15 213 The course that gives CMU its Zip The Memory Hierarchy Oct 3 2002 Topics class12 ppt Storage technologies and trends Locality of reference Caching in the memory hierarchy Random Access Memory RAM Key features RAM is packaged as a chip Basic storage unit is a cell one bit per cell Multiple RAM chips form a memory Static RAM SRAM Each cell stores bit with a six transistor circuit Retains value indefinitely as long as it is kept powered Relatively insensitive to disturbances such as electrical noise Faster and more expensive than DRAM Dynamic RAM DRAM 2 Each cell stores bit with a capacitor and transistor Value must be refreshed every 10 100 ms Sensitive to disturbances Slower and cheaper than SRAM 15 213 F 02 SRAM vs DRAM Summary Tran per bit Access time Persist Sensitive Cost Applications SRAM 6 1X Yes No 100x cache memories DRAM 1 10X No Yes 1X Main memories frame buffers 3 15 213 F 02 Conventional DRAM Organization d x w DRAM dw total bits organized as d supercells of size w bits 16 x 8 DRAM chip 0 addr to CPU 2 3 0 2 bits rows memory controller 1 cols 1 supercell 2 1 2 8 bits 3 data 4 internal row buffer 15 213 F 02 Reading DRAM Supercell 2 1 Step 1 a Row access strobe RAS selects row 2 Step 1 b Row 2 copied from DRAM array to row buffer 16 x 8 DRAM chip 0 RAS 2 2 2 3 0 addr rows memory controller 1 cols 1 2 8 3 data 5 internal row buffer 15 213 F 02 Reading DRAM Supercell 2 1 Step 2 a Column access strobe CAS selects column 1 Step 2 b Supercell 2 1 copied from buffer to data lines and eventually back to the CPU 16 x 8 DRAM chip 0 CAS 1 2 rows memory controller supercell 2 1 2 3 0 addr To CPU 1 cols 1 2 8 3 data 6 supercell 2 1 internal row buffer 15 213 F 02 Memory Modules addr row i col j supercell i j DRAM 0 64 MB memory module consisting of eight 8Mx8 DRAMs DRAM 7 bits bits bits bits bits bits bits 56 63 48 55 40 47 32 39 24 31 16 23 8 15 63 56 55 48 47 40 39 32 31 24 23 16 15 8 7 bits 0 7 0 64 bit doubleword at main memory address A Memory controller 64 bit doubleword 7 15 213 F 02 Enhanced DRAMs All enhanced DRAMs are built around the conventional DRAM core Fast page mode DRAM FPM DRAM Access contents of row with RAS CAS CAS CAS CAS instead of RAS CAS RAS CAS RAS CAS RAS CAS Extended data out DRAM EDO DRAM Enhanced FPM DRAM with more closely spaced CAS signals Synchronous DRAM SDRAM Driven with rising clock edge instead of asynchronous control signals Double data rate synchronous DRAM DDR SDRAM Enhancement of SDRAM that uses both clock edges as control signals Video RAM VRAM Like FPM DRAM but output is produced by shifting row buffer Dual ported allows concurrent reads and writes 8 15 213 F 02 Nonvolatile Memories DRAM and SRAM are volatile memories Lose information if powered off Nonvolatile memories retain value even if powered off Generic name is read only memory ROM Misleading because some ROMs can be read and modified Types of ROMs Programmable ROM PROM Eraseable programmable ROM EPROM Electrically eraseable PROM EEPROM Flash memory Firmware Program stored in a ROM Boot time code BIOS basic input ouput system graphics cards disk controllers 9 15 213 F 02 Typical Bus Structure Connecting CPU and Memory A bus is a collection of parallel wires that carry address data and control signals Buses are typically shared by multiple devices CPU chip register file ALU system bus bus interface 10 I O bridge memory bus main memory 15 213 F 02 Memory Read Transaction 1 CPU places address A on the memory bus register file eax Load operation movl A eax ALU I O bridge bus interface 11 A main memory 0 x A 15 213 F 02 Memory Read Transaction 2 Main memory reads A from the memory bus retreives word x and places it on the bus register file eax Load operation movl A eax ALU I O bridge bus interface 12 x main memory 0 x A 15 213 F 02 Memory Read Transaction 3 CPU read word x from the bus and copies it into register eax register file eax x Load operation movl A eax ALU I O bridge bus interface 13 main memory 0 x A 15 213 F 02 Memory Write Transaction 1 CPU places address A on bus Main memory reads it and waits for the corresponding data word to arrive register file eax y Store operation movl eax A ALU I O bridge bus interface 14 A main memory 0 A 15 213 F 02 Memory Write Transaction 2 CPU places data word y on the bus register file eax y Store operation movl eax A ALU I O bridge bus interface 15 y main memory 0 A 15 213 F 02 Memory Write Transaction 3 Main memory read data word y from the bus and stores it at address A register file eax y Store operation movl eax A ALU I O bridge bus interface 16 main memory 0 y A 15 213 F 02 Disk Geometry Disks consist of platters each with two surfaces Each surface consists of concentric rings called tracks Each track consists of sectors separated by gaps tracks surface track k gaps spindle sectors 17 15 213 F 02 Disk Geometry Muliple Platter View Aligned tracks form a cylinder cylinder k surface 0 platter 0 surface 1 surface 2 platter 1 surface 3 surface 4 platter 2 surface 5 spindle 18 15 213 F 02 Disk Capacity Capacity maximum number of bits that can be stored Vendors express capacity in units of gigabytes GB where 1 GB 10 9 Capacity is determined by these technology factors Recording density bits in number of bits that can be squeezed into a 1 inch segment of a track Track density tracks in number of tracks that can be squeezed into a 1 inch radial segment Areal density bits in2 product of recording and track density Modern disks partition tracks into disjoint subsets called recording zones 19 Each track in a zone has the same number of sectors determined by the circumference of innermost track Each zone has a different number of sectors track 15 213 F 02 Computing Disk Capacity Capacity bytes sector x avg sectors track x tracks surface x surfaces platter x platters disk Example 512 bytes sector 300 sectors track on average 20 000 tracks surface 2 surfaces platter 5 platters disk Capacity 512 x 300 x 20000 x 2 x 5 30 720 000 000 20 30 72 GB 15 213 F 02 Disk Operation Single Platter View The disk surface spins at a fixed rotational rate The read write head is attached to the end of the arm and flies over …
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