IBM Systems Magazine, Mainframe Edition - January/February 2012 - 34

Tech Corner
knows what it can do next, what results must be stored and what results need to be discarded. The z196 can have up to 32 instructions simultaneously in-flight and maintains a queue of up to 40 more that can be selected for execution as soon as their dependencies are resolved. Another concern with OOO is how to deal with multiple in-flight instructions updating the same register. For example, consider the sequence L R1,0(,R2); AR R3,R1; L R1,4(,R2). The second load instruction really shouldn’t be able to execute until the previous value of the register is used in the add instruction. The trick this time is called register renaming. The z196 has 96 physical general-purpose registers, so it can keep the value from the first load in one physical register and the value from the second load in a different physical register. The information about which physical registers map to which program registers at what points in the program is maintained in two tables,

Figure 3

Load/Store Units (LSUs)
AG
Address Generation

D$ STQ D$

D$

FM
Format

WB
Write Back

Fin

AG GR RF

D$

FM

WB

Fin

Fixed Point Units (FPUs)
EX EX CC CC WB WB Fin Fin

Depiction of part of the z196 pipeline showing two load/store units and two fixed point units. The pipeline stages for instructionfetch, instruction-decode, instruction-grouping and result-putaway are not shown. Also not shown are the floating-point units for binary, decimal and arithmetic. Under ideal conditions, the z196 can execute five instructions simultaneously.

one for instructions not yet completed and one for completed instructions that present the latest architectural view. It gets complicated, but whenever the program can observe memory or the registers, it must always appear as though the instructions have executed in the conceptual sequence.

Cracking The mainframe instruction set has always included instructions that perform an operation on a value in memory and a value in a register (they are loosely called RX-type instructions). A RISC architecture has instructions that either transfer data between memory and a

Cracking, Renaming, Superscalar and Out of Order Work Together

F

igure 1 (right) shows a simplistic example of how

cycles if all operands are in level 1 cache. However, if one or more storage operands are not in level 1 data cache, then delays occur in the pipe and the real-time execution grouping will be different. The lower alternative grouping would result if none of the operands were in level 1 cache. This exposes another advantage of superscalar and OOO. The processor can detect earlier that data items need to be loading into cache and launch multiple fetch requests simultaneously. If the instructions in Figure 1 were executed single file and in order, the program would sequentially experience a cache delay on each instruction. But, with superscalar and OOO, using the lower alternative grouping, five of the required fetches would be launched in the first three cycles, and the accesses would be substantially overlapped providing a significant speed-up of the program. —B.R.
Illustration of cracking, renaming, reordering and grouping of a simple program segment. The upper grouping assumes that all operands are in level 1 cache. The lower alternative grouping assumes none of the operands are in level 1 cache.

Figure 1

cracking, renaming, superscalar and out of order (OOO) work together. The short program segment adds two number pairs, stores the sums and then copies a short string. The RX-type add instruction and the move instruction are cracked into two micro-ops each. Even though the original code uses only one register, after renaming, five physical registers are used to reduce inter-instruction dependencies. Grouping creates four groups (see upper alternative grouping). These groups can execute in only four execution

22 34

JANUARY/FEBRUARY 2 012

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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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