NonStop S-Series Server Description Manual (G06.24+)

TNS Execution Modes

HP NonStop S-Series Server Description Manual—520331-003

6-48

Passing Parameters to a Called Procedure

Parameters are passed to a procedure in the top-of-stack area (memory stack).

Naturally, there must be coordination between the caller and the called procedure

when passing parameters. The caller must know the order in which a procedure

expects parameters, and whether a parameter is to be an actual operand (called a

value parameter) or an address pointer (called a reference parameter).

Before the caller invokes a procedure, the parameters are prepared in the register

stack. The actual operands (for value parameters) and the addresses of operands (for

reference parameters) are loaded into the register stack in the order required by the

procedure being called. The address of a reference parameter is obtained by the

execution of a LADR (load address) instruction. The parameters that have been

prepared in the register stack are loaded on the top of the memory stack by executing

a PUSH instruction (which increments the S register accordingly).

The example shown in Figure 6-27 illustrates the instructions used to prepare the top

of the memory stack area for parameter passing. The TAL procedure being called is of

the form:

PROC b (p1,p2);

INT p1,.p2;

Parameter “p1” is a value parameter; therefore, the procedure expects an actual value

to be passed. Parameter “p2” is a reference parameter, and, therefore, the procedure

expects the G-relative address of a variable to be passed.

The call being made from procedure A is:

CALL b (j,i);

The instructions to pass these two parameters are:

LOAD L +002

LADR L +001

PUSH 711

The LOAD instruction puts the contents of the variable “j” (the value 5) on the top of the

B.)

The LADR instruction calculates the G-relative address of the variable “i” and puts the

address on the top of the register stack. (This is the parameter passed as “p2”, a

reference parameter, to procedure B.)

The PUSH instruction places the two parameters from the register stack on the top of

the memory stack and increments the S register setting by 2.