Caching II

Andreas KlappeneckerCPSC321 Computer

Architecture

Verilog Questions & Answers

Verilog Q & A How is the xor instruction encoded?

R-format instruction, function field Ox26 See [PH] page A-59

What is the purpose of Idealmem.v? It models the memory dmeminit.v initializes data memory imeminit.v initializes instruction

memory

Verilog Q&A How do I specify delays?

`define DEL 10 begin a <= #(`DEL) b; c <= #(`DEL) d; end

Delays can be inserted anywhere in an assignment

Delaysmodule iab;integer i, j;initial begin i = 3; j = 4; begin #1 i = #1 j; #1 j = #1 i; endendendmodule

Simulation starts:@time 0: i=3, j=4Simulation continues until first delay #1 and waits until time 1.@time 1, j is sampled

@time 2, assign 4 to i continue w/ next stmt@time 3, i is sampled@time 4, assign 4 to j

Delaysmodule ianb;integer i, j;initial begin i = 3; j = 4; begin i <= #1 j; j <= #1 i; endendendmodule

@time 0: i=3, j=4both non-blocking assignments finish at time 0[intra-assignments delays do not delay the execution of the statement]sample j and schedule to assign to i at time 1sample i and schedule to assign to j

@time 1: i = 4, j = 3

DelaysHint: Using unit delays simplifies

debugging It allows you to find out which signal

depends on which Do not code in the form #1, rather use

define ‘foo_del 1 // Change later a <= #(‘foo_del) b;

Clockmodule m555 (CLK); parameter STime = 0,Ton = 50,Toff = 50,Tcc=Ton+Toff; output CLK; reg CLK; initial begin #STime CLK = 0; end always begin #Toff CLK = ~CLK; #Ton CLK = ~CLK; endendmodule

Project For jal and jr, the datapath of the

book is not enough You need more control signals for

ALUop, so there is no point to stick to the way it is done in the book

ReportInclude some a table explaining yourcontrol signals, e.g.,

Caching

Memory Users want large and fast memories

SRAM is too expensive for main memory DRAM is too slow for many purposes Compromised: Build a memory hierarchy

CPU

Level n

Level 2

Level 1

Levels in thememory hierarchy

Increasing distance from the CPU in

access time

Size of the memory at each level

Locality Temporal locality

A referenced item will be again referenced soon

Spatial locality nearby data will be referenced soon

Mapping: address modulo the number of blocks in the cache, x -> x mod B

Direct Mapped Cache

00001 00101 01001 01101 10001 10101 11001 11101

000

Cache

Memory

001

010

011

100

101

110

111

Cache with 1024=210 words tag from cache is compared against upper portion of

the address If tag=upper 20 bits and valid bit is set, then we have

a cache hit otherwise it is a cache miss

What kind of locality are we taking advantage of?

Direct Mapped CacheAddress (showing bit positions)

20 10

Byteoffset

Valid Tag DataIndex

012

102110221023

Tag

Index

Hit Data

20 32

31 30 13 12 11 2 1 0

Taking advantage of spatial locality:

Direct Mapped Cache

Address (showing bit positions)

16 12 Byteoffset

V Tag Data

Hit Data

16 32

4Kentries

16 bits 128 bits

Mux

32 32 32

2

32

Block offsetIndex

Tag

31 16 15 4 32 1 0

Read hits this is what we want!

Read misses stall the CPU, fetch block from memory, deliver to cache,

restart Write hits:

can replace data in cache and memory (write-through) write the data only into the cache (write-back the cache later)

Write misses: read the entire block into the cache, then write the word

Cache Hits and Misses

What Block Size? A large block size reduces cache misses Cache miss penalty increases We need to balance these two

constraints Next time:

How can we measure cache performance? How can we improve cache performance?