www.nand2tetris.org
Building a Modern Computer From First Principles
High-Level Language
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 2
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
Some milestones in the evolution of programming languages
Machine language (binary code)
Assembly language (low-level symbolic programming)
Simple procedural languages, e.g. Fortran, Basic, Pascal, C
Simple object-based languages (without inheritance), e.g. early versions of Visual Basic, JavaScript
Fancy object-oriented languages (with inheritance):
C++, Java, C#
Jack
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 4
Programming languages
Procedural programming (e.g. C, Fortran, Pascal)
Object-oriented programming (e.g. C++, Java, Python)
Functional programming (e.g. Lisp, ML, Haskell)
Logic programming (e.g. Prolog)
ML
fun fac(x) = if x=0 then 1 else x*fac(x-1);
fun length(L) =
if (L=nil) then 0
else 1+length(tl(L));
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 6
Prolog
Facts
human(kate).
human(bill).
likes(bill,kate).
likes(kate,john).
likes(john,kate).
Rules
friend(X,Y) :- likes(X,Y),likes(Y,X).
Prolog
Absolute value
abs(X, X) :- X>=0, !.
abs(X, Y) :- Y is –X.
?- abs(-9,R).
R=9
?- abs(-9,8).
no
Length of a list my_length([], 0).
my_length([_|T],R) :- my_length(T, R1), R is R1+1.
?- my_length([a, b, [c, d], e], R).
R = 4
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 8
Programming languages
Procedure oriented programming
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 10
Object oriented programming
message
The Jack programming language
Jack: a simple, object-based, high-level language with a Java-like syntax Some sample applications written in Jack:
procedural programming
Pong game
Space Invaders
Tetris
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 12
Disclaimer
Although Jack is a real programming language, we don’t view it as an end Rather, we use Jack as a means for teaching:
How to build a compiler
How the compiler and the language interface with the operating system
How the topmost piece in the software hierarchy fits into the big picture
Jack can be learned (and un-learned) in one hour.
Hello world
/** Hello World program. */
class Main {
function void main () {
// Prints some text using the standard library do Output.printString("Hello World");
do Output.println(); // New line return;
} }
Some observations:
Java-like syntax
Typical comments format
Standard library
Language-specific peculiarities.
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 14
Jack standard library aka language extensions aka Jack OS
class Math {
function void init() function int abs(int x)
function int multiply(int x, int y) function int divide(int x, int y) function int min(int x, int y) function int max(int x, int y) function int sqrt(int x)
}
Class String {
constructor String new(int maxLength) method void dispose()
method int length()
method char charAt(int j)
method void setCharAt(int j, char c) method String appendChar(char c)
method void eraseLastChar() method int intValue()
method void setInt(int j) function char backSpace() function char doubleQuote() function char newLine() }
Class Array {
function Array new(int size) method void dispose()
}
class Output {
function void moveCursor(int i, int j) function void printChar(char c)
function void printString(String s) function void printInt(int i)
function void println() function void backSpace() }
Class Screen {
function void clearScreen()
function void setColor(boolean b)
function void drawPixel(int x, int y) function void drawLine(int x1, int y1,
int x2, int y2)
function void drawRectangle(int x1, int y1, int x2, int y2) function void drawCircle(int x, int y, int r) }
class Memory {
function int peek(int address)
function void poke(int address, int value) function Array alloc(int size)
function void deAlloc(Array o) }
Class Keyboard {
function char keyPressed() function char readChar()
function String readLine(String message) function int readInt(String message) }
Class Sys {
function void halt():
function void error(int errorCode) function void wait(int duration) }
Typical programming tasks in Jack
Jack can be used to develop any app that comes to my mind, for example:
Procedural programming: a program that computes 1 + 2 + ... + n
Object-oriented programming: a class representing bank accounts
Abstract data type representation: a class representing fractions (like 2/5)
Data structure representation: a class representing linked lists
Etc.
We will now discuss the above app examples
As we do so, we’ll begin to unravel how the magic of a high-level object-based language is delivered by the compiler and by the VM
These insights will serve us in the next lectures, when we build the Jack compiler.
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 16
Procedural programming example
Jack program = a collection of
one or more classes Jack class = a collection of
one or more subroutines Execution order: when we execute a Jack program, Main.main() starts running.
Jack subroutine:
method
constructor
function (static method)
(the example on the left has
functions only, as it is “object-less”)
Standard library: a set of OS services
(methods and functions)
organized in 8 supplied classes:
Math,
String.
Array,
Output
,
Keyboard,
Screen,
Memory,
Sys (OS API in the book).class Main {
/** Sums up 1 + 2 + 3 + ... + n */
function int sum (int n) { var int sum, i;
let sum = 0;
let i = 1;
while (~(i > n)) { let sum = sum + i;
let i = i + 1;
}
return sum;
}
function void main () { var int n;
let n = Keyboard.readInt("Enter n: ");
do Output.printString("The result is: ");
do Output.printInt(sum(n));
return;
} }
Object-oriented programming example
/** Represents a bank account.
A bank account has an owner, an id, and a balance.
The id values start at 0 and increment by 1 each time a new account is created. */
class BankAccount {
/** Constructs a new bank account with a 0 balance. */
constructor BankAccount new(String owner)
/** Deposits the given amount in this account. */
method void deposit(int amount)
/** Withdraws the given amount from this account. */
method void withdraw(int amount)
/** Prints the data of this account. */
method void printInfo()
/** Disposes this account. */
method void dispose() }
The
BankAccountclass (skeletal)
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 18
Object-oriented programming example (continues)
/** Represents a bank account. */
class BankAccount {
// class‐level variable static int newAcctId;
// Private variables (aka fields / properties) field int id;
field String owner;
field int balance;
/** Constructs a new bank account */
constructor BankAccount new (String owner) { let id = newAcctId;
let newAcctId = newAcctId + 1;
let this.owner = owner;
let balance = 0;
return this;
}
// More BankAccount methods.
}
// Code in any other class:
var int x;
var BankAccount b;
let b = BankAccount.new("joe");
Explain
return thisThe constructor returns the RAM base address of the memory block that stores the data of the newly created BankAccount object
Explain
b = BankAccount.new("joe") Calls the constructor (which creates a new BankAccount object), then stores in variable b a pointer to the object’s base memory addressBehind the scene
(following compilation):// b = BankAccount.new("joe") push "joe"
call BankAccount.new pop b
Explanation: the calling code pushes an argument and calls the constructor; the constructor’s code (not shown above) creates a new object, pushes its base address onto the stack, and returns;
The calling code then pops the base address into a variable that will now point to the new object.
2
3
1
class BankAccount {
static int nAccounts;
field int id;
field String owner;
field int balance;
// Constructor ... (omitted) /** Handles deposits */
method void deposit (int amount) { let balance = balance + amount;
return;
}
/** Handles withdrawls */
method void withdraw (int amount){
if (~(amount > balance)) {
let balance = balance ‐ amount;
}
return;
}
// More BankAccount methods.
}
...
var BankAccount b1, b2;
...
let b1 = BankAccount.new("joe");
let b2 = BankAccount.new("jane");
do b1.deposit(5000);
do b1.withdraw(1000);
...
Object-oriented programming example (continues)
Explain
do b1.deposit(5000)
In Jack,
voidmethods are invoked using the keyword
do (a compilation artifact)
The object-oriented method invocation style
b1.deposit(5000)is a fancy way to express the procedural semantics
deposit(b1,5000)
Behind the scene
(following compilation):// do b1.deposit(5000) push b1
push 5000
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 20
Object-oriented programming example (continues)
class BankAccount { static int nAccounts;
field int id;
field String owner;
field int balance;
// Constructor ... (omitted)
/** Prints information about this account. */
method void printInfo () { do Output.printInt(id);
do Output.printString(owner);
do Output.printInt(balance);
return;
}
/** Disposes this account. */
method void dispose () { do Memory.deAlloc(this);
return;
}
// More BankAccount methods.
}
// Code in any other class:
...
var int x;
var BankAccount b;
let b = BankAccount.new("joe");
// Manipulates b...
do b.printInfo();
do b.dispose();
...
Explain
do b.dispose()Jack has no garbage collection;
The programmer is responsible for explicitly recycling memory
resources of objects that are no longer needed. If you don’t do so, you may run out of memory.
Explain
do Memory.deAlloc(this) This is a call to an OS function that knows how to recycle the memory block whose base-address is this. We will write this function when we develop the OS (project 12).Abstract data type example
/** Represents a fraction data type.
A fraction consists of a numerator and a denominator, both int values */
class Fraction {
/** Constructs a fraction from the given data */
constructor Fraction new(int numerator, int denominator) /** Reduces this fraction, e.g. changes 20/100 to 1/5. */
method void reduce() /** Accessors
method int getNumerator() method int getDenominator()
/** Returns the sum of this fraction and the other one */
method Fraction plus(Fraction other)
/** Returns the product of this fraction and the other one */
method Fraction product(Fraction other) /** Prints this fraction */
method void print()
/** Disposes this fraction */
method void dispose()
The Fraction class API (method sigantures)
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 22
Abstract data type example (continues)
/** Represents a fraction data type.
A fraction consists of a numerator and a denominator, both int values */
class Fraction {
field int numerator, denominator;
constructor Fraction new (int numerator, int denominator) { let this.numerator = numerator;
let this.denominator = denominator;
do reduce() // Reduces the new fraction return this
}
/** Reduces this fraction */
method void reduce () { // Code omitted }
// A static method that computes the greatest common denominator of a and b.
function int gcd (int a, int b) { // Code omitted
}
method int getNumerator () { return numerator;
}
method int getDenominator () { return denominator;
}
// More Fraction methods follow.
// Code in any other class:
...
var Fraction a, b;
let a = Fraction.new(2,5);
let b = Fraction.new(70,210);
do b.print() // prints "1/3"
...
// (print method in next slide)
Abstract data type example (continues)
/** Represents a fraction data type.
A fraction consists of a numerator and a denominator, both int values */
class Fraction {
field int numerator, denominator;
// Constructor and previously defined methods omitted /** Returns the sum of this fraction the other one */
method Fraction plus (Fraction other) { var int sum;
let sum = (numerator * other.getDenominator()) +
(other.getNumerator() * denominator());
return Fraction.new(sum , denominator * other.getDenominator());
}
// Similar fraction arithmetic methods follow, code omitted.
/** Prints this fraction */
method void print () {
do Output.printInt(numerator);
do Output.printString("/");
do Output.printInt(denominator);
return }
}
// Code in any other class:
var Fraction a, b, c;
let a = Fraction.new(2,3);
let b = Fraction.new(1,5);
// computes c = a + b let c = a.plus(b);
do c.print(); // prints "13/15"
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 24
Data structure example
/** Represents a sequence of int values, implemented as a linked list.
The list consists of an atom, which is an int value, and a tail, which is either a list or a null value. */
class List {
field int data;
field List next;
/* Creates a new list */
constructor List new (int car, List cdr) { let data = car;
let next = cdr;
return this;
}
/* Disposes this list by recursively disposing its tail. */
method void dispose() { if (~(next = null)) {
do next.dispose();
}
do Memory.deAlloc(this);
return;
} ...
} // class List.
// Code in any other class:
...
// Creates a list holding the numbers 2,3, and 5:
var List v;
let v = List.new(5 , null);
let v = List.new(2 , List.new(3,v));
...
v 5
3 5
v 2
Jack language specification
Syntax
Data types
Variable kinds
Expressions
Statements
Subroutine calling
Program structure
Standard library
(for complete language specification, see the book).
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 26
Jack syntax
Jack syntax (continues)
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 28
Jack data types
Primitive types
(Part of the language; Realized by the compiler):
int 16-bit 2’s complement (from ‐32768 to 32767)
boolean 0 and –1, standing for true and false
char unicode character (‘a’, ‘x’, ‘+’, ‘%’, ...)
Abstract data types
(Standard language extensions; Realized by the OS / standard library):
String
Array
...
(extensible)Application-specific types
(User-defined; Realized by user applications):
BankAccount
Fraction
List
Bat / Ball
. . .
(as needed)Jack variable kinds and scope
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 30
Jack expressions
A Jack expression is any one of the following:
A constant
A variable name in scope
(the variable may be static, field, local, or a parameter)
The keyword this, denoting the current object
An array element using the syntax arrayName[expression], where arrayNname is a variable name of type
Arrayin scope
A subroutine call that returns a non-void type
An expression prefixed by one of the unary operators – or ~ :
‐expression
(arithmetic negation)
~expression
(logical negation)
An expression of the form expression op expression where op is one of the following:
+ ‐ * /
(integer arithmetic operators)
& |
(booleanand
andor
operators, bit-wise)
< > =
(comparison operators)
( expression )
(an expression within parentheses)Jack Statements
let
varName
=expression
; orlet
varName
[expression
] =expression
;if (
expression
) {statements
}
else {
statements
}
while (
expression
) {statements
}
do
function-or-method-call
;return
expression
; orElements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 32
Jack subroutine calls
General syntax: subroutineName
(arg0
,arg1
,…
)where each argument is a valid Jack expression
Parameter passing is by-value
(primitive types)or by-reference
(object types)Example 1:
Consider the function
(static method):
function int sqrt(int n)This function can be invoked as follows:
sqrt(17) sqrt(x)
sqrt((b * b) – (4 * a * c)) sqrt(a * sqrt(c ‐ 17) + 3)
Etc. In all these examples the argument value is computed and passed by-value Example 2:
Consider the method:
method Matrix plus (Matrix other);If
uand
vwere variables of type
Matrix, this method can be invoked using:
u.plus(v)The
vvariable is passed by-reference, since it refers to an object.
Noteworthy features of the Jack language
The
(cumbersome) letkeyword, as in
let x = 0;
The
(cumbersome) dokeyword, as in
do reduce();
No operator priority:
1 + 2 * 3
yields
9, since expressions are evaluated left-to-right;
To effect the commonly expected result, use
1 + (2 * 3)
Only three primitive data types:
int,
boolean,
char; In fact, each one of them is treated as a 16-bit value
No casting; a value of any type can be assigned to a variable of any type
Array declaration:
Array x;followed by
x = Array.new();
Static methods are called
function
Constructor methods are called
constructor;
Invoking a constructor is done using the syntax ClassName
.new(argsList
)Q: Why did we introduce these features into the Jack language?
A: To make the writing of the Jack compiler easy!
Any one of these language features can be modified, with a reasonable amount of work,
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 34
Jack program structure
A Jack program:
Each class is written in a
separate file (compilation unit)
Jack program = collection of one or more classes, one of which must be named
Main
The
Mainclass must contain at least one method, named
main() classClassName
{field
variable declarations;
static
variable declarations;
constructor
type
{parameterList
) {local variable declarations;
statements
}
method
type
{parameterList
) {local variable declarations;
statements
}
function
type
{parameterList
) {local variable declarations;
statements
} }
About this spec:
Every part in this spec can appear 0 or more times
The order of the
field/
staticdeclarations is arbitrary
The order of the subroutine declarations is arbitrary
Each type is either
int,
boolean,
char,
or a class name.
A simple game: square
(Demo)
Use Square as an example.
Design a class: think of its
States: data members
Behaviors: function members
Square
x, y, size
MoveUp, MoveDown, IncSize, …
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 36
Perspective
Jack is an object-based language: no inheritance
Primitive type system (3 types)
Standard library
Our hidden agenda: gearing up to learn how to develop the ...
Compiler
(projects 10 and 11)
OS
(project 12).Principles of object-oriented programming
encapsulation (information hiding) polymorphism
inheritance
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 38
Programming languages
Most popular PLs (2014/4)
Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 9: High-Level Language slide 40