crap-derive

for a school assignment thing

mov r0, 3
cmp r0, 8
je true
jmp false
true:
jmp exit

// loop
false:
mov r5, [8]
jmp false


exit:

Specification

The crapderive virtual machine has 16 registers, r0 to r15.

It has 100MB (1024^3) bytes of memory, addressed by the addresses 0x0 to 0x40000000 (not included).

The machine word (register) size is 64 bit.

Arithmetic is defined as wrappings, and division by zero traps (cannot be caught).

The VM has a flag that is used in some operations.

Definitions

Place

A place is a location in the VM. A place can either be a register (r0-r15) or a memory address ([X] where X is an index).

Index

An index is used to dereference memory and can be one of the following:

• A register
• A literal number
• A symbol (TODO)

Value

A value is a value that can be obtained from the VM. A value can either be a place, a literal number or a symbol.

Location

A location is a location in the VM code. It must contain a symbol with the label to redirect execution to.

Instructions

Mov

mov A, B

Mov moves the value B to the place A.

Movb

movb A, B

Mov moves the value B to the place A. Only a single byte is written.

Add

add A, B

Add adds the value B to the place A. The result is stored in the place A.

Sub

sub A, B

Sub subtracts the value B from* the place A. The result is stored in the place A.

Mul

mul A, B

Mul multiplies the place A with the value B. The result is stored in the place A.

Div

div A, B

Div divides the place A by the value B. The result is stored in the place A.

Int

int N

Int performs the interrupt with the number N. N must be a number literal.

For a list of interrupts, see the list below.

Cmp

cmp A, B

Cmp compares the value A with the value B. If they are equal, the flag is set, otherwise the flag is cleared.

Jmp

jmp L

Jmp jumps to the location L. The next instruction executed will be the instruction located at L.

Je

je L

Je jumps to the location L if the flag is set. If the jump happens, the next instruction executed will be the instruction located at L.

Label

<IDENT>:

A label is an identifier (regex: [a-zA-Z]\w+) followed by a colon (:). Jumps (like jmp or je) will be able to jump to this label. A label is applied to the next instruction in the code.

Interrupts

int 0

The interrupt 0 exits the program. The exit code is passed in r0.

int 1

The interrupt 1 writes a string to standard output. r0 contains the address of the string data. r1 contains the length of the string. The string is forwarded as raw bytes to the OS. After the interrupt completed, r0 will contain a 0 if writing was successful, or a 1 if it failed.

int 2

The interrupt 2 reads in a UTF-8 string from stdin. r0 contains the address of the buffer for the data. r1 contains the length of the buffer. The interrupt returns after the buffer has been filled. If reading fails, 1 will be put into r0, if it was successful, 0.