A virtual machine implements a computer in software.

Task[edit]
Write a virtual machine interpreter. This interpreter should be able to run virtual
assembly language programs created via the task. This is a
byte-coded, 32-bit word stack based virtual machine.

The program should read input from a file and/or stdin, and write output to a file and/or
stdout.

Input format:

Given the following program:

count = 1;
while (count < 10) {
    print("count is: ", count, "\n");
    count = count + 1;
}

The output from the Code generator is a virtual assembly code program:



 Output from gen, input to VM



Datasize: 1 Strings: 2
"count is: "
"\n"
    0 push  1
    5 store [0]
   10 fetch [0]
   15 push  10
   20 lt
   21 jz     (43) 65
   26 push  0
   31 prts
   32 fetch [0]
   37 prti
   38 push  1
   43 prts
   44 fetch [0]
   49 push  1
   54 add
   55 store [0]
   60 jmp    (-51) 10
   65 halt

The first line of the input specifies the datasize required and the number of constant
strings, in the order that they are reference via the code.

The data can be stored in a separate array, or the data can be stored at the beginning of
the stack. Data is addressed starting at 0. If there are 3 variables, the 3rd one if
referenced at address 2.

If there are one or more constant strings, they come next. The code refers to these
strings by their index. The index starts at 0. So if there are 3 strings, and the code
wants to reference the 3rd string, 2 will be used.

Next comes the actual virtual assembly code. The first number is the code address of that
instruction. After that is the instruction mnemonic, followed by optional operands,
depending on the instruction.

Registers:

sp:

   the stack pointer - points to the next top of stack.  The stack is a 32-bit integer
   array.

pc:

   the program counter - points to the current instruction to be performed.  The code is an
   array of bytes.

Data:

   data
   string pool

Instructions:

Each instruction is one byte. The following instructions also have a 32-bit integer
operand:

fetch [index]

where index is an index into the data array.

store [index]

where index is an index into the data array.

push n

where value is a 32-bit integer that will be pushed onto the stack.

jmp (n) addr

where (n) is a 32-bit integer specifying the distance between the current location and the
desired location. addr is an unsigned value of the actual code address.

jz (n) addr

where (n) is a 32-bit integer specifying the distance between the current location and the
desired location. addr is an unsigned value of the actual code address.

The following instructions do not have an operand. They perform their operation directly
against the stack:

For the following instructions, the operation is performed against the top two entries in
the stack:

add
sub
mul
div
mod
lt
gt
le
ge
eq
ne
and
or

For the following instructions, the operation is performed against the top entry in the
stack:

neg
not

Print the word at stack top as a character.

prtc

Print the word at stack top as an integer.

prti

Stack top points to an index into the string pool. Print that entry.

prts

Unconditional stop.

halt

 A simple example virtual machine

def run_vm(data_size)
    int stack[data_size + 1000]
    set stack[0..data_size - 1] to 0
    int pc = 0
    while True:
        op = code[pc]
        pc += 1
 
        if op == FETCH:
            stack.append(stack[bytes_to_int(code[pc:pc+word_size])[0]]);
            pc += word_size
        elif op == STORE:
            stack[bytes_to_int(code[pc:pc+word_size])[0]] = stack.pop();
            pc += word_size
        elif op == PUSH:
            stack.append(bytes_to_int(code[pc:pc+word_size])[0]);
            pc += word_size
        elif op == ADD:   stack[-2] += stack[-1]; stack.pop()
        elif op == SUB:   stack[-2] -= stack[-1]; stack.pop()
        elif op == MUL:   stack[-2] *= stack[-1]; stack.pop()
        elif op == DIV:   stack[-2] /= stack[-1]; stack.pop()
        elif op == MOD:   stack[-2] %= stack[-1]; stack.pop()
        elif op == LT:    stack[-2] = stack[-2] <  stack[-1]; stack.pop()
        elif op == GT:    stack[-2] = stack[-2] >  stack[-1]; stack.pop()
        elif op == LE:    stack[-2] = stack[-2] <= stack[-1]; stack.pop()
        elif op == GE:    stack[-2] = stack[-2] >= stack[-1]; stack.pop()
        elif op == EQ:    stack[-2] = stack[-2] == stack[-1]; stack.pop()
        elif op == NE:    stack[-2] = stack[-2] != stack[-1]; stack.pop()
        elif op == AND:   stack[-2] = stack[-2] and stack[-1]; stack.pop()
        elif op == OR:    stack[-2] = stack[-2] or  stack[-1]; stack.pop()
        elif op == NEG:   stack[-1] = -stack[-1]
        elif op == NOT:   stack[-1] = not stack[-1]
        elif op == JMP:   pc += bytes_to_int(code[pc:pc+word_size])[0]
        elif op == JZ:    if stack.pop() then pc += word_size else pc += bytes_to_int(code[pc:pc+word_size])[0]
        elif op == PRTC:  print stack[-1] as a character; stack.pop()
        elif op == PRTS:  print the constant string referred to by stack[-1]; stack.pop()
        elif op == PRTI:  print stack[-1] as an integer; stack.pop()
        elif op == HALT:  break
 Additional examples
Your solution should pass all the test cases above and the additional tests found Here.

Reference
The C and Python versions can be considered reference implementations.

Related Tasks
 Lexical Analyzer task
 Syntax Analyzer task
 Code Generator task
 AST Interpreter task

package main
 
import (
    "bufio"
    "encoding/binary"
    "fmt"
    "log"
    "math"
    "os"
    "strconv"
    "strings"
)
 
type code = byte
 
const (
    fetch code = iota
    store
    push
    add
    sub
    mul
    div
    mod
    lt
    gt
    le
    ge
    eq
    ne
    and
    or
    neg
    not
    jmp
    jz
    prtc
    prts
    prti
    halt
)
 
var codeMap = map[string]code{
    "fetch": fetch,
    "store": store,
    "push":  push,
    "add":   add,
    "sub":   sub,
    "mul":   mul,
    "div":   div,
    "mod":   mod,
    "lt":    lt,
    "gt":    gt,
    "le":    le,
    "ge":    ge,
    "eq":    eq,
    "ne":    ne,
    "and":   and,
    "or":    or,
    "neg":   neg,
    "not":   not,
    "jmp":   jmp,
    "jz":    jz,
    "prtc":  prtc,
    "prts":  prts,
    "prti":  prti,
    "halt":  halt,
}
 
var (
    err        error
    scanner    *bufio.Scanner
    object     []code
    stringPool []string
)
 
func reportError(msg string) {
    log.Fatalf("error : %s\n", msg)
}
 
func check(err error) {
    if err != nil {
        log.Fatal(err)
    }
}
 
func btoi(b bool) int32 {
    if b {
        return 1
    }
    return 0
}
 
func itob(i int32) bool {
    if i != 0 {
        return true
    }
    return false
}
 
func emitByte(c code) {
    object = append(object, c)
}
 
func emitWord(n int) {
    bs := make([]byte, 4)
    binary.LittleEndian.PutUint32(bs, uint32(n))
    for _, b := range bs {
        emitByte(code(b))
    }
}
 
/*** Virtual Machine interpreter ***/
func runVM(dataSize int) {
    stack := make([]int32, dataSize+1)
    pc := int32(0)
    for {
        op := object[pc]
        pc++
        switch op {
        case fetch:
            x := int32(binary.LittleEndian.Uint32(object[pc : pc+4]))
            stack = append(stack, stack[x])
            pc += 4
        case store:
            x := int32(binary.LittleEndian.Uint32(object[pc : pc+4]))
            ln := len(stack)
            stack[x] = stack[ln-1]
            stack = stack[:ln-1]
            pc += 4
        case push:
            x := int32(binary.LittleEndian.Uint32(object[pc : pc+4]))
            stack = append(stack, x)
            pc += 4
        case add:
            ln := len(stack)
            stack[ln-2] += stack[ln-1]
            stack = stack[:ln-1]
        case sub:
            ln := len(stack)
            stack[ln-2] -= stack[ln-1]
            stack = stack[:ln-1]
        case mul:
            ln := len(stack)
            stack[ln-2] *= stack[ln-1]
            stack = stack[:ln-1]
        case div:
            ln := len(stack)
            stack[ln-2] = int32(float64(stack[ln-2]) / float64(stack[ln-1]))
            stack = stack[:ln-1]
        case mod:
            ln := len(stack)
            stack[ln-2] = int32(math.Mod(float64(stack[ln-2]), float64(stack[ln-1])))
            stack = stack[:ln-1]
        case lt:
            ln := len(stack)
            stack[ln-2] = btoi(stack[ln-2] < stack[ln-1])
            stack = stack[:ln-1]
        case gt:
            ln := len(stack)
            stack[ln-2] = btoi(stack[ln-2] > stack[ln-1])
            stack = stack[:ln-1]
        case le:
            ln := len(stack)
            stack[ln-2] = btoi(stack[ln-2] <= stack[ln-1])
            stack = stack[:ln-1]
        case ge:
            ln := len(stack)
            stack[ln-2] = btoi(stack[ln-2] >= stack[ln-1])
            stack = stack[:ln-1]
        case eq:
            ln := len(stack)
            stack[ln-2] = btoi(stack[ln-2] == stack[ln-1])
            stack = stack[:ln-1]
        case ne:
            ln := len(stack)
            stack[ln-2] = btoi(stack[ln-2] != stack[ln-1])
            stack = stack[:ln-1]
        case and:
            ln := len(stack)
            stack[ln-2] = btoi(itob(stack[ln-2]) && itob(stack[ln-1]))
            stack = stack[:ln-1]
        case or:
            ln := len(stack)
            stack[ln-2] = btoi(itob(stack[ln-2]) || itob(stack[ln-1]))
            stack = stack[:ln-1]
        case neg:
            ln := len(stack)
            stack[ln-1] = -stack[ln-1]
        case not:
            ln := len(stack)
            stack[ln-1] = btoi(!itob(stack[ln-1]))
        case jmp:
            x := int32(binary.LittleEndian.Uint32(object[pc : pc+4]))
            pc += x
        case jz:
            ln := len(stack)
            v := stack[ln-1]
            stack = stack[:ln-1]
            if v != 0 {
                pc += 4
            } else {
                x := int32(binary.LittleEndian.Uint32(object[pc : pc+4]))
                pc += x
            }
        case prtc:
            ln := len(stack)
            fmt.Printf("%c", stack[ln-1])
            stack = stack[:ln-1]
        case prts:
            ln := len(stack)
            fmt.Printf("%s", stringPool[stack[ln-1]])
            stack = stack[:ln-1]
        case prti:
            ln := len(stack)
            fmt.Printf("%d", stack[ln-1])
            stack = stack[:ln-1]
        case halt:
            return
        default:
            reportError(fmt.Sprintf("Unknown opcode %d\n", op))
        }
    }
}
 
func translate(s string) string {
    var d strings.Builder
    for i := 0; i < len(s); i++ {
        if s[i] == '\\' && (i+1) < len(s) {
            if s[i+1] == 'n' {
                d.WriteByte('\n')
                i++
            } else if s[i+1] == '\\' {
                d.WriteByte('\\')
                i++
            }
        } else {
            d.WriteByte(s[i])
        }
    }
    return d.String()
}
 
func loadCode() int {
    var dataSize int
    firstLine := true
    for scanner.Scan() {
        line := strings.TrimRight(scanner.Text(), " \t")
        if len(line) == 0 {
            if firstLine {
                reportError("empty line")
            } else {
                break
            }
        }
        lineList := strings.Fields(line)
        if firstLine {
            dataSize, err = strconv.Atoi(lineList[1])
            check(err)
            nStrings, err := strconv.Atoi(lineList[3])
            check(err)
            for i := 0; i < nStrings; i++ {
                scanner.Scan()
                s := strings.Trim(scanner.Text(), "\"\n")
                stringPool = append(stringPool, translate(s))
            }
            firstLine = false
            continue
        }
        offset, err := strconv.Atoi(lineList[0])
        check(err)
        instr := lineList[1]
        opCode, ok := codeMap[instr]
        if !ok {
            reportError(fmt.Sprintf("Unknown instruction %s at %d", instr, opCode))
        }
        emitByte(opCode)
        switch opCode {
        case jmp, jz:
            p, err := strconv.Atoi(lineList[3])
            check(err)
            emitWord(p - offset - 1)
        case push:
            value, err := strconv.Atoi(lineList[2])
            check(err)
            emitWord(value)
        case fetch, store:
            value, err := strconv.Atoi(strings.Trim(lineList[2], "[]"))
            check(err)
            emitWord(value)
        }
    }
    check(scanner.Err())
    return dataSize
}
 
func main() {
    codeGen, err := os.Open("codegen.txt")
    check(err)
    defer codeGen.Close()
    scanner = bufio.NewScanner(codeGen)
    runVM(loadCode())
}