NOTE: this project is much influx and should not be considered usabled

TinyVM is a minimalistic 64bit Virtual Machine. The aim of TinyVM is to make it easy to embed in other Go projects that require a Virtual Machine. TinyVM is not thread-safe, though this is subject to change.

Installation

To install the TinyVM binary please make sure you've got Go properly set up, then run go get github.com/obscuren/tinyvm

Usage

Basic: tinyvm <flags> file. TinyVM allows you to set the registers from the command line using the -r#. Where # is the register number (0 to 15). Please take extra care when setting register 15. This register is used for the program counter and allows you to control the flow of execution. Please refer to the -help option for more information.

Assembler

TinyVM comes with a small set of assembler instructions to make it easy to use. The asm package contains an assembler language definition and a very simple compiler.

VM

TinyVM comes with a small general purpose register (r0..r15), unbounded memory ([addr]) and a general purpose stack mechanism (pop, push). r15 is a special register for the program counter and can be set to jump to arbitrary position in code. TinyVM also has a very simple calling mechanism (call) and keeps an internal call stack to determine the positions for returning (ret).

Setting register r15 to anything other than the default (0) means execution will start from that position and onward. In the future we'll allow labels to be specified in the form of v.Set(asm.Reg, asm.R15, "my_label"), but this has to be implemented in both the vm as well as the compiler who does not yet emit label information during the assembly stage of the "compiler".

See Appendix I for a list op assembly operations.

Conditional execution

TinyVM supports (like ARM) conditional execution e.g. moveq would only be executed if the conditional value were to be set to zero. The conditional value can be set by appending s to the mnemonic (e.g. movs, which sets the 25th bit) or by using the comparison instructions cmp and tst. By default data processing instructions do not set the condition code flag.

An conditional execution must always be preceeded by a comparison instruction or an instruction with the conditional code bit set.

Instruction encoding

The instruction scheme used by TinyVM is based on the ARM instruction scheme though with some arrangement in the ops order. TinyVM uses 4-bit rotation value (9th to 12th bit) to shift the 8-bit immediate value. This clever trick allows us to encode a lot of values in the range of 2^0..32 in only 12-bits. When an immediate value is encoded in Ops2 the 25th bit is set to 1, indicating an immediate value is encoded in the lower 12 bits of the instruction.

The last 4 bits will be used for conditional execution (like ARM). Any instruction can be form of operation[condition] e.g. addeq for add if equal or movgt for mov if greater than.

Instructions can be in several modes stored in bit 27 and 26. TinyVM supports the following modes:

• 00 Data processing
• 01 Data transfer
• 10 Branching
• 11 (reserved / unused)

+--------------+---------+----------+----------+----------+----------+---------+---------+---------+
| Bits         |31 .. 28 | 27 .. 24 | 23 .. 20 | 19 .. 16 | 15 .. 12 | 11 .. 8 | 7 ... 4 | 3 ... 0 |
+--------------+---------+----------+----------+----------+----------+---------+---------+---------+
| Description  |  COND   |   DDSI   |    INS   |    Ds    |   Ops1   |   Ops2  |         |         |
+--------------+---------+----------+----------+----------+----------+---------+---------+---------+
| mov r1 #260  |  0000   |   0001   |   1010   |   0001   |   0000   |   1111  |  0100   |   0001  |
| mov r1 r2    |  0000   |   0000   |   1010   |   0001   |   0002   |   0000  |  0000   |   0000  |
+--------------+---------+----------+----------+----------+----------+---------+---------+---------+

Example

Integraton

The following example is an integration example on how you could embed TinyVM in to your own project.

    mov     r15 main
add:    ; add taket two arguments
	add 	r0 r0 r1
	ret

main:   ; main must be called with r0 and r1 set
	call 	add

	stop

// parse the source code
code, err := asm.Assemble(sourceCode)
if err != nil {
    panic(err)
}


v := vm.New(false) // pass "true" for debug info
// set the registers (as required for "main")
v.Set(asm.Reg, asm.R0, 3) // set r0 to 3
v.Set(asm.Reg, asm.R1, 2) // set r1 to 2

// execute the compiled code
if err := v.Exec(code); err != nil {
    fmt.Println("err", err)
        return
}

// exit: 5
fmt.Println("exit:", v.Get(asm.Reg, asm.R0))

ASM samples

Jumping

Register r15 is used to keep track of the program counter (pc). Setting register r15 allows you to jump to another position during execution (using labels is permitted):

	mov 	r15 main
	mov 	r0  #1
main:

Instruction mov r0 #1 should be ignored and register 0 should be left empty.

Counter

The following code fragment is a loop which runs until the counter in r0 hits zero When it hits zero the condition code ne (not equal to zero) controling branch becomes false and exits the loop.

	mov     r0   #10
loop:
	subs	r0   r0 #1
	movne	r15  loop

Appendix I

All operations take at least 2 argument. The first argument (dst=destination) must be a register (r#).

Opcode	Arg count	Example	Description
mov	2	mov r0 #1	Moves ops1 in to register dst
add	3	add r0 r0 #1	ops1 + ops2 and sets the result to register dst
sub	3	sub r0 r0 #1	ops1 - ops2 and sets the result to register dst
rsb	3	rsb r0 r0 #1	ops2 - ops1 and sets the result to register dst
mul	3	mul r0 r0 #1	ops2 * ops1 and sets the result to register dst
div	3	mul r0 r0 #1	ops2 / ops1 and sets the result to register dst
and	3	and r0 r0 #1	ops1 & ops2 and sets the result to register dst
xor	3	xor r0 r0 #1	ops1 ^ ops2 and sets the result to register dst
orr	3	orr r0 r0 #1	`ops1
cmp	2	cmp r0 r0	ops1 - ops2 and sets the result to the condition value
ldr	2	ldr r0 r1	Load word addressed by ops1 from memory and store in dst
str	2	str r0 r1	Store word indst at address ops1
call	1	call label	sets r15 to dst and pushes pc to the pc stack
ret	0	ret	pops the pc of the pc stack and sets r15. len(stack)==0 halt execution

TODO

• Add conditional-execution tests
• Add assembler tests
• Rewrite memory implementation. Current memory model is temporarily.
• Implement a proper stack mechanism. Current call stack is temporarily.
• Add pc, lr and sp syntatic sugar (pc = r15, lr = r14, sp = r13)