Vax performance V-008 SP User's Guide download pdf (Page 43)

How the MACRO Compiler Functions on Different Platforms

2.8 Declaring Exception Entry Points (OpenVMS Alpha only)

The .EXCEPTION_ENTRY directive, as described in Appendix B, indicates

the entry point of an exception service routine. Use the .EXCEPTION_ENTRY

directive to declare the entry points for routines serving interrupts such as the

following:

• Interval clock

• Interprocessor interrupt

• System/processor correctable error

• Power failure

• System/processor machine abort

• Software interrupt

At routine entry, R3 must contain the address of the procedure descriptor. The

routine must exit with an REI instruction.

At exception entry points, the interrupt dispatcher pushes onto the stack:

registers R2 through R7, the PC, and the PSL. To access the contents of these

registers, specify the stack_base argument in the .EXCEPTION_ENTRY

directive. The compiler generates code that places the value of the SP at routine

entry in the register you specify in stack_base, allowing the exception service

routine to use this register to locate the contents of registers on the stack.

The compiler automatically saves and restores all other registers used in the

routine, plus, if the service routine issues a CALL or a JSB instruction, all

scratch registers, including R0 and R1.

Note

Error handling routines, established when their addresses are stored in

the frame at 0(FP), are not .EXCEPTION_ENTRY routines. Such error

handlers should be declared as .CALL_ENTRY routines and end with

RET instructions.

2.9 Using Packed Decimal Instructions

The packed decimal directive .PACKED and all packed decimal instructions,

except EDITPC, are supported for the MACRO compiler by emulation routines

that exist outside the compiled module.

2.9.1 Differences Between the OpenVMS VAX and OpenVMS Alpha/I64

Implementations

The differences between the implementations on OpenVMS VAX and OpenVMS

Alpha/I64 systems are noted in the following list:

• Packed decimal instructions and atomicity

Since all packed decimal instructions are emulated by means of subroutine

calls, they are not atomic or restartable, and cannot be made atomic by the

.PRESERVE ATOMICITY directive or /PRESERVE=ATOMICITY option.

Also, on OpenVMS Alpha or OpenVMS I64 systems, there is no guarantee

that registers from R16 through R28 are preserved across the instruction.

• Use of argument registers

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Vax performance V-008 SP User's Guide Page 43

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