RosettaCodeData/Task/Compiler-virtual-machine-interpreter/00-TASK.txt

{{task heading|Virtual Machine Interpreter}}

A virtual machine implements a computer in software.

Write a virtual machine interpreter.  This interpreter should be able to run virtual
assembly language programs created via the [[Compiler/code_generator|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.

{{task heading|Input format}}

Given the following program:

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

The output from the [[Compiler/code_generator|Code generator]] is a virtual assembly code program:

{| class="wikitable"
|-
! Output from gen, input to VM
|-

| style="vertical-align:top" |
<b><pre>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</pre></b>
|}

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.

{{task heading|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

{{task heading|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:

<syntaxhighlight lang="python">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</syntaxhighlight>

; Additional examples

Your solution should pass all the test cases above and the additional tests found '''[[Compiler/Sample_programs|Here]]'''.

{{task heading|Reference}}

The C and Python versions can be considered reference implementations.

;Related Tasks

* [[Compiler/lexical_analyzer|Lexical Analyzer task]]
* [[Compiler/syntax_analyzer|Syntax Analyzer task]]
* [[Compiler/code_generator|Code Generator task]]
* [[Compiler/AST_interpreter|AST Interpreter task]]

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