pTAL Conversion Guide

Data Allocation

pTAL Conversion Guide—527302-002

5-11

Undefined Uses of the VOLATILE Attribute

The effect of the VOLATILE attribute is undefined for a volatile variable that is the

target of an intermediate store operation. Use separate assignment statements to

ensure that the VOLATILE attribute has the intended effect.

Difference Between Volatile Data and Atomic Operations

A volatile data item is always maintained in memory (as opposed to a register), but the

object code might use more than one machine instruction to access the data item each

time your source code reads or writes it.

When you access a variable using an atomic operation, that access is treated as if you

had declared the variable volatile—its value is read from or written to memory—but an

atomic operation completes before any other process, or other code within the same

process (for example, a trap handler), accesses the data. If your program has variables

that must be accessed in one memory fetch or one memory store, use one or more of

the atomic operations in Table 18-2 on page 18-4.

VOLATILE INT a[0:9]; ! Direct array

VOLATILE INT .b[0:9]; ! Indirect array

VOLATILE INT .EXT c[0:9]; ! Extended array

VOLATILE INT i = 'P' := "ABCD"; ! Read-only array

Example 5-9. Ensuring That VOLATILE is Effective

VOLATILE INT a;

INT b;

INT c;

b := a := c + 1; ! VOLATILE attribute of a might not be

! honored when assigning the value of

! c + 1 to a

a := a + 1; ! VOLATILE attribute of a is honored

b := a; ! VOLATILE attribute of a is honored

Example 5-8. Invalid Uses of the VOLATILE Attribute (page 2 of 2)