Vax performance V-008 SP User's Guide download pdf (Page 76)

Recommended and Required Source Changes

3.3 Flow Control Mechanisms

The following examples show two ways that the REI instruction is used in VAX

MACRO code for changing modes and one way to accomplish the same thing

using the EXE$REI_INIT_STACK routine for OpenVMS Alpha or OpenVMS

I64:

Before (1)

PUSHL new_PSL

PUSHL new_PC

REI

Before (2)

PUSHL new_PSL

JSB 10$

CONTINUE ;With new PSL

10$: REI

After

PUSHL Continuation_routine

PUSHL new_mode ;Not a PSL

CALLS #2, exe$rei_init_stack

Continuation routine: .JSB_ENTRY

When your program reaches the continuation routine, it will be executing in a

new mode at a new location and the old mode stack will be reinitialized. The

memory stack and (on OpenVMS I64) the register stack will be set to their bases

for the new mode.

Note that there is the issue of passing arguments to the outer mode routine. On

OpenVMS Alpha systems, most registers are preserved across REI. On OpenVMS

I64 systems, only R26, R27, and R10 (corresponding to Alpha registers R8, R9,

and R10) are preserved. If you need to pass more data, you cannot make a data

structure on the inner mode stack and pass the address to the outer mode routine

through a register, because the inner mode stack is reset to its base by EXE$REI_

INIT_STACK. You should instead allocate space on the outer mode stack (using

MFPR_xSP and MTPR_xSP) and place the data there.

3.3.8 Loop Nesting Limit

The compiler must identify loops in program code in order to generate code

correctly. A loop is a ‘‘nested’’ loop if it is inside another loop or if it partially

overlaps another loop. If loops are nested more than 32 levels deep, the

compiler will report a fatal error, and compilation will terminate. The VAX

MACRO assembler did not need to identify loops, and so did not enforce such a

restriction.

Recommended Change

If the compiler reports this error, restructure the code so that the program does

not exceed the limit.

3–14 Recommended and Required Source Changes

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Vax performance V-008 SP User's Guide Page 76

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