Jack Reference

Syntax Reference

Keywords

class	method	function	constructor
int	boolean	char	void
var	static	field
let	do	if	else	while	return
true	false	null	this

Symbols

{	}	(	)	[	]
.	,	;	+	-	*	/
&	\|	<	>	=	~

Integer Constants

A decimal number in the range 0..32767

String Constant

A sequence of unicode characters not including double quote or newline, surrounded by double quotes

Identifiers

A sequence of letters, digits and underscore, not starting with a digit

The Jack OS API

The Jack language comes with a collection of eight built-in classes that extend the language's capabilities. This standard library, which can also be viewed as a basic operating system, includes the following classes, all implemented in Jack:

Math

This class enables various mathematical operations.

function int abs(int x)	returns the absolute value of x.
function int multiply(int x, int y)	returns the product of x and y.
function int divide(int x, int y)	returns the integer part of x/y.
function int min(int x, int y)	returns the minimum of x and y.
function int max(int x, int y)	returns the maximum of x and y.
function int sqrt(int x)	returns the integer part of the square root of x.

String

This class implements the String data type and various string-related operations.

constructor String new(int maxLength)	constructs a new empty string (of length zero) that can contain at most maxLength characters.
method void dispose()	disposes this string.
method int length()	returns the length of this string.
method char charAt(int j)	returns the character at location j of this string.
method void setCharAt(int j, char c)	sets the j-th element of this string to c.
method String appendChar(char c)	appends c to this string and returns this string.
method void eraseLastChar()	erases the last character from this string.
method int intValue()	returns the integer value of this string (or the string prefix until a non- digit character is detected).
method void setInt(int j)	sets this string to hold a representation of j.
function char backSpace()	returns the backspace character.
function char doubleQuote()	returns the double quote (") character.
function char newLine()	returns the newline character.

Array

This class enables the construction and disposal of arrays.

function Array new(int size)	constructs a new array of the given size.
method void dispose()	disposes this array.

Output

This class allows writing text on the screen.

function void moveCursor(int i, int j)	moves the cursor to the j-th column of the i-th row, and erases the character displayed there.
function void printChar(char c)	prints c at the cursor location and advances the cursor one column forward.
function void printString(String s)	prints s starting at the cursor location and advances the cursor appropriately.
function void printInt(int i)	prints i starting at the cursor location and advances the cursor appropriately.
function void println()	advances the cursor to the beginning of the next line.
function void backSpace()	moves the cursor one column back.

Screen

This class allows drawing graphics on the screen. Column indices start at 0 and are left-to-right. Row indices start at 0 and are top-to-bottom. The screen size is hardware-dependant (in the Hack platform: 256 rows by 512 columns).

function void clearScreen()	erases the entire screen.
function void setColor(boolean b)	sets a color (white=false, black=true) to be used for all further drawXXX commands.
function void drawPixel(int x, int y)	draws the (x,y) pixel.
function void drawLine(int x1, int y1, int x2, int y2)	draws a line from pixel (x1,y1) to pixel (x2,y2).
function void drawRectangle(int x1, int y1, int x2, int y2)	draws a filled rectangle whose top left corner is (x1, y1) and bottom right corner is (x2,y2).
function void drawCircle(int x, int y, int r)	draws a filled circle of radius r<=181 around (x,y).

Keyboard

This class allows reading inputs from a standard keyboard.

function char keyPressed()	returns the character of the currently pressed key on the keyboard; if no key is currently pressed, returns 0.
function char readChar()	waits until a key is pressed on the keyboard and released, then echoes the key to the screen and returns the character of the pressed key.
function String readLine(String message)	prints the message on the screen, reads the line (text until a newline character is detected) from the keyboard, echoes the line to the screen, and returns its value. This function also handles user backspaces.
function int readInt(String message)	prints the message on the screen, reads the line (text until a newline character is detected) from the keyboard, echoes the line to the screen, and returns its integer value (until the first non-digit character in the line is detected). This function also handles user backspaces.

Special Keys:

Key Pressed	Keyboard output
newline	128
backspace	129
left arrow	130
up arrow	131
right arrow	132
down arrow	133
home	134
end	135
page up	136
page down	137
insert	138
delete	139
escape	140
f1-f12	141-152

Memory

This class allows direct access to the main memory of the host platform.

function int peek(int address)	returns the value of the main memory at this address.
function void poke(int address, int value)	sets the contents of the main memory at this address to value.
function Array alloc(int size)	finds and allocates from the heap a memory block of the specified size and returns a reference to its base address.
function void deAlloc(Array o)	De-allocates the given object and frees its memory space.

Sys

This class supports some execution-related services.

function void halt()	halts the program execution.
function void error(int errorCode)	prints the error code on the screen and halts.
function void wait(int duration)	waits approximately duration milliseconds and returns.

CSE365

Elements of Computing Systems