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Overview

This document is a complete specification for the VVhitespace language.

Since Whitespace and VVhitespace are closely related, this document is heavily copied/adapted from the "Whitespace Tutorial" published by e.c.brady@dur.ac.uk.

Reference

The only lexical tokens in the VVhitespace language are

  • Tab (ASCII 9),
  • Line Feed (ASCII 10),
  • Vertical Tab (ASCII 11), and
  • Space (ASCII 32).

The language itself is an imperative, stack based language. Each command consists of a series of tokens, beginning with the Instruction Modification Parameter (IMP). These are listed in the table below.

IMP Meaning
[Space] Stack Manipulation
[Tab][Space] Arithmetic
[Tab][Tab] Heap access
[LF] Flow Control
[Tab][LF] I/O

The virtual machine on which programs run has a stack and a heap, both of fixed, implementation defined size and both designed around a 64-bit word. The programmer may push and pop words from the stack in addition to accessing the heap on a per-word basis as a permanent store of variables and data structures.

Many commands require numbers or labels as parameters.

Numbers are integers ranging from -(2^63) to +(2^63)-1 and are represented in sign-magnitude format as a sign, either [Space] for positive or [Tab] for negative, followed by a series of [Space] (zero) and [Tab] (one) digits terminated by a [LF]. Note that positive and negative zero are considered equivalent. As an example, the decimal number +42 has the following representation.

[Space] [Tab][Space][Tab][Space][Tab][Space] [LF]

Labels consist of an [LF] terminated list of up to sixteen spaces and tabs. The program must not begin with a label. There is only one global namespace so all labels must be unique. Labels are left-padded with [Space] up to sixteen characters; the following two labels are interchangeable.

       [Tab][Space][Tab] [LF]
[Space][Tab][Space][Tab] [LF]

Stack Manipulation (IMP: [Space])

Stack manipulation is one of the more common operations, hence the shortness of the IMP [Space]. There are four stack instructions.

Command Params Meaning
[Space] Number Push the number onto the stack
[LF][Space] --- Duplicate the top item on the stack
[LF][Tab] --- Swap the top two items on the stack
[LF][LF] --- Discard the top item on the stack

Arithmetic (IMP: [Tab][Space])

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 left of the operator. The modulo command will always return a positive result.

Command Params Meaning
[Space][Space] --- Addition
[Space][Tab] --- Subtraction
[Space][LF] --- Multiplication
[Tab][Space] --- Integer Division
[Tab][Tab] --- Modulo

Heap Access (IMP: [Tab][Tab])

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.

Command Params Meaning
[Space] --- Store
[Tab] --- Retrieve

Flow Control (IMP: [LF])

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.

Command Params Meaning
[Space][Space][VTab] Label Mark a location in the program
[Space][Tab] Label Call a subroutine
[Space][LF] Label Jump unconditionally to a label
[Tab][Space] Label Jump to label if TOS is zero
[Tab][Tab] Label Jump to label if TOS is negative
[Tab][LF] --- Return from subroutine
[LF][LF] --- End the program

I/O (IMP: [Tab][LF])

Finally, we need to be able to interact with the user. There are IO instructions for reading and writing numbers and individual characters.

The read instructions take the heap address in which to store the result from the top of the stack.

Command Params Meaning
[Space][Space] --- Output the character at the TOS
[Space][Tab] --- Output the number at the TOS
[Tab][Space] --- Read character and store at location given by TOS
[Tab][Tab] --- Read number and store at location given by TOS