README.md

go.vm

This project is a golang based compiler and intepreter for a simple virtual machine. It is a port of the existing project:

• https://github.com/skx/simple.vm

(The original project has a perl based compiler/decompiler and an interpreter written in C.)

You can get a feel for what it looks like by refering to either the parent project, or the examples contained in this repository.

This particular virtual machine is intentionally simple, but despite that it is hopefully implemented in a readable fashion. ("Simplicity" here means that we support only a small number of instructions, and the 16-registers the virtual CPU possesses can store strings and integers, but not floating-point values.)

Installation

Install the code via:

$ go get -u  github.com/skx/go.vm
$ go install github.com/skx/go.vm

Once installed there are three sub-commands of interest:

• go.vm compile $file.in
  • Compiles the given program into bytecode.
• go.vm execute $file.raw
  • Given the path to a file of bytecode, then interpret it.
• go.vm run $file.in
  • Compiles the specified program, then directly executes it.

So to compile the input-file examples/hello.in into bytecode:

 $ go.vm compile examples/hello.in

Then to execute the resulting bytecode:

 $ go.vm execute examples/hello.raw

Or you can handle both steps at once:

 $ go.vm run examples/hello.in

Opcodes

The virtual machine has 16 registers, each of which can store an integer or a string. For example to set the first two registers you might write:

 store #0, "This is a string"
 store #1, 0xFFFF

In addition to this there are several mathematical operations which have the general form:

 $operation $result, $src1, $src2

For example to add the contents of register #1 and register #2, storing the result in register #0 you would write:

 add #0, #1, #2

Strings and integers may be displayed to STDOUT via:

 print_str #1
 print_int #3

Control-flow is supported via call, ret (for subroutines) and jmp for absolute jumps. You can also use the Z-flag which is set by comparisons and the inc/dec instructions and make conditional jumps:

    store #1, 0x42
    cmp #1, 0x42
    jmpz ok

    store #1, "Something weird happened!\n"
    print_str #1
    exit
  :ok
    store #1, "Comparing register #01 to 0x42 succeeded!\n"
    print_str #1
    exit

Further instructions are available and can be viewed beneath examples/. The instruction-set is pretty limited, for example there is no notion of reading from STDIN - however this is supported via the use of traps, as documented below.

Notes

Some brief notes on parts of the code / operation:

The compiler

The compiler is built in a traditional fashion:

• Input is split into tokens via lexer.go
  • This uses the token.go for the definition of constants.
• The stream of tokens is iterated over by compiler.go
  • This uses the constants in opcode.go for the bytecode generation

The approach to labels is the same as in the inspiring-project: Every time we come across a label we output a pair of temporary bytes in our bytecode. Later, once we've read the whole program and assume we've found all existing labels, we go back up and fix the generated addresses.

You can use the dump command to see the structure the lexer generates:

 $ go.vm dump ./examples/hello.in
 {STORE store}
 {IDENT #1}
 {, ,}
 {STRING Hello, World!
 }
 {PRINT_STR print_str}
 {IDENT #1}
 {EXIT exit}

The interpreter

The core of the intepreter is located in the file cpu.go and is as simple and naive as you would expect. There are some supporting files in the same directory:

• register.go
  • The implementation of the register-related functions.
• stack.go
  • The implementation of the stack.
• traps.go
  • The implementation of the traps, to be described below.

Changes

Compared to the original project there are two main changes:

• The DB/DATA operation allows storing string data directly in the generated bytecode.
• There is a notion of traps.
  • Rather than defining opcodes for complex tasks it is now possible to callback into the CPU-emulator to do work.

DB/DATA Changes

For example in simple.vm project this is possible:

DB 0x01, 0x02,

But this is not:

 DB "This is a string, with terminator to follow"
 DB 0x00

go.vm supports this, and it is demonstrated in examples/peek-strlen.in.

Traps

The instruction int can be used to call back to the emulator to do some work on behalf of a program. Only two traps are defined right now:

• int 0x00
  • Set the contents of the register #0 with the length of the string in register #0.
• int 0x01
  • Set the contents of the register #0 with a string entered by the user.
  • See examples/trap.stdin.in.

Adding your own trap-functions should be as simple as editing cpu/traps.go.

Steve