User`s guide

How the MACRO Compiler Functions on Different Platforms

2.5 Declaring JSB Routine Entry Points

2.5.2 Two General Cases for Using .JSB32_ENTRY

There are two general cases where you can use .JSB32_ENTRY:

• If a user-mode application or self-contained subsystem is written entirely in

Macro-32 you can use .JSB32_ENTRY throughout the application.

• If you have one major Macro-32 routine that is called from another language

or from any source which requires 64-bit register preservation, and that

routine calls several other Macro-32 routines, you can set a barrier of 64-bit

preservation. You can do this by using .JSB_ENTRY or .CALL_ENTRY on the

major routine and preserving all registers that are not explicit outputs. The

internal subroutines can then use the .JSB32_ENTRY directive.

Note that it is not sufﬁcient to just use .JSB_ENTRY or .CALL_ENTRY for

the barrier without explicitly preserving all registers, since by default the

compiler will only save and restore the registers that are explicitly modiﬁed

in the major routine. The internal subroutines that use .JSB32_ENTRY may

then modify registers that are not being preserved. You must also make sure

that none of the internal subroutines can be called in any way that bypasses

the barrier.

Caution

The .JSB32_ENTRY directive can be a great time-saver if you are sure

that you can use it. If you use .JSB32_ENTRY in a situation where the

upper 32 bits of a register are being used, it may cause very obscure and

difﬁcult-to-track bugs by corrupting a 64-bit value that may be several

calling levels above the offending routine.

.JSB32_ENTRY should never be used in an AST routine, condition

handler, or any other code that can be executed asynchronously.

2.5.3 PUSHR and POPR Instructions Within JSB Routines

There will be cases when you add a .JSB_ENTRY directive to a routine which

already saves/restores some registers by means of PUSHR/POPR. Depending on

routine usage, some registers may end up being saved/restored twice, once by the

compiler and again by the PUSHR/POPR. Do not attempt to optimize this unless

the code is extremely performance sensitive. The compiler attempts to detect this

and eliminate the redundant save/restores.

2.5.4 Establishing Dynamic Condition Handlers in JSB Routines

The compiler will ﬂag, as illegal, any code in a .JSB_ENTRY routine that

attempts to modify 0(FP).

2.6 Declaring a Routine’s Register Use

The compiler provides four register declaration arguments that you can specify

in a .CALL_ENTRY, .JSB_ENTRY, .JSB32_ENTRY, .CALL_LINKAGE (OpenVMS

I64 only), .USE_LINKAGE (OpenVMS I64 only), or .DEFINE_LINKAGE

(OpenVMS I64 only) entry-point directive:

• Input

• Output

• Scratch

2–12 How the MACRO Compiler Functions on Different Platforms