1. 1 Important Announcements  Midterm 3 is on Wednesday, April 20 from 7pm to 8:30pm —Practice Midterm 1 released tonight —Please email me ASAP in case you need a conflict  Final Exam (cumulative) is on Monday, May 9 from 1:30pm to 4:30pm —Please email me ASAP in case you need a conflict

2. 2 Pipelined datapath Read address Instruction cache Instruction [31-0] Address Write data Data cache Read data MemWrite MemRead 1010 MemToReg 4 Shift left 2 Add Sign extend ALUSrc Result Zero ALU ALUOp Instr [15 - 0] RegDst Read register 1 Read register 2 Write register Write data Read data 2 Read data 1 Registers RegWrite Add Instr [15 - 11] Instr [20 - 16] 0101 0 1 IF/IDID/EXEX/MEMMEM/WB 1010 PCSrc PCPC

3. 3 PC register stores virtual address  The PC registers stores PC v = virtual address of the next instruction Read address Instruction cache Instruction [31-0] 4 Add 1010 PCSrc PCvPCv

4. 4 PC register stores both virtual and physical address  The PC registers stores both PC v and PC p Read address Instruction cache Instruction [31-0] 4 Add 1010 PCSrc PCvPCv PCpPCp

5. 5 Virtual Memory system virtual address data physical address TLB page table memory cache disk page offset virtual page number (VPN) PPN tag index block offset

6. 6 Hard drives  The textbook shows the ugly guts of a hard disk —Data is stored on double-sided magnetic disks called platters —Each platter is arranged like a record, with many concentric tracks —Tracks are further divided into individual sectors, which are the basic unit of data transfer —Each surface has a read/write head like the arm on a record player, but all the heads are connected and move together  A 75GB IBM Deskstar has roughly: —5 platters (10 surfaces), —27,000 tracks per surface, —512 bytes per sector, —~512 sectors per track… …but this number increases going from the center to the rim

7. 7  There are two fundamental performance metrics for I/O systems: 1.LatencyTime to initiate data-transfer(units = sec) 2.BandwidthRate of initiated data-transfer(units = bytes/sec) Time = latency + transfer_size / bandwidth sec bytes / (bytes/sec) I/O Performance Dominant term for small transfers Dominant term for large transfers

8. 8 Accessing data on a hard disk  Factors affecting latency: —Seek time measures the delay for the disk head to reach the track —A rotational delay accounts for the time to get to the right sector  Factors affecting bandwidth: —Usually the disk can read/write as fast as it can spin —Bandwidth is determined by the rotational speed, which also determines the rotational delay  We can compute average seek time/rotational delay/etc. but careful placement of data on the disk can speed things up considerably: —head is already on or near the desired track —data in contiguous sectors —in other words, locality!  Even so, loading a page from the hard-disk can take tens of milliseconds

9. 9 Parallel I/O  Many hardware systems use parallelism for increased speed  A redundant array of inexpensive disks or RAID system allows access to several hard drives at once, for increased bandwidth —similar to interleaved memories from last week