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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML
4.01 Transitional//EN" "http://www.w3.org/TR/1999/REC-html401-19991224/loose.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> <title>Whitespace tutorial</title>
</head> <body> <h2>Whitespace tutorial</h2> <p> The
only lexical tokens in the whitespace language are <em>Space</em> (ASCII
32), <em>Tab</em> (ASCII 9) and <em>Line Feed</em> (ASCII 10).
By only allowing line feed as a token, CR/LF problems are
avoided across DOS/Unix file conversions. (<em>Um, not sure. Maybe
we'll sort this in a later version.</em>). </p> <p>
The language itself is an imperative, stack based language. Each
command consists of a series of tokens, beginning with the
<em>Instruction Modification Parameter</em> (IMP). These are listed in the table
below. </p> <table border> <tr><th>IMP</th><th>Meaning</th></tr> <tr><td>[Space]</td><td>Stack Manipulation</td></tr> <tr><td>[Tab][Space]</td><td>Arithmetic</td></tr> <tr><td>[Tab][Tab]</td><td>Heap
access</td></tr> <tr><td>[LF]</td><td>Flow Control</td></tr> <tr><td>[Tab][LF]</td><td>I/O</td></tr> </table> <p> The virtual
machine on which programs run has a stack and a heap.
The programmer is free to push arbitrary width integers onto
the stack (only integers, currently there is no implementation of
floating point or real numbers). The heap can also be accessed
by the user as a permanent store of variables and data
structures. </p> <p> Many commands require numbers or
labels as parameters. Numbers can be any number of bits
wide, and are simply represented as a series of [Space] and
[Tab], terminated by a [LF]. [Space] represents the binary digit
0, [Tab] represents 1. The sign of a number is given
by its first character, [Space] for positive and [Tab] for negative.
Note that this is <em>not</em> twos complement, it just indicates
a sign. </p> <p> Labels are simply [LF] terminated lists of
spaces and tabs. There is only one global namespace so
all labels must be unique. </p> <h3>Stack Manipulation (IMP: [Space])
</h3> <p> Stack manipulation is one of the more common
operations, hence the shortness of the IMP [Space]. There are
four stack instructions. </p> <table border> <tr><th>Command</th><th>Parameters</th><th>Meaning</th></tr> <tr><td>[Space]</td><td>Number</td><td>Push the number onto
the stack</td></tr> <tr><td>[LF][Space]</td><td>-</td><td>Duplicate the top item on the stack</td></tr> <tr><td>[LF][Tab]</td><td>-</td><td>Swap
the top two items on the stack</td></tr> <tr><td>[LF][LF]</td><td>-</td><td>Discard the top item on
the stack</td></tr> </table> <h3>Arithmetic
(IMP: [Tab][Space]) </h3> <p> Arithmetic commands operate on the top two items
on the stack, and replace them with the result
of the operation. The first item pushed is considered to be
<em>left</em> of the operator. </p> <table border> <tr><th>Command</th><th>Parameters</th><th>Meaning</th></tr> <tr><td>[Space][Space]</td><td>-</td><td>Addition</td></tr> <tr><td>[Space][Tab]</td><td>-</td><td>Subtraction</td></tr>
<tr><td>[Space][LF]</td><td>-</td><td>Multiplication</td></tr> <tr><td>[Tab][Space]</td><td>-</td><td>Integer Division</td></tr> <tr><td>[Tab][Tab]</td><td>-</td><td>Modulo</td></tr> </table> <h3>Heap Access (IMP: [Tab][Tab])</h3> <p> Heap
access commands look at the stack to find the address
of items to be stored or retrieved. To store an item,
push the address then the value and run the store
command. To retrieve an item, push the address and run the retrieve
command, which will place the
value stored in the location at the top
of
the stack. </p> <table border> <tr><th>Command</th><th>Parameters</th><th>Meaning</th></tr> <tr><td>[Space]</td><td>-</td><td>Store</td></tr> <tr><td>[Tab]</td><td>-</td><td>Retrieve</td></tr> </table> <h3>Flow Control (IMP: [LF])</h3> <p> Flow control operations are also common. Subroutines are marked by labels, as well as the targets of conditional and unconditional jumps, by which loops can be implemented. Programs
must be ended by means of [LF][LF][LF] so that the interpreter
can exit
cleanly. </p> <table border> <tr><th>Command</th><th>Parameters</th><th>Meaning</th></tr> <tr><td>[Space][Space]</td><td>Label</td><td>Mark a location in the program</td></tr> <tr><td>[Space][Tab]</td><td>Label</td><td>Call
a subroutine</td></tr> <tr><td>[Space][LF]</td><td>Label</td><td>Jump unconditionally
to a label</td></tr> <tr><td>[Tab][Space]</td><td>Label</td><td>Jump to a label if the top of the stack is zero</td></tr> <tr><td>[Tab][Tab]</td><td>Label</td><td>Jump to a label if the top of the stack is negative</td></tr> <tr><td>[Tab][LF]</td><td>-</td><td>End a subroutine and transfer control back to
the caller</td></tr> <tr><td>[LF][LF]</td><td>-</td><td>End the program</td></tr> </table> <h3>I/O (IMP: [Tab][LF])</h3>
<p> Finally, we need to
be able to interact with the user. There are IO
instructions
for reading and writing numbers and individual characters. With these, string manipulation routines can be written. </p> <p> The <em>read</em> instructions take the heap address in which to store the result from the top of the stack. </p> <table border> <tr><th>Command</th><th>Parameters</th><th>Meaning</th></tr>
<tr><td>[Space][Space]</td><td>-</td><td>Output the
character at the
top of the stack</td></tr> <tr><td>[Space][Tab]</td><td>-</td><td>Output the number at the top of the stack</td></tr> <tr><td>[Tab][Space]</td><td>-</td><td>Read a character and place it in the location given by the top of the stack</td></tr> <tr><td>[Tab][Tab]</td><td>-</td><td>Read a number and place it in the location given by the top of the stack</td></tr> </table> <h3>Annotated Example</h3> <p> Here is an annotated example of a program which
counts
from
1
to
10, outputting the current value as it goes. </p> <table border> <tr> <td>[Space][Space][Space][Tab][LF]</td><td> Put a 1 on the stack</td></tr> <tr> <td>[LF][Space][Space][Space][Tab][Space][Space] [Space][Space][Tab][Tab][LF] </td><td>Set a Label at this point</td></tr> <tr> <td>[Space][LF][Space]</td><td>Duplicate the top stack item</td></tr>
<tr> <td>[Tab][LF][Space][Tab]</td><td>Output
the current value</td></tr> <tr> <td>[Space][Space][Space][Tab][Space][Tab][Space][LF]</td> <td>Put
10 (newline) on the stack...</td></tr>
<tr> <td>[Tab][LF][Space][Space]</td><td>...and output the newline</td></tr> <tr> <td>[Space][Space][Space][Tab][LF]</td><td>Put a 1 on the stack</td></tr> <tr> <td>[Tab][Space][Space][Space]</td><td>Addition. This increments our current value.</td></tr> <tr> <td>[Space][LF][Space]</td><td>Duplicate that value so we can test it</td></tr> <tr> <td>[Space][Space][Space][Tab][Space][Tab][Tab][LF]</td> <td>Push 11 onto the stack</td></tr> <tr> <td>[Tab][Space][Space][Tab]</td><td>Subtraction. So if we've reached the end, we have a zero on the stack.</td></tr> <tr> <td>[LF][Tab][Space][Space][Tab][Space][Space] [Space][Tab][Space][Tab][LF]</td><td>If we have a zero, jump to the end </td></tr> <tr> <td>[LF][Space][LF][Space][Tab][Space] [Space][Space][Space][Tab][Tab][LF]</td> <td>Jump to
the start</td></tr>
<tr> <td> [LF][Space][Space][Space][Tab][Space] [Space][Space][Tab][Space][Tab][LF] </td><td>Set the
end label</td></tr> <tr> <td>[Space][LF][LF]</td><td>Discard our
accumulator, to be tidy</td></tr> <tr> <td>[LF][LF][LF]</td><td>Finish</td></tr> </table> <p> What could be simpler? The source code for this program is available <a href="../count.ws">here</a>. Have fun! </p> <hr> <p> <a href="mailto:e.c.brady@dur.ac.uk"><em>e.c.brady@dur.ac.uk</em></a> </p> </td>
</tr> </table> </body></html>
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