]```

By default, the output is written to `out.txt`. It is saved as a string of space-separated decimal numbers.


### Run simulator

```./cardiograph.js < machine_code.txt```

### Dependencies

- Node.js


## CPU

### Registers and Flags

- `A` - accumulator
- `IP` - instruction pointer (aka program counter)
- `FLAGS` - flags: **O**verflow, **N**egative, **Z**ero, **C**arry
    - in machine language, each flag is given a number:
        - O = 3  
          N = 2  
          Z = 1  
          C = 0
    - (bitwise, `0000 = ONZC`)


### Instruction set

#### Operations 

```
Hex   Mnem.  Operand      Effect

00    END    (ignored)    Halt CPU
01    STO    literal #    mem[lit#] = A
02    STO    address      mem[mem[addr]] = A
03    LDA    literal #    A = lit#
04    LDA    address      A = addr
05    ADD    literal #    A = A + lit#
06    ADD    address      A = A + mem[addr]
07    SUB    literal #    A = A - lit#
08    SUB    address      A = A - mem[addr]
09    HOP    literal #    If A == lit#, skip next op (IP += 4)
0A    HOP    address      If A == mem[addr], skip next instruction (IP += 4)
0B    JMP    literal #    IP = lit#
0C    JMP    address      IP = mem[addr]
0D    FTG    literal #    Toggle flag, where flag number == lit#
0E    FHP    literal #    Skip next op if flag is set, where flag number == lit#
0F    NOP    (ignored)    None
```

- Instructions are two bytes long:
  one byte for the opcode, one for the operand    


#### Effects on memory, flags, registers

```
op   mem  flags  IP

END              +2
NOP              +2

STO  w           +2
LDA  r    NZ     +2
ADD       ONZC   +2
SUB       ONZC   +2
HOP              +2/+4
JMP              arg
FTG       ONZC   +2
FHP       ONZC   +2/+4

STO  r,w         +2
LDA  r,r  NZ     +2
ADD  r    ONZC   +2
SUB  r    ONZC   +2
HOP  r           +2/+4
JMP  r           arg
FTG  r    ONZC   +2
FHP  r    ONZC   +2/+4
```

### Start-up

When starting up, the CPU executes a `JMP $FF`.

Put differently: it starts executing instructions at the address contained in `$FF`.

### Assembly language

    ADD $01         ; comments follow a `;`

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

    END             ; END and NOP don't require operands
                    ; (the assembler will fill in a default value of 0)

    @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 operand
                    ; the label will be replaced with
                    ; the address of the label

    #foo $FF        ; define a constant
                    ; (must be defined before it is referenced)
    
    ADD #foo        ; use a constant as an operand

    LDA *           ; `*` is a special label referencing the memory address
                    ; where the current line will be stored after assembly

- Prefix hexadecimal numbers with `$` (or `0x`)
- Prefix binary numbers with `0b`
- Whitespace is ignored

## Cardiograph memory map

- `00-19` - display (5x5)
- `1A   ` - pointer to display memory
- `1B   ` - keypad: value of latest key pressed
- `1C   ` - reserved for future use (bank switching flag)
- `1D   ` - initial IP
- `1D-FE` - free
- `FF   ` - ROM (unwriteable) pointer to initial IP (not yet implemented)

## Cardiograph peripherals

### Keypad

The value of the latest keypress on a hex keypad is stored at `$1B`.

The keypad uses the same layout as the COSMAC VIP (and CHIP-8). The CPU simulator maps those keys onto a Qwerty set:

```
1 2 3 C    1 2 3 4
4 5 6 D    Q W E R
7 8 9 E    A S D F
A 0 B F    Z X C V

hex pad    simulator
```

The arrow keys are also mapped onto the hex keypad:

```
   5           ↑
 7 8 9       ← ↓ →

hex pad    simulator
```