Computer Architecture PC Structure and Peripherals Dr. Lihu Rappoport

DRAM

Basic DRAM chip DRAM access sequence Row RAS# Row Address Latch decoder Column addr CAS# RAS# Data Memory array Addr Column Address Latch DRAM access sequence Put Row on addr. bus and assert RAS# (Row Addr. Strobe) to latch Row Put Column on addr. bus and assert CAS# (Column Addr. Strobe) to latch Col Get data on address bus

DRAM Operation DRAM cell consists of transistor + capacitor AL DL C M DRAM cell consists of transistor + capacitor Capacitor keeps the state; Transistor guards access to the state Reading cell state: raise access line AL and sense DL Capacitor charged  current to flow on the data line DL Writing cell state: set DL and raise AL to charge/drain capacitor Charging and draining a capacitor is not instantaneous Leakage current drains capacitor even when transistor is closed DRAM cell periodically refreshed every 64ms

DRAM Access Sequence Timing tRCD – RAS/CAS delay tRP – Row Precharge RAS# Data A[0:7] CAS# Data n Row i Col n Row j X CL – CAS latency Put row address on address bus and assert RAS# Wait for RAS# to CAS# delay (tRCD) between asserting RAS and CAS Put column address on address bus and assert CAS# Wait for CAS latency (CL) between time CAS# asserted and data ready Row precharge time: time to close current row, and open a new row

DRAM controller DRAM controller gets address and command Splits address to Row and Column Generates DRAM control signals at the proper timing DRAM data must be periodically refreshed DRAM controller performs DRAM refresh, using refresh counter DRAM address decoder Time delay gen. mux RAS# CAS# R/W# A[20:23] A[10:19] A[0:9] Memory address bus D[0:7] Select Chip select

Improved DRAM Schemes Paged Mode DRAM Multiple accesses to different columns from same row Saves RAS and RAS to CAS delay Extended Data Output RAM (EDO RAM) A data output latch enables to parallel next column address with current column data RAS# Data A[0:7] CAS# Data n D n+1 Row X Col n Col n+1 Col n+2 D n+2 RAS# Data A[0:7] CAS# Data n Data n+1 Row X Col n Col n+1 Col n+2 Data n+2

Improved DRAM Schemes (cont) Burst DRAM Generates consecutive column address by itself RAS# Data A[0:7] CAS# Data n Data n+1 Row X Col n Data n+2

Synchronous DRAM – SDRAM All signals are referenced to an external clock (100MHz-200MHz) Makes timing more precise with other system devices 4 banks – multiple pages open simultaneously (one per bank) Command driven functionality instead of signal driven ACTIVE: selects both the bank and the row to be activated ACTIVE to a new bank can be issued while accessing current bank READ/WRITE: select column Burst oriented read and write accesses Successive column locations accessed in the given row Burst length is programmable: 1, 2, 4, 8, and full-page May end full-page burst by BURST TERMINATE to get arbitrary burst length A user programmable Mode Register CAS latency, burst length, burst type Auto pre-charge: may close row at last read/write in burst Auto refresh: internal counters generate refresh address

SDRAM Timing clock cmd Bank Data Addr NOP X Data j Data k ACT Bank 0 Row i RD Col j tRCD > 20ns RD+PC Col k Row l t RC>70ns Bank 1 Row m t RRD > CL=2 Col q Col n Data n Data q BL = 1 tRCD: ACTIVE to READ/WRITE gap = tRCD(MIN) / clock period tRC: successive ACTIVE to a different row in the same bank tRRD: successive ACTIVE commands to different banks

DDR-SDRAM 2n-prefetch architecture Uses 2.5V (vs. 3.3V in SDRAM) DRAM cells are clocked at the same speed as SDR SDRAM cells Internal data bus is twice the width of the external data bus Data capture occurs twice per clock cycle Lower half of the bus sampled at clock rise Upper half of the bus sampled at clock fall Uses 2.5V (vs. 3.3V in SDRAM) Reduced power consumption n:2n-1 0:n-1 200MHz clock 0:2n-1 SDRAM Array 400M xfer/sec

DDR SDRAM Timing 133MHz clock cmd Bank Addr Data tRCD >20ns NOP X ACT Bank 0 Row i RD Col j tRCD >20ns Row l t RC>70ns Bank 1 Row m t RRD >20ns CL=2 Col n j +1 +2 +3 n

DIMMs DIMM: Dual In-line Memory Module A small circuit board that holds memory chips 64-bit wide data path (72 bit with parity) Single sided: 9 chips, each with 8 bit data bus Dual sided: 18 chips, each with 4 bit data bus Data BW: 64 bits on each rising and falling edge of the clock Other pins Address – 14, RAS, CAS, chip select – 4, VDC – 17, Gnd – 18, clock – 4, serial address – 3, …

DDR Standards DRAM timing, measured in I/O bus cycles, specifies 3 numbers CAS Latency – RAS to CAS Delay – RAS Precharge Time CAS latency (latency to get data in an open page) in nsec CAS Latency × I/O bus cycle time Total BW for DDR400 3200M Byte/sec = 64 bit2200MHz / 8 (bit/byte) 6400M Byte/sec for dual channel DDR SDRAM Standard name Mem. clock (MHz) I/O bus clock (MHz) Cycle time (ns) Data rate (MT/s) VDDQ (V) Module name  transfer rate (MB/s) Timing (CL-tRCD-tRP) CAS Latency DDR-200 100 10 200  2.5  PC-1600 1600 DDR-266 133⅓ 7.5 266⅔ PC-2100 2133⅓ DDR-333 166⅔ 6 333⅓ PC-2700 2666⅔ DDR-400 5 400 2.6 PC-3200 3200 2.5-3-3 3-3-3 3-4-4 12.5 15

DDR2 DDR2 doubles the bandwidth Smaller page size: 1KB vs. 2KB 4n pre-fetch: internally read/write 4× the amount of data as the external bus DDR2-533 cell works at the same freq. as a DDR266 cell or a PC133 cell Prefetching increases latency Smaller page size: 1KB vs. 2KB Reduces activation power – ACTIVATE command reads all bits in the page 8 banks in 1Gb densities and above Increases random accesses 1.8V (vs 2.5V) operation voltage Significantly lower power Memory Cell Array I/O Buffers Data Bus Memory Cell Array I/O Buffers Data Bus Memory Cell Array I/O Buffers Data Bus

DDR2 Standards Standard name Mem clock (MHz) Cycle time I/O Bus clock Data rate (MT/s) Module name Peak transfer rate Timings CAS Latency DDR2-400 100 10 ns 200 400 PC2-3200 3200 MB/s 3-3-3 4-4-4 15 20 DDR2-533 133 7.5 ns 266 533 PC2-4200 4266 MB/s 11.25 DDR2-667 166 6 ns 333 MHz 667 PC2-5300 5333 MB/s 4-4-4 5-5-5 12 DDR2-800 5 ns 400 MHz 800 PC2-6400 6400 MB/s 4-4-4 5-5-5 6-6-6 10 12.5 DDR2-1066 3.75 ns 533 MHz 1066 PC2-8500 8533 MB/s 6-6-6 7-7-7 13.125

DDR3 30% power consumption reduction compared to DDR2 1.5V supply voltage, compared to DDR2's 1.8V 90 nanometer fabrication technology Higher bandwidth 8 bit deep prefetch buffer (vs. 4 bit in DDR2 and 2 bit in DDR) Transfer data rate Effective clock rate of 800–1600 MHz using both rising and falling edges of a 400–800 MHz I/O clock DDR2: 400–800 MHz using a 200–400 MHz I/O clock DDR: 200–400 MHz based on a 100–200 MHz I/O clock DDR3 DIMMs 240 pins, the same number as DDR2, and are the same size Electrically incompatible, and have a different key notch location

Peak transfer rate (MB/s) Timings (CL-tRCD-tRP) DDR3 Standards Standard Name  Mem clock (MHz) I/O bus clock (MHz) I/O bus Cycle time (ns) Data rate (MT/s) Module name  Peak transfer rate (MB/s) Timings (CL-tRCD-tRP) CAS Latency (ns) DDR3-800 100 400 2.5 800 PC3-6400 6400 5-5-5 6-6-6 12 1⁄2 15 DDR3-1066 133⅓ 533⅓ 1.875 1066⅔ PC3-8500 8533⅓ 6-6-6 7-7-7 8-8-8 11 1⁄4 13 1⁄8 15 DDR3-1333 166⅔ 666⅔ 1.5 1333⅓ PC3-10600 10666⅔ 8-8-8 9-9-9 12 13 1⁄2 DDR3-1600 200 1.25 1600 PC3-12800 12800 9-9-9 10-10-10 11-11-11 11 1⁄4 12 1⁄2 13 3⁄4 DDR3-1866 233⅓ 933⅓ 1.07 1866⅔ PC3-14900 14933⅓ 11-11-11 12-12-12 11 11⁄14 12 6⁄7  DDR3-2133 266⅔ 0.9375 2133⅓ PC3-17000 17066⅔ 12-12-12 13-13-13 11 1⁄4  12 3⁄16

DDR2 vs. DDR3 Performance The high latency of DDR3 SDRAM has negative effect on streaming operations Source: xbitlabs

SRAM – Static RAM True random access High speed, low density, high power No refresh Address not multiplexed DDR SRAM 2 READs or 2 WRITEs per clock Common or Separate I/O DDRII: 200MHz to 333MHz Operation; Density: 18/36/72Mb+ QDR SRAM Two separate DDR ports: one read and one write One DDR address bus: alternating between the read address and the write address QDRII: 250MHz to 333MHz Operation; Density: 18/36/72Mb+

SRAM vs. DRAM Random Access: access time is the same for all locations DRAM – Dynamic RAM SRAM – Static RAM Refresh Refresh needed No refresh needed Address Address muxed: row+ column Address not multiplexed Access Not true “Random Access” True “Random Access” density High (1 Transistor/bit) Low (6 Transistor/bit) Power low high Speed slow fast Price/bit Typical usage Main memory cache

Read Only Memory (ROM) Random Access Non volatile ROM Types PROM – Programmable ROM Burnt once using special equipment EPROM – Erasable PROM Can be erased by exposure to UV, and then reprogrammed E2PROM – Electrically Erasable PROM Can be erased and reprogrammed on board Write time (programming) much longer than RAM Limited number of writes (thousands)

Hard Disks

Hard Disk Structure Rotating platters coated with a magnetic surface Track Sector Rotating platters coated with a magnetic surface Each platter is divided to tracks: concentric circles Each track is divided to sectors Smallest unit that can be read or written Disk outer tracks have more space for sectors than the inner tracks Constant bit density: record more sectors on the outer tracks speed varies with track location Moveable read/write head Radial movement to access all tracks Platter rotation to access all sectors Buffer Cache A temporary data storage area used to enhance drive performance Platters

Hard Disk Structure

Disk Access Seek: position the head over the proper track Average: Sum of the time for all possible seek / total # of possible seeks Due to locality of disk reference, actual average seek is shorter: 4 to 12 ms Rotational latency: wait for desired sector to rotate under head The faster the drives spins, the shorter the rotational latency time Most disks rotate at 5,400 to 15,000 RPM At 7200 RPM: 8 ms per revolution An average latency to the desired information is halfway around the disk At 7200 RPM: 4 ms Transfer block: read/write the data Transfer time is a function of: sector size, rotation speed, and recording density: bits per inch on a track Typical values: 100 MB / sec Disk Access Time = Seek time + Rotational Latency + Transfer time + Controller Time + Queuing Delay

EIDE Disk Interface EIDE, PATA, UltraATA, ATA 100, ATAPI: all the same interface Uses for connecting hard disk drives and CD/DVD drives 80-pin cable, 40-pin dual header connector 100 MB/s EIDE controller integrated with the motherboard EIDE controller has two channels Primary and a secondary, which work independently Two devices per channel: master and slave, but equal The 2 devices take turns in controlling the bus If there are two device on the system (e.g., a hard disk and a CD/DVD) It is better to put them on different channels Avoid mixing slower (DVD) and faster devices (HDD) on the same channel If doing a lot of copying from drive to drive Better performance by separating devices to separate channels

Disk Interface – Serial ATA (SATA) Point-to-point connection Dedicated BW per device (no sharing) No master/slave jumper configuration needed when a adding a 2nd SATA drive Increased BW SATA rev 1: 150 MB/sec SATA rev 2: 300 MB/sec SATA rev 3: 600 MB/sec Thinner (7 wires), flexible, longer cables Easier routing, easier installation, better reliability, improved airflow 1/6 the board area compared to EIDE connector 4 wires for signaling + 3 ground to minimize impedance and crosstalk Current HDDs still do not utilize SATA rev 3 BW HDD peak (not sustained) gets to 157 MB/s SSD gets to 250 MB/sec

Flash Memory Flash is a non-volatile, rewritable memory NOR Flash Supports per-byte data read and write (random access) Erasing (setting all the bits) done only at block granularity (64-128KB) Writing (clearing a bit) can be done at byte granularity Suitable for storing code (e.g. BIOS, cell phone firmware) NAND Flash Supports page-mode read and write (0.5KB – 4KB per page) Suitable for storing large data (e.g. pictures, songs) Similar to other secondary data storage devices  Reduced erase and write times Greater storage density and lower cost per bit

Flash Memory Principles of Operation Information is stored in an array of memory cells In single-level cell (SLC) devices, each cell stores one bit Multi-level cell (MLC) devices store multiple bits per cell using multiple levels of electrical charge Each memory cell is made from a floating-gate transistor Resembles a standard MOSFET, with two gates instead of one A control gate (CG), as in other MOS transistors, placed on top A floating gate (FG), interposed between the CG and the MOSFET channel The FG is insulated all around by an oxide layer  electrons placed on it are trapped Under normal conditions, will not discharge for many years When the FG holds a charge, it partially cancels the electric field from the CG Modifies the cell’s threshold voltage (VT): more voltage has to be applied to the CG to make the channel conduct Read-out: apply a voltage intermediate between the possible threshold voltages to the CG Test the channel's conductivity by sensing the current flow through the channel In a MLC device, sense the amount of current flow

Flash Write Endurance Typical number of write cycles Bad block management (BBM) Performed by the device driver software, or by a HW controller E.g., SD cards include a HW controller perform BBM and wear leveling Map logical block to physical block Mapping tables stored in dedicated flash blocks or Each block checked at power-up to create a bad block map in RAM Each write is verified, and block is remapped in case of write failure Memory capacity gradually shrinks as more blocks are marked as bad ECC compensates for bits that spontaneously fail 22 (24) bits of ECC code correct a one bit error in 2048 (4096) data bits If ECC cannot correct the error during read, it may still detect the error SLC MLC NAND flash 100K 1K – 3K NOR flash 100K to 1M

Flash Write Endurance (cont) Wear-leveling algorithms Evenly distribute data across flash memory and move data around Prevent from one portion to wear out faster than another SSD's controller keeps a record of where data is set down on the drive as it is relocated from one portion to another Dynamic wear leveling Map Logical Block Addresses (LBAs) to physical Flash memory addresses Each time a block of data is written, it is written to a new location Link the new block Mark original physical block as invalid data Blocks that never get written remain in the same location Static wear leveling Periodically move blocks which are not written Allow these low usage cells be used by other data

Solid State Drive – SSD Most manufacturers use "burst rate" for Performance numbers Not its steady state or average read rate Any write operation requires an erase followed by the write When SSD is new, NAND flash memory is pre-erased Consumer-grade multi-level cell (MLC) Allows ≥2 bit per flash memory cell Sustains 2,000 to 10,000 write cycles Notably less expensive than SLC drives Enterprise-class single-level cell (SLC) Allows 1 bit per flash memory cell Lasts 10× write cycles of an MLC The more write/erase cycle  the shorter the drive's lifespan Use wear-leveling algorithms to evenly distribute writes DRAM cache to buffer data writes to reduce number of write/erase cycles Extra memory cells to be used when blocks of flash memory wear out

SSD (cont.) Data in NAND flash memory organized in fixed size in blocks When any portion of the data on the drive is changed Mark block for deletion in preparation for the new data Read current data on the block Redistribute the old data Lay down the new data in the old block Old data is rewritten back Typical write amplification is 15 to 20 For every 1MB of data written to the drive, 15MB to 20MBs of space is actually needed Using write combining reduces write amplification to ~10% Flash drives compared to HD drives: Smaller size, faster, lighter, noiseless, lower power Withstanding shocks up to 2000 Gs (like 10 foot drop onto concrete) More expensive (cost/byte): ~2$/1GB vs ~0.1$/1GB in HDD

The Motherboard

Computer System Structure – 2009 External Graphics Card HDMI PCI express ×16 North Bridge (GMCH) LLC CPU BUS DDRII Channel 1 On-board Graphics Memory controller Core Mem BUS Core DDRII Channel 2 IO Controller South Bridge (ICH) PCI express ×1 USB controller IDE controller SATA controller PCI Sound Card Lan Adap Floppy Drive keybrd mouse Old DVD Drive Hard Disk Parallel Port Serial Port speakers LAN

Computer System – Nehalem External Graphics Card PCI express ×16 DDRIII Channel 1 North Bridge Cache CPU BUS Memory controller Mem BUS On-board Graphics Core HDMI DDRIII Channel 2 Core IO Controller South Bridge PCI express ×1 USB controller SATA controller SATA controller PCI Sound Card Lan Adap Floppy Drive keybrd mouse DVD Drive Hard Disk Parallel Port Serial Port speakers LAN

Computer System – Westmere External Graphics Card MCP – Multi Chip Package PCI express ×16 DDRIII Channel 1 North Bridge Memory controller Cache Mem BUS On-board Graphics Core DDRIII Channel 2 Core Display link HDMI IO Controller South Bridge (PCH) PCI express ×1 USB controller SATA controller SATA controller PCI Sound Card Lan Adap Floppy Drive keybrd mouse DVD Drive Hard Disk Parallel Port Serial Port speakers LAN

Computer System – Sandy Bridge External Graphics Card PCI express ×16 GFX Display link 2133-1066 MHz DDRIII Channel 1 Memory controller Cache System Agent 4×DMI Mem BUS Core DDRIII Channel 2 Core Line out Audio Codec Line in South Bridge (PCH) D-sub, HDMI, DVI, Display port S/PDIF out S/PDIF in exp slots BIOS PCI express ×1 LPC Super I/O Lan Adap USB SATA SATA Parallel Port Serial Port Floppy Drive PS/2 keybrd/ mouse mouse DVD Drive Hard Disk LAN

PCH Connections LPC (Low Pin Count) Bus Direct Media Interface (DMI) Supports legacy, low BW I/O devices Typically integrated in a Super I/O chip Serial and parallel ports, keyboard, mouse, floppy disk controller Other: Trusted Platform Module (TPM), Boot ROM Direct Media Interface (DMI) The link between an Intel north bridge and an Intel south bridge Replaces the Hub Interface DMI shares many characteristics with PCI-E Using multiple lanes and differential signaling to form a point-to-point link Most implementations use a ×4 link, providing 10Gb/s in each direction DMI 2.0 (introduced in 2011) doubles the BW to 20Gb/s with a ×4 link Flexible Display Interface (FDI) Connects the Intel HD Graphics integrated GPU with the PCH south bridge where display connectors are attached Supports 2 independent 4-bit fixed frequency links/channels/pipes at 2.7GT/s data rate

Motherboard Layout – 1st Gen Core2TM IEEE-1394a header PCI add-in card connector PCI express x1 connector PCI express x16 connector audio header Back panel connectors Processor core power connector Rear chassis fan header High Def. Audio header PCI add-in card connector LGA775 processor socket Parallel ATA IDE connector GMCH: North Bridge + integ GFX Speaker Processor fan header Front panel USB header DIMM Channel A sockets Serial port header DIMM Channel B sockets Diskette drive connector 4 × SATA connectors ICH: South Bridge + integ Audio Battery Main Power connector

Motherboard Layout (Sandy Bridge) IEEE-1394a header PCI add-in card connector PCI express x1 connector PCI express x16 connector Back panel connectors audio header High Def. Audio header Processor core power connector S/PDIF Rear chassis fan header LGA775 processor socket Processor fan header DIMM Channel A sockets Front panel USB headers DIMM Channel B sockets Front chassis fan header Chassis intrusion header Bios setup config jumper speaker Serial port header SATA connectors Main Power connector Rear chassis fan header Battery PCH

ASUS Sabertooth P67 B3 Sandy Bridge Motherboard

How to get the most of Memory ? Single Channel DDR Dual channel DDR Each DIMM pair must be the same Balance FSB and memory bandwidth 800MHz FSB provides 800MHz × 64bit / 8 = 6.4 G Byte/sec Dual Channel DDR400 SDRAM also provides 6.4 G Byte/sec L2 Cache CPU FSB – Front Side Bus Memory Bus DRAM Ctrlr DDR DIMM CH A DDR DIMM CH B L2 Cache CPU FSB – Front Side Bus DRAM Ctrlr

How to get the most of Memory ? Each DIMM supports 4 open pages simultaneously The more open pages, the more random access It is better to have more DIMMs n DIMMs: 4n open pages DIMMs can be single sided or dual sided Dual sided DIMMs may have separate CS of each side In this case the number of open pages is doubled (goes up to 8) This is not a must – dual sided DIMMs may also have a common CS for both sides, in which case, there are only 4 open pages, as with single side

Motherboard Back Panel USB 2.0 ports LAN port USB 3.0 ports eSATA DisplayPort HDMI DVI-I IEEE 1394A Rear Surround Line in Line out/ Front speakers S/PDIF Center / subwoofer Mic in / Side surround

System Start-up Upon computer turn-on several events occur: 1. The CPU "wakes up" and sends a message to activate the BIOS 2. BIOS runs the Power On Self Test (POST): make sure system devices are working ok Initialize system hardware and chipset registers Initialize power management Test RAM Enable the keyboard Test serial and parallel ports Initialize floppy disk drives and hard disk drive controllers Displays system summary information

System Start-up (cont.) 3. During POST, the BIOS compares the system configuration data obtained from POST with the system information stored on a memory chip located on the MB A CMOS chip, which is updated whenever new system components are added Contains the latest information about system components 4. After the POST tasks are completed the BIOS looks for the boot program responsible for loading the operating system Usually, the BIOS looks on the floppy disk drive A: followed by drive C: 5. After boot program is loaded into memory It loads the system configuration information contained in the registry in a Windows® environment, and device drivers 6. Finally, the operating system is loaded

Backup

Western Digital HDDs Caviar Green 2TB Caviar Blue 1TB Caviar Black 2TB Maximum external transfer rate 300MB/s Maximum sustained data rate 126MB/s 138MB/s Average rotational latency 4.2 ms Spindle speed 7,200 RPM Cache size 32MB 64MB Platter size 500GB Areal density 400 Gb/in² Available capacities 2TB Idle power 6.1W 8.2W Read/write power 6.8W 10.7W Idle acoustics 28 dBA 29 dBA Seek acoustics 33dBA 30-34 dBA

HDD Example Rotational Speed 7,200 RPM (nominal) Buffer Size 64 MB Performance Specifications   Rotational Speed 7,200 RPM (nominal) Buffer Size 64 MB Average Latency 4.20 ms (nominal) Load/unload Cycles 300,000 minimum Transfer Rates Buffer To Host (Serial ATA) 6 Gb/s (Max) Physical Specifications Formatted Capacity 2,000,398 MB Capacity 2 TB Interface SATA 6 Gb/s User Sectors Per Drive 3,907,029,168 Acoustics Idle Mode 29 dBA (average) Seek Mode 0 34 dBA (average) Seek Mode 3 30 dBA (average) Current Requirements Power Dissipation Read/Write 10.70 Watts Idle 8.20 Watts Standby 1.30 Watts Sleep

DDR Comparison DDR SDRAM Standard Bus clock (MHz) Internal rate (MHz) Prefetch (min burst) Transfer Rate (MT/s) Voltage DIMM pins DDR  100–200  100–200 2n 200–400  2.5 184 DDR2 200–533  100–266 4n 400–1066 1.8 240 DDR3 8n 800–2133 1.5

Attribute or characteristic SSD vs HDD Attribute or characteristic Solid-state drive Hard disk drive Random access time[57] ~0.1 ms 5–10 ms Consistent read performance[61] Read performance does not change based on where data is stored on an SSD If data is written in a fragmented way, reading back the data will have varying response times Fragmentation Non-issue due Files may fragment; periodical defragmentation is required to maintain ultimate performance. Acoustic levels SSDs have no moving parts and make no sound Mechanical reliability No moving part Maintenance of temperature Less heat Susceptibility to environmental factors No flying heads or rotating platters to fail as a result of shock, altitude, or vibration Magneticsusceptibility No impact on flash memory Magnets or magnetic surges can alter data on the media Weight and size very light compared to HDDs Parallel operation Some flash controllers can have multiple flash chips reading and writing different data simultaneously HDDs have multiple heads (one per platter) but they are connected, and share one positioning motor. Write longevity Flash-based SSDs have a limited number of writes (1-5 million or more) over the life of the drive. Magnetic media do not have a similar limited number of writes but are susceptible to eventual mechanical failure. Cost per capacity $.90–2.00 per GB $0.05/GB for 3.5 in and $0.10/GB for 2.5 in drives Storage capacity Typically 4-256GB typically up to 1 – 2 TB Read/write performance symmetry Less expensive SSDs typically have write speeds significantly lower than their read speeds. Higher performing SSDs have a balanced read and write speed. HDDs generally have slightly lower write speeds than their read speeds. Free block availability and TRIM SSD write performance is significantly impacted by the availability of free, programmable blocks. Previously written data blocks that are no longer in use can be reclaimed by TRIM; however, even with TRIM, fewer free, programmable blocks translates into reduced performance.[29][75][76] HDDs are not affected by free blocks or the operation (or lack) of the TRIM command Power consumption High performance flash-based SSDs generally require 1/2 to 1/3 the power of HDDs High performance HDDs require 12-18 watts; drives designed for notebook computers are typically 2 watts.

Raw bandwidth (Mbit/s) PC Connections Name Raw bandwidth (Mbit/s) Transfer speed (MB/s) Max. cable length (m) Power provided Devices per channel eSATA 3,000 300 2 with eSATA HBA (1 with passive adapter) No 1 (15 with port multiplier) eSATAp 5 V/12 V[33] SATA revision 3.0 6,000 600[34] 1 SATA revision 2.0 SATA revision 1.0 1,500 150[35] 1 per line PATA 133 1,064 133.5 0.46 (18 in) 2 SAS 600 600 10 1 (>65k with expanders) SAS 300 SAS 150 150 IEEE 13943200 3,144 393 100 (more with special cables) 15 W, 12–25 V 63 (with hub) IEEE 1394800 786 98.25 100[36] IEEE 1394400 49.13 4.5[36][37] USB 3.0* 5,000 400[38] 3[39] 4.5 W, 5 V 127 (with hub)[39] USB 2.0 480 60 5[40] 2.5 W, 5 V USB 1.0 12 1.5 3 Yes SCSI Ultra-640 5,120 640 15 (plus the HBA) SCSI Ultra-320 2,560 320 Fibre Channel over optic fibre 10,520 1,000 2–50,000 126 (16,777,216 with switches) Fibre Channel over copper cable 4,000 400 InfiniBand Quad Rate 10,000 5 (copper)[41][42]<10,000 (fiber) 1 with point to point Many withswitched fabric Thunderbolt 1,250 100 10 W 7