User`s guide

How the MACRO Compiler Functions on Different Platforms

2.11 Preserving VAX Atomicity and Granularity

an OpenVMS AXP System: Recompiling and Relinking Applications

for a more

complete discussion of the programming issues involved in read-modify-write

operations in an Alpha system.

Warning

When preserving atomicity, the compiler generates aligned memory

instructions that cannot be handled by the Alpha PALcode unaligned

fault handler. They will cause a fatal reserved operand fault on unaligned

addresses. Therefore, all memory references for which .PRESERVE

ATOMICITY is speciﬁed must be to aligned addresses (see Section 2.11.5).

2.11.2 Preserving Granularity

To preserve the granularity of a VAX MACRO memory write instruction on a byte,

word, or unaligned longword on Alpha means to guarantee that the instruction

executes successfully on the speciﬁed data and preserves the integrity of the

surrounding data.

The VAX architecture includes instructions that perform independent access to

byte, word, and unaligned longword locations in memory so two processes can

write simultaneously to different bytes of the same aligned longword without

interfering with each other.

The Alpha architecture, as originally implemented, deﬁned instructions that

could address only aligned longword and quadword operands. However, byte and

word operands for load and store were later added.

On Alpha, code that writes a data ﬁeld to memory that is less than a longword

in length or is not aligned can do so only by using an interruptible instruction

sequence that involves a quadword load, an insertion of the modiﬁed data into the

quadword, and a quadword store. In this case, two processes that intend to write

to different bytes in the same quadword will actually load, perform operations on,

and store the whole quadword. Depending on the timing of the load and store

operations, one of the byte writes could be lost.

The Itanium architecture has byte, word, longword, and quadword addressibility,

so granularity of access is easily obtained without having to request it with an

option or declaration.

The compiler provides the /PRESERVE=GRANULARITY option to

guarantee the integrity of byte, word, and unaligned longword writes. The

/PRESERVE=GRANULARITY option causes the compiler to generate Alpha

instructions that provide granularity preservation for any VAX instructions that

write to bytes, words, or unaligned longwords. Alternatively, you can insert the

.PRESERVE GRANULARITY and .NOPRESERVE GRANULARITY directives in

sections of VAX MACRO source code as required to enable and disable granularity

preservation.

This manual has been archived. It is no longer maintained and is not part

of the OpenVMS documentation set. However, you can view it online at

http://www.hp.com/go/openvms/doc.

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