IBM Systems Magazine, Mainframe Edition - January/February 2012 - 32

Tech Corner
since many hazards can inhibit full-speed execution in the pipeline. Some of the greatest causes of delays are failure to fetch and decode instructions in time and failure to fetch required operands from storage in time. We’ll defer these topics for now. Instead, let’s look at another major hazard and see how the engineers deal with it. This hazard is based on the need to determine the address of an operand before it can be fetched from memory. In Figure 1, imagine the first instruction loads a register that the second instruction needs to compute the address of an operand, e.g., LA 2,256(,1); L 3,0(,2). The second instruction (at the operand address stage) needs the new value of the register before the first instruction stores it away (at the Putaway stage). There would be a three-cycle delay waiting for the value of the register to become available. Called an address generation interlock (AGI) delay, it would appear to hold up all subsequent instructions in the pipeline and cause three cycles of processing time to be lost. But engineers don’t let that happen. They use a trick called a load address AGI bypass to reduce the delay. It takes advantage of the fact that for a load address instruction, the new value of the register is already known at the end of the operand address stage. So, by adding special circuitry, the new value can be used in the second instruction as soon as it is produced in the first instruction. This is highlighted in Figure 1 by an arrow connecting the operand address stage of the first instruction to the operand address stage of the second instruction. Hazard avoided. An AGI bypass also exists for load instructions. However, because a load instruction must actually fetch an operand before the new value of the register is known, it can’t eliminate the delay; it can only reduce it to a single cycle. The z196 has additional AGI bypasses for other instructions.

Figure 1

Instruction Instruction Fetch Decode

Operand Address AGI Bypass

Operand Fetch

Execute

Putaway Result

Instruction Instruction Fetch Decode

Operand Address

Operand Fetch

Execute

Putaway Result

Instruction Instruction Fetch Decode

Operand Address

Operand Fetch

Execute

Putaway Result

Instruction Instruction Fetch Decode

Operand Address

Operand Fetch

Execute

Putaway Result

Time
An idealized pipeline similar to the IBM zSeries z900. The pipeline of the z10 is more than twice as long. The pipeline of the z196 is too complicated to give a single number as its length, but it’s even longer than the pipeline of the z10.

Figure 2

Instruction Instruction Fetch Decode

Operand Address

Operand Fetch

Execute

Putaway Result

Operand Forwarding Instruction Instruction Fetch Decode Operand Address Operand Fetch Execute Putaway Result

Instruction Instruction Fetch Decode

Operand Address

Operand Fetch

Execute

Putaway Result

Operand Forwarding Instruction Instruction Fetch Decode Operand Address Operand Fetch Execute Putaway Result

Two examples of operand forwarding. By performing operand forwarding, the z196 can group instructions for simultaneous execution, even when the second instruction has an operand dependency upon the first.

Superscalar The next technique for evading the conception sequence is to have multiple pipelines, which allow groups of instructions to flow through them. This is exactly what was done on the z990 with the introduction of superscalar processing. The z196 is very ambitious with five execution pipes, so first, we’ll look at the much simpler System z10* server with just two pipes. In the System z10 pipeline, up to two instructions can be sent through at a time. The two instructions go down the two pipes in lockstep. Some early systems required that only instructions with a single cycle of execution could be grouped for superscalar processing. Even though the mainframe has some complex instructions, almost all of the commonly used ones have a single cycle of execution. That is, the modern mainframes are almost like RISC machines in terms of instruction execution. We’ll see later that the z196 extends this RISCness even further.

22 32

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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