Agenda Types of Storage Media Semiconductor ROM and RAM Magnetic Tapes, Floppy Disks, and Hard Disks Optical CS C446 Data Storage Technologies & Networks

Sundar B. Semiconductor Memories Random Access Memory Volatile Data retained only when powered on Static vs. Dynamic RAM Read-Only-Memory Non-Volatile Data retained even while not powered PROM, EPROM, EEPROM (Flash ROM)

Sundar B. Static RAM (SRAM) A single bit is stored in a bi-stable memory cell i.e. a cell that can stay indefinitely in either of two different voltage configurations as long as it is powered on Memory cell characteristics: (typically) made of 6 transistors (typical) access time close to 10ns Used for: Caches due to fast access Not used for: Main Memory due to low density and high cost

Sundar B. Dynamic RAM (DRAM) A single bit memory cell is a small capacitor (30 femtofarads i.e. 30x10^-15 farads) and a single access transistor DRAM in comparison with SRAM Has higher density Less costly and consumes less power Higher access time (close to 60ns) Is susceptible to optical or other disturbances Loses charge faster (due to capacitive leakage) - in 10 to 100 ms Has to be refreshed periodically

Sundar B. Memory access in DRAMs Memory (DRAM chip) is arranged as a matrix of cells Main reason is to reduce address pins (see below) Read Sequence: CPU sends address to memory controller Controller sends row address to DRAM chip The required row is copied into the internal row buffer Controller sends column address to DRAM chip DRAM chip sends the contents of the required cell to the Controller Controller forwards the contents to the CPU.

Sundar B. Enhanced DRAMs Fast Page Mode DRAM: Serves consecutive requests from the same row from internal row buffer Extended Data Out DRAM: Individual column access signals for the same row can be squeezed in time Synchronous DRAM: Access control signals are synchronized with the controller’s clock signal – improves data transfer rate Double Data-Rate SDRAM: Use both the edges of the clock as control signals.

Sundar B. ROM (Non-volatile memory) Originally ROMs were read-only devices: Data was hard-wired (as arrangements of transistors) and shipped; data is stored indefinitely even if not powered on. Also known as Masked ROMs Programmable ROMs: Can be programmed once; stores all 1’s by default; A fuse attached to a cell can be blown to store a 0 Erasable PROM: Data Can be stored using shining light through a transparent quartz windows; needs a separate device (device programmer) Access time: 450 ns (approx).

Sundar B. ROM (Non-volatile memory) Electrically-Erasable Programmable Memory No separate programming device needed Reprogrammed (i.e. erased and written) close to 10^5 times Flash ROMs Special EEPROM devices - Close to 1 million erase cycles Special file system support Techniques for amortizing wear but still limited in number of writes Used in PDAs, music players, game consoles, pen drives etc. Capcaity upto a few GBytes.

Sundar B. Magnetic Memories Non-Volatile Power not needed for retention Floppy Disks Flexible, can be carried, limited size Hard Disks Hard cased Originally mean for PCs and mainframes Now available in various shapes: Mini disks for laptops, gaming consoles and as external pocket storage Micro disks for iPods / Cameras / other handheld devices

Sundar B. Floppy Disk Originally A single surface disk (8 in.) Later 2 surfaces; 4X density; (5.25in.) 2 surfaces; High density; (3.5 in) 1.44MB capacity; Access rate 10 ms (complete sector) Geometry: 80 tracks per surface; 18 sectors per track (160 x 18 = 2880 sectors) 1 sector = 512 bytes

Sundar B. Hard Disks Disk Geometry Made of platters – each platter has two sides/surfaces Platters are stacked on top of each other in a cylinder A rotating spindle in the centre spins the platter at a fixed rotational rate Each surface has a read/write head that is attached to an actuator arm radial movement of the arm allows access to a specific track

Sundar B. Hard Disks [2] Capacity depends on Recording density (bits/inch) Track Density (tracks/inch) Areal Density is Product of recording density and track density Total capacity is (# bytes / sector) * (# sectors / track) * (#tracks / surface) * (# surfaces / platter) * (#platters / disk) Early days: Fixed number of sectors / track – sectors were spaced apart in outer tracks

Sundar B. Hard Disks [3] Modern disks High areal densities – inter-sector gaps would be a huge wastage. Multiple zones Each zone is a contiguous collection of tracks each zone has fixed #sectors / track (determined by the innermost track in the zone) Capacity equation Replace (#sectors/track) with (average # sectors / track) or Computer per zone and add

Sundar B. Hard Disks [4] Read and writing in sector-sized blocks Typically 512 bytes Access time Seek time (t seek ) Required to move the arm and position over required track Average seek time (typically 6 to 9ms) Max. single seek time (upto 20 ms) Rotational Latency (t avg-rotation ) Head on track, wait time for first bit of the sector to be over/under the head. Max. wait time = 1 single rotation i.e. t max-rotation Avg. wait time = ½ of t max-rotation i.e. t avg-rotation Transfer time (t avg-transfer) Time for rotating one sector over/under the head

Sundar B. Magnetic Tapes Magnetic recording Tape is spooled over head Tape Width ~3mm to ~19mm. Higher density and lower cost than disks E.g. DAT 72 (circa ’03) 170 m long 36GB capacity 3.5 MB/s speed E.g SAIT-4 (circa ’08?) 600m long 4000GB capacity 240 MB/s speed

Sundar B.