Memory Ordering in Modern Microprocessors, Part II

The zSeries machines make up the IBM mainframe family previously known as the 360, 370 and 390. Parallelism came late to zSeries, but given that these mainframes first shipped in the mid-1960s, this is not saying much. The bcr 15,0 instruction is used for the Linux smp_mb(), smp_rmb() and smp_wmb() primitives. It also has comparatively strong memory-ordering semantics, as shown in Table 1. This should allow the smp_wmb() primitive to be a no-op, and by the time you read this, this change may have happened.

As with most CPUs, the zSeries architecture does not guarantee a cache-coherent instruction stream. Hence, self-modifying code must execute a serializing instruction between updating the instructions and executing them. That said, many actual zSeries machines do in fact accommodate self-modifying code without serializing instructions. The zSeries instruction set provides a large set of serializing instructions, including compare-and-swap, some types of branches—for example, the aforementioned bcr 15,0 instruction—and test-and-set, among others.

This final installment of the memory-barrier series has given an overview of how a number of CPUs implement memory barriers. Although these overviews should by no means be considered a substitute for carefully reading the architecture manuals (see Resources), I hope that it has served as a useful introduction.

I owe thanks to many CPU architects for patiently explaining the instruction and memory-reordering features of their CPUs, particularly Wayne Cardoza, Ed Silha, Anton Blanchard, Tim Slegel, Juergen Probst, Ingo Adlung and Ravi Arimilli. Wayne deserves special thanks for his patience in explaining Alpha's reordering of dependent loads, a lesson that I resisted quite strenuously!

This work represents the view of the author and does not necessarily represent the view of IBM. IBM, zSeries and PowerPC are trademarks or registered trademarks of International Business Machines Corporation in the United States, other countries or both. Linux is a registered trademark of Linus Torvalds. i386 is a trademark of Intel Corporation or its subsidiaries in the United States, other countries or both. Other company, product, and service names may be trademarks or service marks of such companies. Copyright (c) 2005 by IBM Corporation.

Resources for this article: /article/8406.