About

This is an exploratory project into virtual machines and assembly language. By no means is this ready for production use or particularly well maintained. The language is inspired by x86 and ARM assembly and does very little hand holding.

Checkout the bin/example.wasm example source to get an overview of the language, or keep on reading!

Design

From Text To Runtime Behaviour

In order to turn the source text into executable code we use 3 passes:

Pass 1: tokenization (syntax check)
Pass 2: interpretation (semantics check)
Pass 3: execution (runtime check)

After pass 2 ties to the source code are lost, meaning that any error occurring afterwards can be a bit cryptic as to where it originated.

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
square brackets ([ and ]) for addressing memory

Memory Model

The stack, with which you interact through pop/push operations, grows from memory location 0 to the end of the memory. There is no strict checking on whether your own memory operations through [] affect the stack: this is a feature, not a bug. Keep in mind that the stack can underflow and overflow and that the memory uses byte units (8 bits), whereas the registers are all 32 bits wide. This means that reading from location $900 overlaps with 3 bytes when reading from location $901 (the first byte of $901 is the second byte of location $900).

Symbols

All symbols are reserved keywords and can therefore NOT be used as labels. There is currently no strict checking, so be careful.

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
call put the next statement to execute on the stack and jump to the label given by the first argument
ret pop the the next statement to execute off the stack, e.g. returning to the next execution statement before calling call
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..3] 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
[4..4] Input from STDIN
- 4 get a single ASCII character from STDIN and store it in register A

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