VM

Includes code for an assembler for a assembly like language and a VM to run the binary produced by the assembler.

Registers

A, B, C, D, E and F are all 8 bit registers. However by design the VM has support for grouped registers that is AB, CD and EF to form 16 bit registers. The PC is a 16 bit register as well. F is a special register, it used a flag register for cmp.

Instruction encoding

Instructions are encoded as 2 bytes. Mainly like the way described in the table below.

4 bit	4 bit	4 bit	4 bit
instruction id	register or integer	register or integer	register or integer

Obviously if the instruction doesn't require 3 arguments the last bits will be zero. Some instructions are special like the jmp, it requires that another 16 bit is fetched (if the jump location isn’t at PC=0). Meaning that the VM first will read the instruction

4 bit	4 bit	4 bit	4 bit
jmp	0	0	0

then it will fetch the next instruction with the location.

16 bit
jmp location

push works the same way if the value pushed onto the stack is greater than 8 bits.

Instruction set

Instruction	Argument 1	Argument 2	Argument 3	Action	Notes
lr	register	integer	None	Loads registers with value	None
push	register or integer	None	None	Push value onto the stack	None
pop	register	None	None	Pop value off the stack and into a register	Can only pop 16 bit values to a 16 bit registers.
add	register	register	register	Addition	None
sub	register	register	register	Subtraction	None
and	register	register	register	Bitwise and	None
or	register	register	register	Bitwise or	None
xor	register	register	register	Bitwise xor	None
cmp	register	register	None	Compare value of to registers	Results is saved to the F register.
jne	label	None	None	Jump to a declared label if the comparison was not equal	Depends on value of the F register. Run cmp first.
je	label	None	None	Jump to a declared label if the comparison was equal	Depends on value of the F register. Run cmp first.
jmp	label	None	None	Jump to a declared label	None

Example code

Example code using a variety of the instruction set for the purpose of demonstration the VM in action.

lr a, 10
lr c, 20
jmp middle

start:
	add a, b, b
	cmp a, c
	jne start
	je end

middle:
	lr b, 10
	cmp a, b
	je start

end:
	push c
	push ab
	push 255
	push 0xffff
	pop ef