IBM Systems Magazine, Mainframe Edition - January/February 2012 - 33

There are other restrictions on which instructions can be pipelined simultaneously. On the System z10, for example, either of the two instructions can be a branch instruction, but not both. And, of course, there are problems if the second instruction depends on a value computed by the first. So if two add instructions accumulate a sum into a register, like A R1,0(,R2); A R1,4(,R2), they cannot be grouped to go through the pipe together. However, due to a clever trick, many pairs that appear to be disqualified can be processed as a group. The trick is called operand forwarding, and it can be used in some cases when the first instruction needs no execution stage or the second stage is a simple store. You can see some examples in Figure 2 (page 32). Being able to execute these kinds of pairs in parallel makes the superscalar pipelines more efficient.

Speculative Execution Pipeline and superscalar processing can really increase speed—until the program does a conditional branch instruction. The processor doesn’t really know which instruction to execute next after the branch until the branch instruction has made it through the pipe. But why wait until you really know? Why not guess and hope for the best? That’s exactly what the last several generations of IBM mainframes have done. Success at this hinges on accurate branch prediction. The z196 has extremely advanced branch prediction logic. Suffice it to say that the z196 takes a guess at which way the branch will go and keeps executing along that path. Of course, it must hold onto any updates made after the branch because, if it turns out the prediction was wrong, all of the work done on speculation must

be discarded. Instructions executed past a branch that was mispredicted must appear not to have been executed at all. But when the prediction is correct, considerable delay is avoided.

Out of Order So far, we’ve seen some ways that processor designers have been able to speed program execution, but data dependencies can still hold things up. The next bit of magic, introduced in the z196, is really mind-bending—executing instructions out of order (OOO). This leads to some significant challenges, the primary of which is to maintain the illusion that the instructions have been executed in their proper order. The z196 manages all of this with a Global Completion Table, which it uses to keep track of all of the in-flight instructions. In accordance with various queues, it

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IBM Systems Magazine, Mainframe Edition - January/February 2012

Table of Contents for the Digital Edition of IBM Systems Magazine, Mainframe Edition - January/February 2012

IBM Systems Magazine, Mainframe Edition - January/February 2012
Table of Contents
Editor’s Desk: Simplicity Itself
IBM Perspective: The Changing Data Center
Insider: Solve the DevOps Challenge With Orchestrated IT
Q&A: Guru Rao Explains How a Better World Begins With Smarter Computing
Case Study: A Comfortable Place: Furniture Brands rests easier with SecureAgent's backup and recovery solution
Cover Story: Smarter By Design: "That's the way to build capacity and hold costs flat," says IBM VP Doug Brown
Feature: Another Dimension: What sets the zEnterprise environment apart?
Tech Corner: How You Do What You Do When You're a z196 CPU
Tips & Techniques: Virtualization Reshapes How Data Is Stored
Solutions: CA Mainframe Application Turner - M4Workbench Eclipse - Virtual Machine Backup
Resources: Advertisers’ Index
Stop Run: Through Three Generations, One Family's Story Shows That While Technology Has Changed, IBM's Fundamental Mission Hasn't
Reference Point - Global events, Education, Resources of Mainframe
IBM Systems Magazine, Mainframe Edition - January/February 2012 - Intro
IBM Systems Magazine, Mainframe Edition - January/February 2012 - IBM Systems Magazine, Mainframe Edition - January/February 2012
IBM Systems Magazine, Mainframe Edition - January/February 2012 - Cover2
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 1
IBM Systems Magazine, Mainframe Edition - January/February 2012 - Table of Contents
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 3
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 4
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 5
IBM Systems Magazine, Mainframe Edition - January/February 2012 - Editor’s Desk: Simplicity Itself
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 7
IBM Systems Magazine, Mainframe Edition - January/February 2012 - IBM Perspective: The Changing Data Center
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 9
IBM Systems Magazine, Mainframe Edition - January/February 2012 - Insider: Solve the DevOps Challenge With Orchestrated IT
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 11
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 12
IBM Systems Magazine, Mainframe Edition - January/February 2012 - Q&A: Guru Rao Explains How a Better World Begins With Smarter Computing
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 14
IBM Systems Magazine, Mainframe Edition - January/February 2012 - CT1
IBM Systems Magazine, Mainframe Edition - January/February 2012 - CT2
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 15
IBM Systems Magazine, Mainframe Edition - January/February 2012 - Case Study: A Comfortable Place: Furniture Brands rests easier with SecureAgent's backup and recovery solution
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 17
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 18
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 19
IBM Systems Magazine, Mainframe Edition - January/February 2012 - Cover Story: Smarter By Design: "That's the way to build capacity and hold costs flat," says IBM VP Doug Brown
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 21
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 22
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 23
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 24
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 25
IBM Systems Magazine, Mainframe Edition - January/February 2012 - Feature: Another Dimension: What sets the zEnterprise environment apart?
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 27
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 28
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 29
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 30
IBM Systems Magazine, Mainframe Edition - January/February 2012 - Tech Corner: How You Do What You Do When You're a z196 CPU
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 32
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 33
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 34
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 35
IBM Systems Magazine, Mainframe Edition - January/February 2012 - Tips & Techniques: Virtualization Reshapes How Data Is Stored
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 37
IBM Systems Magazine, Mainframe Edition - January/February 2012 - 38
IBM Systems Magazine, Mainframe Edition - January/February 2012 - Resources: Advertisers’ Index
IBM Systems Magazine, Mainframe Edition - January/February 2012 - Stop Run: Through Three Generations, One Family's Story Shows That While Technology Has Changed, IBM's Fundamental Mission Hasn't
IBM Systems Magazine, Mainframe Edition - January/February 2012 - Cover3
IBM Systems Magazine, Mainframe Edition - January/February 2012 - Cover4
IBM Systems Magazine, Mainframe Edition - January/February 2012 - Reference Point - Global events, Education, Resources of Mainframe
IBM Systems Magazine, Mainframe Edition - January/February 2012 - BG1
IBM Systems Magazine, Mainframe Edition - January/February 2012 - BG2
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