MOUSE-83

Contents

The Stack and Arithmetic

In standard mathematical notation, we write a mathematical operator between its two operands; for example:

   2 + 3

In RPN, we write the operator after the two operands:

   2 3 +

In computer implementations of RPN, each of the operators is stored on a stack before the operation takes place, and the operator operates on the numbers stored on the stack. The stack may be thought of as an array of storage registers, each of which can hold a single number. Placing a number on the stack bumps each of the other numbers further into the stack, and the last number placed on the stack is the first one taken off. By convention, the register on the "top" of the stack is called X; the next one is called Y; the next one Z; and the next one T.

The operation of the stack is perhaps best illustrated with an example. Suppose we wish to compute 2 + 3 using RPN and a stack. We would perform the following steps:

Step	Stack				Description
	X	Y	Z	T
2	2				Put 2 on the stack.
3	3	2			Put 3 on the stack;  2 is bumped up to Y.
+	5				Add the numbers in X and Y;  put the result in X.

More complex operations may be performed similarly. For example, suppose we wish to compute

   (2 + 3) * (4 + 6)

Here the * operator indicates multiplication. This calculation would be performed in RPN as follows:

   2 3 + 4 6 + *

The following table shows how the stack changes with each step:

Step	Stack				Description
	X	Y	Z	T
2	2				Put 2 on the stack.
3	3	2			Put 3 on the stack;  2 is bumped up to Y.
+	5				Add the numbers in X and Y;  put the result in X.
4	4	5			Put 4 on the stack;  5 is bumped to Y.
6	6	4	5		Put 6 on the stack;  4 is bumped up to Y;  5 is bumped to Z.
+	10	5			Add the numbers in X and Y;  put the result in X;  5 drops down to Y.
*	50				Multiply the numbers in X and Y;  put the result in X.

Notice that the + operator always adds the numbers in X and Y, leaves the result in X, and "drops" the stack so that each number moves down one. Similarly, the * operator multiplies the numbers in X and Y, leaves the result in X, and drops the stack.

Expressions of considerable complexity may be performed in RPN in this fashion, leaving intermediate results on the stack. Parentheses are never needed. With a little practice, you will find that RPN is a very efficient means of evaluating expressions.

Five mathematical operators are available in Mouse-83:

Operator	Description
+	Addition (add Y + X)
-	Subtraction (subtract Y - X)
*	Multiplication (multiply Y × X)
/	Division (divide Y ÷ X)
\	Remainder (of Y ÷ X)

Notice the order of operations for subtraction and division. For example, to perform

   (2 - 3)/(4 - 5)

in RPN, would would use the sequence

   2 3 - 4 5 - /

The Mouse-83 language operates only with integers. Consequently, the division operator (/) performs integer division: only the integer part of the quotient is returned, and any remainder is discarded. (The \ operator may be used to compute the remainder.)

Input and Output

Operator	Description
?	Input a number from the terminal
?'	Input a character from the terminal
!	Print a number to the terminal
!'	Print a character to the terminal
"	Print a string to the terminal

For example, to read a number from the terminal and print it to the terminal, we would use the sequence

   ? !

(Spaces are not significant in Mouse, so we could eliminate the space between the operators and write ?!.)

To be more specific, the ? operator reads a number (an integer) from the terminal, and pushes it onto the top of the stack (the X register); any other numbers already on the stack are "bumped" further into the stack. The ! operator prints the number in the X register to the terminal, then "drops" the stack.

The operators ?' and !'!' operator prints the character whose ASCII code is on the top of the stack.

To print the string "Hello" to the terminal, we would use

   "Hello"

The " operator will print all of the following characters to the terminal until it finds a matching ". If there is an exclamation mark (!) embedded in the string, a carriage return will be executed, and the following text will appear on the next line. For example,

   "Hello!Hello again"

will print

   Hello
   Hello again

Simple Programs

For example, a simple program to input a number from the terminal and print it at the terminal would be:

   ?!$

Within a mouse program, two numbers are separated by a space. For example, the following program adds 2 + 3:

   2 3 + $

The following program reads a number from the terminal, adds 3 to it, and prints the result to the terminal:

   ? 3 + ! $

Variables

When Mouse encounters a variable name in a program, is pushes that variable's address onto the stack. To print the address of variable D, for example, we could use the program

   D ! $$

The addresses of variables A through Z are the integers 0 through 25, respectively, so this program would print 3 for the address of D.

To store a number into a variable, you use the : (colon) operator. The : operator takes the number in the stack's Y register, and stores it into the variable whose address is in the X register. For example, we could write a program to store a 37 into variable D as follows:

   37 D: $

This program works as follows:

Step	Stack				Description
	X	Y	Z	T
37	37				Put 37 on the stack;  address of D is bumped up to Y.
D	3	37			Put address of D on the stack.
:					Store 37 into D.
$					End of program.

To recall a number from a variable, you use the . (period) operator. The . operator takes the address of a variable from the X register, and returns the number stored in that variable. The result replaces the address in the X register.

For example, the sequence

   D. !

Prints the value stored in variable D to the terminal.

The following program stores a 23 into variable G, then recalls the value from G and prints it to the terminal:

   23 G: G. ! $

Here's a step-by-step stack diagram for this program:

Step	Stack				Description
	X	Y	Z	T
23	23				Put 23 on the stack;  address of G is bumped up to Y.
G	6	23			Put address of G on the stack.
:					Store 23 into G.
G	6				Put address of G on the stack.
.	23				Fetch contents of G.
!					Print contents of G.
$					End of program.

Conditional Statements

   B [ S ]

is equivalent to: if B then S , where B is a boolean value (0 or 1), and S is a series of statements.

Descrition in detail. When Mouse sees a [ operator, it executes the code between the [ and the matching ] only if the number in the X register is greater than zero. If X is less than or equal to zero, the code between [ and ] is skipped. The [ operator drops the stack in either case. For example, the following sequence adds 3 to variable A if variable E is greater than zero:

   A. E. [ 3 + ]

Here's a step-by-step stack diagram for this sequence, assuming A=7 and E=1:

Step	Stack				Description
	X	Y	Z	T
A	0				Put address of A on the stack.
.	7				Fetch contents of A.
E	4	7			Put address of E on the stack.
.	1	7			Fetch contents of E.
[	7				Test X register and drop stack.
3	3	7			Condition true; put 3 on stack.
+	10				Add X + Y.
]	10				End of conditional.

But if E=0, we get the following:

Step	Stack				Description
	X	Y	Z	T
A	0				Put address of A on the stack.
.	7				Fetch contents of A.
E	4	7			Put address of E on the stack.
.	0	7			Fetch contents of E.
[	7				Test X register and drop stack;  X was false, so skip to ].
]	7				End of conditional.

Loops

A simple infinite loop has the form

   ( S )

where S is a statement or series of statements.

The carat ( ^ ) operator is used to break out of a loop. If the boolean value on the top of the stack is false (0), the ^ operator will break out of the loop and continue execution following the right parenthesis. The carat operator may be used to construct a "while" loop. The Mouse-83 statement

   ( B ^ S )

is equivalent to: while B do S , where B is a boolean value (0 or 1), and S is a series of statements.

Similarly, a "repeat" loop may be constructed in the form

   ( S B ^ )

This is equivalent to: repeat S until (not B) .

See the section on sample programs for examples on the use of loops in Mouse.

Macros

To call the macro, the main program uses the # operator. For example, the macro A may be called from the main program using the instructions

   #A;

Here the # operator marks a call to macro A, and the semicolon marks the end of the macro call.

A macro is defined using the $ operator. A simple macro (named A in this example) has the form

   $A <instructions> @

Here the $ operator defines the start of macro A and the @ operator returns exectution from the macro back to the main program.

A program may also pass parameters to a macro. For example, suppose we wish to write a macro B that will add two numbers together. To add 3 and 5, we call the macro using

   #B,3,5;

A comma is used to separate the macro name from its parameters, and to separate the parameters.

The % operator is used to define parameters in a macro. In this example, macro B may be defined using

   $B 1% 2% + @

Here 1% and 2% are dummy variables that hold the values of the parameters. In this example, 1% will be assigned the value 3, and 2% will be 5.

Common Idioms

X:           ~ store into variable X
X.           ~ recall variable X
X. Y:        ~ assign X to Y
N. 1 + N:    ~ increment N by 1
P. Q. P: Q:  ~ swap values of P and Q
? A:         ~ input a number and store in A
P. !         ~ print variable P

Sample Programs

"Hello world."
$

This program displays the squares of the integers from 1 to 10.

1 N:              ~ initialize N to 1
( N. N. * ! " "   ~ begin loop; print squares of numbers
  N. 10 - 0 < ^   ~ exit loop if N >= 10
  N. 1 + N: ) $   ~ increment N and repeat loop

Bibliography

1. Grogono, Peter. "Mouse: A Language for Microcomputers", Byte Magazine, July 1979, pp. 198 ff.
2. Grogono, Peter. Mouse: A Language for Microcomputers. Petrocelli Books, New York, 1983.

Web sites

1. Grogono, Peter.   The Mouse Programming Language
2. Fuller, Sean.   The Great Mouse Programming Language Revival
3. Hunt, Tom.   Friends of the Mouse
4. Bradley, Lee.   Mouse, the Language
5. Wikipedia article

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