m16vm - 16 bit processor virtual machine

This is a virtual machine for a RISC-processor designed for educational purposes.

The design is taken from the real world and tries to mimic existing RISC-architectures. So it should in theory be possible to construct actual hardware based on the instruction set. Maybe not build a transistor-chip (it's hard, unless you know someone working at Intel/Amd or something) but by putting together a bunch of gate-chips on a breadboard.

The instruction set is by no means designed to be "fast" or "optimal" but focus more on being easy to understand and modify/play with.

Specification

The cpu has 16 general purpose registers, r0 - r15. Each register is 16-bit.

Memory addresses are aligned by 2 bytes (16 bit) and the machine has a total of 65536 bytes of memory (per default, can be changed at compile time).

instruction set overview

There are 16 different instructions.

opcode	Name	Type	Description
0000	noop	-	No operation
0001	add	R	Addition
0010	movl	I	move value to register (lowest 8-bits)
0011	movh	I	move value to register (highest 8-bits)
0100	ld	RI	load word
0101	sw	RI	store word
0110	beq	RI	Branch on equal
0111	jmp	J	Jump
1000	jr	I	Jump (register)
1001	-	-	Reserved
1010	-	-	Reserved
1011	-	-	Reserved
1100	-	-	Reserved
1101	-	-	Reserved
1110	-	-	Reserved
1111	int	I	Interrupt

NOTE: Subtraction can be implemented via add and negative register values, so no special opcode is needed.

Instruction set format

Register operation (R):

opcode (4) | rs (4)  | r0 (4) | r1 (4)

r0,r1      - Operand registers.
rs         - save register

Register operation offset (RI):

opcode (4) | rs (4)  | r0 (4) | offset (signed 4)

r0 	- Operand registers.
offset  - constant offset from r0 value.
rs   	- save register

Constant operation format (I):

opcode (4) | reg (4) | data (signed 8)

reg - Register
data - Constant data to insert into reg.

Jump format (J):

opcode (4) | addr (12)

Interrupts

There can be a total of 16 Interrupts and the instruction uses the I-format.

The interrupt number is stored in reg (note that for interrupts this is a constant and not a register).

The data field can be used as an argument to the interrupt.

Number	Name	data	Description
10	I/O Write	format	Outputs a value to the screen, value are stored in r15

I/O Write - int 10

The formatting of the output can be controlled by the number in data as follows:

Argument	Datatype
0	Integer (16 bit signed)
1	Integer (8 bit signed)
2	Character

Example programs.

in the programs directory, there is some example programs to run.