HP NonStop G-Series : TAL Reference Manual : Page 235

Statements

TAL Reference Manual—526371-001

12-39

Usage Considerations

The compiler associates each identifier specified in a USE statement with an index

register starting with R[7] down to R[5]. Thus, you can use at most three reserved

registers at a time.

Statements that appear between a USE statement and a corresponding DROP

statement or the end of the procedure or subprocedure can access the reserved index

register. If a global or local item with the same identifier as a USE identifier already

exists, the compiler issues an error message.

You can, for example, use a reserved index register to optimize a FOR statement as

described in the description of the FOR statement. Before referring to the identifier of a

reserved index register, be sure to assign a value to it.

You can also assign a value to a reserved index register and then pass the index

register content as an implicit parameter to a procedure or subprocedure. This

practice, however, is not portable to future software platforms. For more information,

see the

TAL Programmer’s Guide.

If evaluation of an expression overwrites the value in a reserved register, the compiler

issues a diagnostic message. For example, multiplication of two FIXED values will

overwrite the reserved register.

To determine whether using the index register is beneficial, you can use the

INNERLIST directive and then review the code generated by statements in the range

of a USE statement.

If the compiler needs an index register and none is available, the compiler emits a

diagnostic message.

When you are finished using a reserved index register, release (or drop) it by issuing a

DROP statement, as described in DROP Statement

on page 12-20.

Examples of USE Statements

1. This example reserves two index registers. The compiler associates each identifier

with an index register, starting with R[7]:

USE a_index; !Reserve R[7]

USE b_index; !Reserve R[6]

a_index := 0;

b_index := -1;

2. This example contrasts a standard FOR loop with an optimized FOR loop (if no

procedure or function calls occur within the loop):