L756 Language Manual

An L756 program is a sequence of (possibly labeled) statements. Normally, statements are executed in their order of appearance in the sequence unless a transfer of control is specified by a GOTO statement, an IF statement, or a FOR statement.

Any number of blanks can appear between adjacent tokens, and no blank may separate adjacent characters of a token. Line break characters are treated as space characters.

Available Features

Numbers

A number is a sequence of decimal digits such as: 1235 and 22

Variables/Labels

A variable is denoted by a string of letters and digits whose first character is a letter. Strings which agree on their first seven characters are assumed to denote the same variable. Labels are similarly defined but they must be followed by ”:”.

Example. I, I1AB3, and Z9:

Each variable must be defined at the start of the program using a DCL instruction of the form ‘DCL <id>’.

Arithmetic Expressions

Arithmetic expressions are formulas for computing a value. They are made up using ṉumbers, variables, the four arithmetic operators +, -, *, /, and the left and right parentheses.

Example. (of general expressions)

A1 + B * (C + D * (C + E))
A1 + 3114159 / 4 * (C + 12)

The order in which operations are performed is normally from left to right except that

Operations within parentheses are carried out before the parenthesized quantity is use din further computations.
Multiplications and divisions are done before additions or subtractions.

Example. (order of operations is indicated below the expression)

A + (B /(C + D)) * (F * G * (H + B)) + C
7 2 1 6 3 5 4 8

Assignment Statements

The general form of an Assignment statement is

LET <variable> = <arithmetic expression>

The value of the arithmetic expression is computed and assigned to the variable.

Example. : LET X1 = y1 + 1206 * Z + XL

GOTO Statements

The general form of a GOTO statement is ‘GOTO <id>’. Control is transferred to the statement with the specified label.

Example. : GOTO L5

Conditional Statements

The general form of a Conditional Statement is:

IF <arithmetic expression><relational operator>
<arithmetic expression> THEN GOTO <label>

where the relational operators are =, <>, <, <=, >, and >=.

If the relation holds between the values of the two arithmetic expressions then the control is transferred to the specified label. Otherwise, control passes to the code that appear after the instruction.

IF X > y THEN GOTO L2
IF Z + (X*y) = X1 + 12i THEN GOTO L5

FOR Statements

FOR statements are used to set up program loops. There are two general forms of FOR Statements:

   FOR <variable> = <arithmetic expression 1>
       STEP<arithmetic expression 3> TO
           <arithmetic expression 2>
   DO ... statements ... NEXT

   FOR <variable> = <arithmetic expression 1> TO
           <arithmetic expression 2>
   DO ... statements ... NEXT

The second form is interpreted in the same way as the first except that all the step size (arithmetic expression 3) is assumed equal to 1. The FOR statement forms the entrance of a program loop that continues through any number of subsequent statements until a NEXT to close the loop is reached.

Example.

   FOR X = 1 STEP 5 TO 100 DO
       LET W = X + y + Z
       LET Z = X * y
   NEXT

Informally, the statements in the loop between the FOR and the NEXT are executed over and over again a number of times as specified by the FOR statement. Each time around the loop the value of the variable is incremented by the step and compared with the final value to determine whether to enter the loop again or to terminate the FOR statement.

Specifically, the execution of the FOR Statement is as follows:

Arithmetic expression 1 (the initial value ), arithmetic expression 2 (the final value ), and arithmetic expression 3 (the step size ) are all computed.
The initial value is assigned to the variable.
A test is made, depending on the value of the step size.
- If the step size is greater than or equal to zero, and the value of the variable is less than or equal to the final value, control passes to the statement following the FOR statement (i.e., the loop is entered). Otherwise, control passes to the statement following the associated NEXT (i.e., the loop is not entered and the FOR statement is completed).
- If the step size is negative, and the value of the variable is greater than or equal to the final value, control passes to the statement following the FOR statement (i.e., the loop is entered). Otherwise, control passes to the statement following the associated NEXT (i.e., the loop is not entered and the FOR statement is completed).
If the loop is entered, the statements between the FOR statement and the associated NEXT are executed.
When NEXT is reached, the value of the step size is added to the current value of the variable and the result assigned to the variable. The procedure then continues at step (c). Note the initial value, final value, and step size are computed only once, at the beginning, not every time around the loop. However, the value of the controlled variable can be changed by the statements within the loop and the latest value is always used for the incrementing and testing steps.
It is allowable to have loops within loops, but they must be properly nested.

       FOR X = 1 TO N
       ...
       FOR y = 1 TO N
             ...
             NEXT
             ...
             NEXT

Input/Output Statements

READ <identifier>
WRITE <expression>

END Statements

Every program must have a single END statement – the last (highest-numbered) statement in the program.

Comments

Comments can be added to a program using the REM statement, the general form of which is

REM <any sequence of characters but;>;

The compiler ignores the input stream between (and including) the REM and its first following semicolon.

The Syntax of the L756 Language

1.	<program>	$\to$	<dcl> <body> END
2.	<dcl>	$\to$	DCL <id> <dcl>
3.		$\to$	$ϵ$
4.	<body>	$\to$	<stat> <body>
5.		$\to$	$ϵ$
6.	<stat>	$\to$	<stat $_{1}$ >
7.		$\to$	<label> <stat $_{1}$ >
8.	<label>	$\to$	<id> :
9.	<stat $_{1}$ >	$\to$	LET <id> = <expr>
10.		$\to$	GOTO <id>
11.		$\to$	IF <expr> <relop> <expr> THEN GOTO <id>
12.		$\to$	<for> <body> NEXT
13.		$\to$	READ <id>
14.		$\to$	WRITE <expr>
15.	<for>	$\to$	FOR <id> = <expr> <step> TO <expr> DO
16.	<step>	$\to$	STEP <expr>
17.		$\to$	$ϵ$
18.	<relop>	$\to$	<relop $_{1}$ >
19.		$\to$	=
20.	<expr>	$\to$	<expr $_{1}$ >
21.	<expr $_{1}$ >	$\to$	<expr $_{1}$ > <+-> <term>
22.		$\to$	<term>
23.	<term>	$\to$	<term> <*/> <oprnd>
24.		$\to$	<oprnd>
25.	<oprnd>	$\to$	<number>
26.		$\to$	<id>
27.		$\to$	( <expr $_{1}$ > )

Note:

<

relop

_{1}

>

stands for the class of relation operations:

>, <, < >, > =, < = .