Review: New-School Machine Structures. Review: Direct-Mapped Cache. TIO Dan s great cache mnemonic. Memory Access without Cache

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1 In st r uct io n Un it ( s) A+B A1+B1 A+B A3+B3 Guest Lecturer Alan Christopher inst.eecs.berkeley.edu/~cs61c UCB CS61C : Machine Structures Lecture 13 Caches II MEMRISTOR MEMORY ON ITS WAY (HOPEFULLY) HP has begun testing research prototypes of a novel non-volatile memory element, the memristor. They have double the storage density of flash, and has 1x more read-write cycles than flash (1 6 vs 1 5 ). Memristors are (in principle) also capable of being memory and logic, how cool is that? Originally slated to be ready by 13, HP later pushed that date to some time Parallel Requests Assigned to computer e.g., Search Katz Parallel Threads Assigned to core e.g., Lookup, Ads Review: New-School Machine Structures Software Parallel Instructions >1 one time e.g., 5 pipelined instructions Parallel Data >1 data one time e.g., Add of 4 pairs of words Hardware descriptions All one time Programming Languages Harness Parallelism & Achieve High Performance Hardware Warehouse Scale Computer /19/14 Fall 1 -- Lecture #14 Smart Phone Computer Core Core Memory (Cache) Today s Input/Output Lecture Core Functional Unit(s) Cache Memory Logic Gates Review: Direct-Mapped Cache All fields are read as unsigned integers. Index specifies the cache index (or row /block) Tag distinguishes betw the addresses that map to the same location Offset specifies which byte within the block we want tttttttttttttttttt iiiiiiiiii oooo tag index byte to check to offset if have select within correct block block block TIO Dan s great cache mnemonic AREA (cache size, B) = HEIGHT (# of blocks) (H+W) = H * W * WIDTH (size of one block, B/block) WIDTH Tag Index Offset (size of one block, B/block) Addr size (often 3 bits) HEIGHT (# of blocks) AREA (cache size, B) CS61C L31 Caches II (3) Garcia, Spring 13 UCB CS61C L31 Caches II (4) Garcia, Spring 13 UCB Memory Access without Cache Load word instruction: lw $t, ($t1) $t1 contains 1 ten, Memory[1] = Processor issues address 1 ten to Memory. Memory reads word at address 1 ten (99) 3. Memory sends 99 to Processor 4. Processor loads 99 into register $t1 Memory Access with Cache Load word instruction: lw $t, ($t1) $t1 contains 1 ten, Memory[1] = 99 With cache (similar to a hash) 1. Processor issues address 1 ten to Cache. Cache checks to see if has copy of data at address 1 ten a. If finds a match (Hit): cache reads 99, sends to processor b. No match (Miss): cache sends address 1 to Memory I. Memory reads 99 at address 1 ten II. Memory sends 99 to Cache III. Cache replaces word with new 99 IV. Cache sends 99 to processor 3. Processor loads 99 into register $t1 CS61C L31 Caches II (5) Garcia, Spring 13 UCB CS61C L31 Caches II (6) Garcia, Spring 13 UCB

2 CS61C L31 Caches II (7) Caching Terminology When reading memory, 3 things can happen: cache hit: cache block is valid and contains proper address, so read desired word cache miss: nothing in cache in appropriate block, so fetch from memory cache miss, block replacement: wrong data is in cache at appropriate block, so discard it and fetch desired data from memory (cache always copy) Cache Terms Hit rate: fraction of access that hit in the cache Miss rate: 1 Hit rate Miss penalty: time to replace a block from lower level in memory hierarchy to cache Hit time: time to access cache memory (including tag comparison) Abbreviation: $ = cache (A Berkeley innovation!) Garcia, Spring 13 UCB CS61C L31 Caches II (8) Garcia, Spring 13 UCB Accessing data in a direct mapped cache Ex.: 16KB of data, direct-mapped, 4 word blocks Can you work out height, width, area? Read 4 addresses 1. x14. x1c 3. x34 4. x814 Memory vals here: CS61C L31 Caches II (9) Memory Address (hex)value of Word 1 a 14 b 18 c 1C d C C e f g h i j k l Garcia, Spring 13 UCB Accessing data in a direct mapped cache 4 Addresses: x14, x1c, x34, x814 4 Addresses divided (for convenience) into Tag, Index, Byte Offset fields Tag Index Offset CS61C L31 Caches II (1) Garcia, Spring 13 UCB 16 KB Direct Mapped Cache, 16B blocks bit: determines whether anything is stored in that row (when computer initially turned on, all entries invalid) Index Tag xc-f x8-b x4-7 x Read x Tag field Index field Offset Index Tag xc-f x8-b x4-7 x CS61C L31 Caches II (11) Garcia, Spring 13 UCB CS61C L31 Caches II (1) Garcia, Spring 13 UCB

3 CS61C L31 Caches II (13) 1 13 So we read block 1 (1) 1 1 Index Tag xc-f x8-b x4-7 x No valid data 1 1 Index Tag xc-f x8-b x4-7 x Garcia, Spring 13 UCB CS61C L31 Caches II (14) Garcia, Spring 13 UCB So load that data into cache, setting tag, valid 1 1 Index Tag xc-f x8-b x4-7 x d c b a Read from cache at offset, return word b 1 1 Tag field Index field Offset Index Tag xc-f x8-b x4-7 x d c b a CS61C L31 Caches II (15) Garcia, Spring 13 UCB CS61C L31 Caches II (16) Garcia, Spring 13 UCB. Read x1c = Index Tag xc-f x8-b x4-7 x d c b a Index is 1 11 Index Tag xc-f x8-b x4-7 x d c b a CS61C L31 Caches II (17) Garcia, Spring 13 UCB CS61C L31 Caches II (18) Garcia, Spring 13 UCB

4 CS61C L31 Caches II (19) Index valid, Tag Matches 1 11 Index Tag xc-f x8-b x4-7 x d c b a Index, Tag Matches, return d 1 11 Index Tag xc-f x8-b x4-7 x d c b a Garcia, Spring 13 UCB CS61C L31 Caches II () Garcia, Spring 13 UCB 3. Read x34 = Index Tag xc-f x8-b x4-7 x d c b a So read block Index Tag xc-f x8-b x4-7 x d c b a CS61C L31 Caches II (1) Garcia, Spring 13 UCB CS61C L31 Caches II () Garcia, Spring 13 UCB No valid data 11 1 Index Tag xc-f x8-b x4-7 x d c b a Load that cache block, return word f 11 1 Index Tag xc-f x8-b x4-7 x d c b a 3 1 h g f e CS61C L31 Caches II (3) Garcia, Spring 13 UCB CS61C L31 Caches II (4) Garcia, Spring 13 UCB

5 CS61C L31 Caches II (5) 4. Read x814 = Index Tag xc-f x8-b x4-7 x d c b a 3 1 h g f e So read Cache Block 1, Data is Index Tag xc-f x8-b x4-7 x d c b a 3 1 h g f e Garcia, Spring 13 UCB CS61C L31 Caches II (6) Garcia, Spring 13 UCB Cache Block 1 Tag does not match (!= ) Index Tag xc-f x8-b x4-7 x d c b a 3 1 h g f e Miss, so replace block 1 with new data & tag Index Tag xc-f x8-b x4-7 x l k j i 3 1 h g f e CS61C L31 Caches II (7) Garcia, Spring 13 UCB CS61C L31 Caches II (8) Garcia, Spring 13 UCB And return word J Index Tag xc-f x8-b x4-7 x l k j i 3 1 h g f e Do an example yourself. What happens? Chose from: Cache: Hit, Miss, Miss w. replace Values returned: a,b, c, d, e,, k, l Read address x3? 11 Read address x1c? 1 11 Cache Index Tag xc-f x8-b x4-7 x l k j i 3 1 h g f e CS61C L31 Caches II (9) Garcia, Spring 13 UCB CS61C L31 Caches II (3) Garcia, Spring 13 UCB

6 CS61C L31 Caches II (31) Answers x3 a hit Index = 3, Tag matches, Offset =, value = e x1c a miss Index = 1, Tag mismatch, so replace from memory, Offset = xc, value = d Since reads, values must = memory values whether or not cached: x3 = e x1c = d Memory Address (hex)value of Word 1 a 14 b 18 c 1C d C e f g h 81 i 814 j 818 k 81C l Garcia, Spring 13 UCB Administrivia Proj 1- due Sunday CS61C L31 Caches II (3) Garcia, Spring 13 UCB Multiword-Block Direct-Mapped Cache Four words/block, cache size = 4K words Byte Hit offset Tag Index Tag AND 18 Index 1 MUX 4 1 Multiplexor Data Block offset What kind of locality are we taking advantage of? 3 Data Peer Instruction 1) Mem hierarchies were invented before 195. (UNIVAC I wasn t delivered til 1951) ) All caches take advantage of spatial locality. 3) All caches take advantage of temporal locality. CS61C L31 Caches II (34) 13 a) FFF a) FFT b) FTF b) FTT c) TFF d) TFT e) TTF e) TTT Garcia, Spring 13 UCB 1 3 And in Conclusion Mechanism for transparent movement of data among levels of a storage hierarchy set of address/value bindings address index to set of candidates compare desired address with tag service hit or miss load new block and binding on miss address: tag index offset 1 11 Tag xc-f x8-b x4-7 x-3 1 d c b a CS61C L31 Caches II (36) Garcia, Spring 13 UCB