Chapter 4 Introduction

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Morgan Kaufmann Publishers 31 January 2013Data Hazardsn An instruction depends on completion ofdata access by a previous instructionn add $s0, $t0, $t1sub $t2, $s0, $t3CSE 420 Chapter 4 — The Processor — 39Forwarding (a.k.a. Bypassing)n Use result when it is computedn Don’t wait for it to be stored in a registern Requires extra connections in the datapathCSE 420 Chapter 4 — The Processor — 40
Morgan Kaufmann Publishers 31 January 2013Load-Use Data Hazardn Can’t always avoid stalls by forwardingn If value not computed when neededn Can’t forward backward in time!CSE 420 Chapter 4 — The Processor — 41Code Scheduling to Avoid Stallsn Reorder code to avoid use of load resultin the next instructionn C code for A = B + E; C = B + F;stallstalllw $t1, 0($t0)lw $t2, 4($t0)add $t3, $t1, $t2sw $t3, 12($t0)lw $t4, 8($t0)add $t5, $t1, $t4sw $t5, 16($t0)lw $t1, 0($t0)lw $t2, 4($t0)lw $t4, 8($t0)add $t3, $t1, $t2sw $t3, 12($t0)add $t5, $t1, $t4sw $t5, 16($t0)Eliminates 2 stall cyclesCSE 420 Chapter 4 — The Processor — 42
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Chapter 4 Introduction

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