# Design

## Parsing

- First we tokenize the input (syntax check)
- Then we interpret the input (semantics check)
- Then we execute the input

## Notation

- `[operation][number type]`, e.g. `divi` for divide (div) integer
- `%[register]` for addressing registers
- `$[value]` for using literals/immediate values
- `;` for end of statement (mandatory)
- `[label]:` for labels
- `#[text]` for comments: any text is ignored till a newline (`\n`) is found
- `[[%register|$value]]` for accessing memory
- Elements must be separated by whitespace character
	- Good: `add $2 $5 %A;`
	- Bad: `add $2$5%A;`

## Examples

Divide register A by 5 and store the result in register A:
`divi %A $5 %A;`

Increment B until it is 10:
```
# Set B to zero
addi $0 $0 %B;

loop:
addi $1 %B %B;
lti %B $10;
jmp loop;
```

Read the integer at memory location `1024` into register A:
```
seti %A [$1024];
```
Remember not to use spaces inside the `[` brackets.

## Reserved symbols

The following whitespace characters are used to separate symbols:
- space (` `)
- tab (`\t`)
- return carriage (`\r`)
- newline (`\n`)

The following characters are used as identifiers:
- dollar (`$`) for immediate (literal) values
- percentage (`%`) for register identifiers
- colon (`:`) for jump labels
- semicolon (`;`) for statement termination
- hash (`#`) for comments

All symbols are reserved keywords and can therefore NOT be used as labels.

## Symbols

## Directives

- `DECLARE` declares the first label argument to equal the second, immediate value, argument and is used to declare a constant for the virtual machine.

### Operands

- `addi` add the first to the second argument and store the result in the third argument
- `subi` subtract the first from the second argument and store the result in the third argument
- `divi` divide the first by the second argument and store the result in the third argument
- `muli` multiply the first by the second argument and store the result in the third argument
- `shli` shift left the first argument by the number of positions given by the second argument and store the result in the third argument
- `shri` shift right the first argument by the number of positions given by the second argument and store the result in the third argument
- `seti` set the first register argument to the second argument
- `int` calls the interrupt specified by the first (integer) argument

### Control Flow

- `jmp` jump to the label given by the first argument
- `lti` execute next statement if argument 1 is less than argument 2 else skip the next statement
- `gti` execute next statement if argument 1 is greater than argument 2 else skip the next statement
- `eqi` execute the next statement if argument 1 is equal to argument 2 else skip the next statement

## Memory

- `popi` pops the first value on the stack into the register specified as the first argument
- `pushi` pushes the value on the stack from the register or immediate value as the first argument

## Interupts

- [0..9] Output to STDOUT
	- `0` put value of register A as ASCII character on stdout
	- `1` put value of register A as decimal integer on stdout
	- `2` put value of register A as hexadecimal integer on stdout
	- `3` put the string pointed at by register A for the amount of characters defined by register B on stdout