User`s guide

How the MACRO Compiler Functions on Different Platforms

2.5 Declaring JSB Routine Entry Points

In assembled VAX MACRO code and compiled OpenVMS Alpha and OpenVMS

I64 object code alike, JSB routine parameters are typically passed in registers.

A JSB routine that writes to registers sometimes saves and restores the contents

of those registers, unless the new contents are intended to be returned to a caller

as output. However, because it is fairly common for a VAX MACRO module to

save and restore register contents by issuing instructions such as PUSHL and

POPL, saving only the low 32 bits of the register contents, the compiler must add

64-bit register saves/restores at routine’s entry and exit points.

The compiler can compute the set of used registers, but without complete

knowledge of the routine’s callers (which may be in other modules), it cannot

determine which registers are intended to contain output values. For this

reason, the user must add, at each JSB routine entry point, a .JSB_ENTRY or a

.JSB32_ENTRY directive that declares the routine’s register usage.

2.5.1 Differences Between .JSB_ENTRY and .JSB32_ENTRY

The compiler provides two different ways of declaring a JSB entry point. The

.JSB_ENTRY directive is the standard declaration, and the .JSB32_ENTRY

directive is provided for cases when you know that the upper 32 bits of the 64-bit

In routines declared with the .JSB_ENTRY directive, the compiler, by default,

saves at routine entry and restores at routine exit the full 64-bit contents of

any register (except R0 and R1) that is modiﬁed by the routine. If a register is

not explicitly modiﬁed by the routine, the compiler will not preserve it across the

routine call. You can override the compiler’s default behavior by means of register

arguments that can be speciﬁed with the .JSB_ENTRY directive, as described in

Section 2.6.

When .JSB32_ENTRY is used, the compiler does not automatically save or restore

any registers, thus leaving the current 32-bit operation untouched. The compiler

will only save and restore the full 64-bit value of registers you explicitly speciﬁed

in the preserve argument.

If the routine you are porting is in an environment where you know that no caller

ever needs to have the upper 32 bits of a register preserved, it is recommended

that you use the .JSB32_ENTRY directive. It can be much faster for porting code

because you do not have to examine the register usage of each routine. You can

specify the .JSB32_ENTRY directive with no arguments, and the existing 32-bit

Note

The OpenVMS Alpha or OpenVMS I64 compilers for other languages may

use 64-bit values. Therefore, any routine which is called from another

language — or which is called from another MACRO routine that is in

turn called from another language — cannot use .JSB32_ENTRY.

How the MACRO Compiler Functions on Different Platforms 2–11