pTAL Guidelines for TAL Programmers

Coding Guidelines

pTAL Guidelines for TAL Programmers—527256-002

2-54

USE and DROP Statements

Guideline: Always assume that the value of a USE variable is initially undefined.

TAL programs can rely on the ways in which USE and DROP statements use TNS

registers. Within the DO-UNTIL loop in the following example, the program executes, in

succession, a DROP statement, a USE statement, a second DROP statement, and a

second USE statement.

Although Example 2-67 on page 2-54 uses two different names for the USE

variables—i and i_prime—both i and i_prime use the same hardware stack

In pTAL, the initial value of a variable named in a USE statement is undefined. pTAL

reports a warning message if you use a USE variable in an expression without first

storing a value in it.

pTAL supports the USE and DROP statements so that programs that need to allocate

a register when running as TNS processes can do so and still be source-code

compatible with pTAL; however, in general, pTAL allocates memory for and optimizes

variables declared in USE statements just as it does a variable declared INT.

RP Directive

Guideline: Avoid using code sequences that require you to use the RP directive.

pTAL does not support the RP directive.

Example 2-67. USE and DROP Statements

PROC p;

BEGIN

USE i_prime;

BEGIN

DROP i_prime;

USE i; ! Allocate USE variable for local

... ! optimization

DROP i; ! Drop USE variable

...

USE i_prime; ! Reaccess value in register

END

UNTIL i_prime = 0;

END;