Guardian Procedure Calls Reference Manual

where num-locals is a LITERAL defining the number of words of local storage needed (see

the next consideration).

• Base-address equivalencing and declaring local variables

Any local variables in the application program's trap-handling procedure must be declared

relative to the L register by using base-address equivalencing. For example:

INT I='L' + 9;

STRING .EXT X(X_TEMPLATE)='L' + 12;

Assuming that the trap handler begins with the statement CODE ( PUSH %777 ), the first local

variable is placed at ‘L'+9.

For details on base-address equivalencing, see the TAL Reference Manual.

NOTE: Variables declared in this form cannot be initialized.

The trap-handling procedure must contain a statement that explicitly allocates storage for any

locally declared variables (see the preceding consideration).

• Space ID, P register, and hardware ENV register

The space ID consists of a code space bit that signifies system or user space, a library space

bit that signifies code or library space, and a five-bit index to indicate which of the 32 possible

segments you are referring to. For more information about space IDs, see the appropriate

system description manual for your system.

At the time of a trap, the space ID of the calling procedure is placed in the stack at ‘L'[-5]; the

stack marker ENV register at ‘L'[-1] also contains the system code and library code bits but

not the index. When exiting the trap handler, execution resumes at the location identified by

the P register at ‘L'[-2], the space index in the stack at ‘L'[-5], and the library space and code

space bits of ‘L'[-1].

• Value for the P register

The value for the P register at the time of the trap depends upon the trap condition. In this

table, I represents the address of the instruction being executed at the time of the trap, and ?

means undefined.

P RegisterTrap

I + 12

I11

I12

?13

• Terminating a trap handler

A trap handler can terminate in these ways on either a TNS or native system:

◦ Clear the overflow (or trap) bit in the trap ENV variable and resume from a trap 2

(arithmetic overflow). See the “Resuming at the point of the trap” consideration below.

(If the overflow and trap bits of ENV are both set upon exit from the trap handler, then

72 Guardian Procedure Calls (A-B)