Virtual Machine

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Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 7: Virutal Machine, Part I slide 1

www.nand2tetris.org

Building a Modern Computer From First Principles

Virtual Machine

Part I: Stack Arithmetic

Where we are at:

Assembler

Chapter 6 H.L. Language

&

Operating Sys.

abstract interface

Compiler

Chapters 10 - 11

VM Translator Chapters 7 - 8

Computer Architecture Chapters 4 - 5

Gate Logic

Chapters 1 - 3 Electrical

Engineering

Physics Virtual

Machine abstract interface

Software hierarchy

Assembly Language abstract interface

Hardware hierarchy Machine

Language abstract interface

Hardware Platform abstract interface

Chips &

Logic Gates abstract interface Human

Thought

Abstract design Chapters 9, 12

Motivation

class Main { static int x;

function void main() {

// Inputs and multiplies two numbers var int a, b, x;

let a = Keyboard.readInt(“Enter a number”);

let b = Keyboard.readInt(“Enter a number”);

let x = mult(a,b);

return;

} }

// Multiplies two numbers.

function int mult(int x, int y) { var int result, j;

let result = 0; let j = y;

while ~(j = 0) {

let result = result + x;

let j = j – 1;

}

return result;

} }

Jack code (example)

Our ultimate goal:

Translate high-level programs into executable code.

Compiler

0000000000010000 1110111111001000 0000000000010001 1110101010001000 0000000000010000 1111110000010000 0000000000000000 1111010011010000 0000000000010010 1110001100000001 0000000000010000 1111110000010000 0000000000010001 0000000000010000 1110111111001000 0000000000010001 1110101010001000 0000000000010000 1111110000010000 0000000000000000 1111010011010000 0000000000010010 1110001100000001 0000000000010000 1111110000010000 0000000000010001

...

Hack code

Compilation models . . .

requires n m translators hardware platform 2 hardware

platform 1

hardware platform m

. . .

language 1 language 2 language n

direct compilation:

.

. . .

hardware platform 2 hardware

platform 1

hardware platform m

. . .

language 1 language 2 language n

intermediate language

requires n + m translators

2-tier compilation:

Two-tier compilation:

First stage: depends only on the details of the source language

Second stage: depends only on the details of the target language.

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The big picture

. . .

RISC machine

Intermediate code

other digital platforms, each equipped with its own VM implementation RISC

machine language

computerHack Hack machine language CISC

machine language

CISC machine

. . .

a high-level^{written in} language

Any computer

. . .

VM implementation

over CISC platforms

VM imp.

over RISC platforms

VM imp.

over the Hack platform VM

emulator Some Other

language

Jack language

compilerSome Some Other

compiler

Jack compiler

. . .

Some

language

. . .

The intermediate code:

 The interface between the 2 compilation stages

 Must be sufficiently general to support many

<high-level language, machine-language>

pairs

 Can be modeled as the language of an abstract virtual machine (VM)

 Can be implemented in several different ways.

Focus of this lecture

(yellow)

:

. . .

RISC machine

VM language

other digital platforms, each equipped with its VM implementation RISC

machine language

computerHack Hack machine language CISC

machine language

CISC machine

. . .

a high-level^{written in} language

Any computer

. . .

VM implementation

over CISC platforms

VM imp.

over RISC platforms

VM imp.

over the Hack platform VM

emulator Some Other

language Jack

language

compiler

compilerJack

. . .

Some

language

. . .

1, 2, 3, 4, 5, 6 7, 8 9, 10, 11, 12 Book chapters and Course projects:

(this and the next lecture)

Virtual machines

 A virtual machine (VM) is an emulation of a particular (real or hypothetical) computer system.

 System virtual machine (full virtualization VMs): a complete substitute for the targeted real machine and a level of

functionality required for the execution of a complete operating system, e.g., VirtualBox.

Virtual machines

 A virtual machine (VM) is an emulation of a particular (real or hypothetical) computer system.

 System virtual machine (full virtualization VMs): a complete substitute for the targeted real machine and a level of

functionality required for the execution of a complete operating system, e.g., VirtualBox.

 Process virtual machine: designed to execute a single computer

program by providing an abstracted and platform-independent program execution environment, e.g., Java virtual machine (JVM).

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The VM model and language

Perspective:

From here till the end of the next lecture we describe the VM model used in the Hack-Jack platform

Other VM models (like Java’s JVM/JRE and .NET’s IL/CLR) are similar in spirit, but differ in scope and details.

The VM model and language

Several different ways to think about the notion of a virtual machine:

 Abstract software engineering view:

the VM is an interesting abstraction that makes sense in its own right (a hypothetical machine closer to high-level language, but could still be built easily. Sometimes, no need to worry about how to implement it in hardware.)

 Practical software engineering view:

the VM code layer enables “managed code” (e.g. enhanced security)

 Pragmatic compiler writing view:

a VM architecture makes writing a compiler much easier (as we’ll see later in the course)

 Opportunistic empire builder view:

a VM architecture allows writing high-level code once and have it run on many target platforms with little or no modification.

Hack virtual machine

Arithmetic / Boolean commands add

sub neg eq gt lt and or not

Memory access commands pop x (pop into x, which is a variable) push y (y being a variable or a constant)

Program flow commands label (declaration) goto (label) if‐goto (label)

Function calling commands function (declaration) call (a function) return (from a function)

Chapter 7 Chapter 8

Goal: Specify and implement a VM model and language:

Our game plan: (a) describe the VM abstraction (3 types of instructions) (b) propose how to implement it over the Hack platform.

The stack

The stack:

 A classical LIFO data structure

 Elegant and powerful

 Several hardware / software implementation options.

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The stack

The stack:

 A classical LIFO data structure

 Elegant and powerful

 Several hardware / software implementation options.

 Several flavors: next empty/available, upward/downward

push(x)

stack[top]=x;

top++;

pop()

top‐‐;

return stack[top];

peek(), empty()

What is the stack good for?

 Stack can be used for evaluating arithmetic expressions

 Expression: 5 * (6+2) – 12/4

 Infix

 Prefix

 Postfix

Stack is also good for implementing function call structures, such as subroutines, local variables and recursive calls. Will discuss it later.

Our VM model is stack-oriented

 All operations are done on a stack

 Data is saved in several separate memory segments

 All the memory segments behave the same

 One of the memory segments m is called static, and we will use it (as an arbitrary example) in the following examples:

Data types

Our VM model features a single 16-bit data type that can be used as:

an integer value (16-bit 2’s complement: ‐32768, ... , 32767)

a Boolean value (0 and ‐1, standing for true and false)

a pointer (memory address)

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Memory access operations

(before)

push static 2

(after)

Evaluation of arithmetic expressions

// z=(2‐x)‐(y+5) push 2

push x sub push y push 5 add sub pop z

VM code (example) (suppose that

xrefers to static 0, yrefers to static 1, zrefers to static 2)

Evaluation of Boolean expressions

// (x<7) or (y=8) push x

push 7 lt push y push 8 eq or

VM code (example) (suppose that

xrefers to static 0, yrefers to static 1)

(actually trueand false are stored as ⁰and ^‐1, respectively)

Arithmetic and Boolean commands in the VM language

(wrap-up)

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A VM program is designed to provide an interim abstraction of a program written in some high-level language.

Modern OO languages normally feature the following variable kinds:

Class level:

Static variables (class-level variables)

Private variables (aka “object variables” / “fields” / “properties”) Method level:

Local variables

Argument variables

When translated into the VM language,

The static, private, local and argument variables are mapped by the compiler on the four memory segments static, this, local, argument In addition, there are four additional memory segments, whose role will

be presented later: that, constant, pointer, temp.

The VM’s Memory segments

Memory segments and memory access commands

Memory access VM commands:

 pop memorySegment index

 push memorySegment index

Where memorySegment is static, this, local, argument, that, constant, pointer, or temp

And index is a non-negative integer

The VM abstraction includes 8 separate memory segments named:

static, this, local, argument, that, constant, pointer, temp As far as VM programming commands go, all memory segments look and

behave the same

To access a particular segment entry, use the following generic syntax:

(In all our code examples thus far, memorySegment was static)

The different roles of the eight memory segments will become relevant when we’ll talk about the compiler

At the VM abstraction level, all memory segments are treated the same way.

VM programming

VM programs are normally written by compilers, not by humans However, compilers are written by humans ...

In order to write or optimize a compiler, it helps to first

understand the spirit of the compiler’s target language – the VM language

So, we’ll now see an example of a VM program

VM programming

The example includes three new VM commands:

 function functionSymbol // function declaration

 label labelSymbol // label declaration

 if‐goto labelSymbol // pop x

// if x=true, jump to execute the // command after labelSymbol // else proceed to execute the next // command in the program

For example, to effect if (x > n) goto loop, we can use the following VM commands:

push x

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function mult (x,y) { int result, j;

result = 0;

j = y;

while ~(j = 0) { result = result + x;

j = j ‐ 1;

}

return result;

}

High-level code

function mult(x,y) push 0

pop result push y pop j label loop

push j push 0 eq

if‐goto end push result push x add pop result push j push 1 sub pop j goto loop label end

push result return VM code (first approx.)

function mult 2 push constant 0 pop local 0 push argument 1 pop local 1 label loop

push local 1 push constant 0 eq

if‐goto end push local 0 push argument 0 add

pop local 0 push local 1 push constant 1 sub

pop local 1 goto loop label end

push local 0 return

VM code

...

loop:

if (j=0) goto end result=result+x j=j‐1

goto loop end:

...

Pseudo code

function mult (x,y) { int result, j;

result = 0;

j = y;

while ~(j = 0) { result = result + x;

j = j ‐ 1;

}

return result;

}

High-level code

function mult(x,y) push 0

pop result push y pop j label loop

push j push 0 eq

if‐goto end push result push x add pop result push j push 1 sub pop j goto loop label end

push result return VM code (first approx.)

function mult 2 push constant 0 pop local 0 push argument 1 pop local 1 label loop

push local 1 push constant 0 eq

if‐goto end push local 0 push argument 0 add

pop local 0 push local 1 push constant 1 sub

pop local 1 goto loop label end

push local 0 return

VM code

VM programming:

multiple functions Compilation:

A Jack application is a set of 1 or more class files (just like .java files).

When we apply the Jack compiler to these files, the compiler creates a set of 1 or more .vm files (just like .class files). Each method in the Jack app is translated into a VM function written in the VM language

Thus, a VM file consists of one or more VM functions.

VM programming:

multiple functions Execution:

At any given point of time, only one VM function is executing (the

“current function”), while 0 or more functions are waiting for it to terminate (the functions up the “calling hierarchy”)

For example, a main function starts running; at some point we may reach the command call factorial, at which point the factorial function starts running;

then we may reach the command call mult, at which point the mult function starts running, while both main and factorial are waiting for it to terminate

The stack:a global data structure, used to save and restore the resources (memory segments) of all the VM functions up the calling hierarchy (e.g. main and factorial). The tip of this stack if the working stack of the current function (e.g. mult).

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VM programming:

multiple functions (files)

VM programming:

multiple functions (memory)

Handling array

int foo() { // some language, not Jack int bar[10];

...

bar[2] = 19;

}

Handling array

Alternative

push local 0

pop pointer 1

push constant 19

pop that 2

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Handling objects

Class Ball { // some language, not Jack int x, y, radius, color;

int SetR(int r) {radius = r;}

} Ball b;

b.SetR(10);

Handling objects

Lecture plan

Summary: Hack VM has the following instructions and eight memory segments.

Method: (a) specify the abstraction (stack, memory segments, commands) (b) how to implement the abstraction over the Hack platform.

Arithmetic / Boolean commands add

sub neg eq gt lt and or not

Memory access commands pop x (pop into x, which is a variable) push y (y being a variable or a constant)

Program flow commands label (declaration) goto (label) if‐goto (label)

Function calling commands function (declaration) call (a function) return (from a function)

Chapter 7 Chapter 8

Implementation of VM on Hack

 Each VM instruction must be translated into a set of Hack assembly code

 VM segments need to be realized on the host memory

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Implementation

VM implementation options:

 Emulator-based (e.g. emulate the VM model using Java)

 Translator-based (e. g. translate VM programs into the Hack machine language)

 Hardware-based (realize the VM model using dedicated memory and registers)

Two well-known translator-based implementations:

JVM: Javac translates Java programs into bytecode;

The JVM translates the bytecode into the machine language of the host computer

CLR: C# compiler translates C# programs into IL code;

The CLR translated the IL code into the machine language of the host computer.

Software implementation: VM emulator

(part of the course software suite)

VM implementation on the Hack platform (memory)

The stack:a global data structure, used to save and restore the resources of all the VM functions up the calling hierarchy.

The tip of this stack if the working stack of the current function

static, constant, temp, pointer:

Global memory segments, all functions see the same four segments

local,argument,this,that:

these segments are local at the function level;

each function sees its own, private copy of each one of these four segments

The challenge:

represent all these logical constructs on the same single physical address space -- the host RAM.

Host RAM

VM implementation on the Hack platform (memory)

Basic idea: the mapping of the stack and the global segments on the RAM is easy (fixed);

the mapping of the function-level segments is dynamic, using pointers

The stack:mapped on RAM[256 ... 2047];

The stack pointer is kept in RAM address SP static:mapped on RAM[16 ... 255];

each segment reference statici appearing in a VM file named f is compiled to the assembly language symbol f.i (recall that the assembler further maps such symbols to the RAM, from address 16 onward)

local,argument,this,that:these method-level segments are mapped somewhere from address 2048onward, in an area called “heap”. The base addresses of these segments are kept in RAM addresses LCL, ARG, THIS, and THAT. Access to the i-th entry of any of these segments is Statics

3

12

. . .

4 5

14 15 0 1

13 2 THIS THAT SP LCL ARG

TEMP

255

. . .

16 General purpose

. . .²⁵⁶

Host

RAM

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VM implementation on the Hack platform (memory)

Statics 3

12

. . .

4 5

14 15 0

1

13 2 THIS THAT SP

LCL ARG

TEMP

255

. . .

16 General purpose

2047

. . .²⁵⁶

2048

Stack

. . . Heap

Host RAM

Practice exercises

Now that we know how the memory segments are mapped on the host RAM, we can write Hack commands that realize the various VM commands.

for example, let us write the Hack code that implements the following VM commands:

push constant 1

pop static 7 (suppose it appears in a VM file named f)

push constant 5

add

pop local 2

eq Tips:

1.The implementation of any one of these VM commands requires several Hack assembly commands involving pointer arithmetic (using commands like ^A=M)

2.If you run out of registers (you have only two ...), you may use R13, R14, and R15.

VM implementation on the Hack platform (translator)

push constant 1

@1 D=A

@SP A=M M=D

@SP M=M+1

add

@SP AM=M‐1 D+M A=A‐1 M=M+D

pop local 2

@LCL D=M

@2 D=D+A

@R15 M=D

@SP AM=M‐1 D=M

@R15 A=M M=D

Perspective

 In this lecture we began the process of building a compiler

 Modern compiler architecture:

 Front-end (translates from a high-level language to a VM language)

 Back-end (translates from the VM language to the machine language of some target hardware platform)

 Brief history of virtual machines:

 1970’s: p-Code

 1990’s: Java’s JVM

 2000’s: Microsoft .NET

 A full blown VM implementation typically also includes a common software library (can be viewed as a mini, portable OS).

 We will build such a mini OS later in the course.

. . . VM language

RISC machine

language Hack

CISC machine

language . . . a high-level^{written in}

language

. . .

VM implementation

over CISC platforms

VM imp.

over RISC

platforms emulator^VM Translator

Some Other

language Jack

compiler compiler

. . .

Some language. . .

The big picture

 JVM

 Java

 Java compiler

 JRE

 CLR

 C#

 C# compiler

 .NET base class library

 VM

 Jack

 Jack compiler

 Mini OS

 7, 8

 9

 10, 11

 12

(Book chapters and Course projects)