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05 Internal Memory William Stallings Computer Organization and Architecture 8th Edition Chapter 5 Internal Memory Semiconductor Memory Types Memory Type Category Erasure Write Mechanism Volatility Random-access Read-write memory Electrically, byte-level Electrically Volatile memory (RAM) Read-only Masks memory (ROM) Read-only memory Not possible Programmable ROM (PROM) Erasable PROM UV light, chip-level (EPROM) Nonvolatile Electrically Electrically Erasable Read-mostly memory Electrically, byte-level PROM (EEPROM) Flash memory Electrically, block-level Semiconductor Memory • RAM —Misnamed as all semiconductor memory is random access —Read/Write —Volatile —Temporary storage —Static or dynamic Memory Cell Operation Dynamic RAM • Bits stored as charge in capacitors • Charges leak • Need refreshing even when powered • Simpler construction • Smaller per bit • Less expensive • Need refresh circuits • Slower • Main memory • Essentially analogue —Level of charge determines value DRAM Cell Structure DRAM Operation • Address line active when bit read or written —Transistor switch closed (current flows) • Write —Voltage to bit line – High for 1 low for 0 —Then signal address line – Transfers charge to capacitor • Read —Address line selected – transistor turns on —Charge from capacitor fed via bit line to sense amplifier – Compares with reference value to determine 0 or 1 —Capacitor charge must be restored Static RAM • Bits stored as on/off switches • No charges to leak • No refreshing needed when powered • More complex construction • Larger per bit • More expensive • Does not need refresh circuits • Faster • Cache • Digital —Uses flip-flops SRAM Cell Structure Static RAM Operation • Transistor arrangement gives stable logic state • State 1 —C1 high, C2 low —T1 T4 off, T2 T3 on • State 0 —C2 high, C1 low —T2 T3 off, T1 T4 on • Address line transistors T5 T6 is switch • Write – apply value to B & compliment to B • Read – value is on line B SRAM v DRAM • Both volatile —Power needed to preserve data • Dynamic cell —Simpler to build, smaller —More dense —Less expensive —Needs refresh —Larger memory units • Static —Faster —Cache Read Only Memory (ROM) • Permanent storage —Nonvolatile • Used to store: —Microprogramming (see later) —Library subroutines —Systems programs (e.g. BIOS) —Function tables Types of ROM • Written during manufacture, a.k.a. ROM —Very expensive for small runs • Programmable (once) —PROM —Needs special equipment to program • Read ―mostly‖ —Erasable Programmable (EPROM) – Erased by UV – One transistor/bit high density —Electrically Erasable (EEPROM) – Takes much longer to write (100 s) than read (100 ns) – Can read and write individual Bytes – 3 transistors/bit + ―charge pump‖ low density Types of ROM —Flash memory: in between EPROM and EEPROM – Erase whole memory or one block electrically – Cannot erase individual Bytes – One transistor/bit high density Although it’s FGMOS, not the usual MOS Source and explanations: http://www.techquark.com/2009/12/inside-flash-memory-and-drives.html Memory Organisation vs. Architecture Example architecture: • The programmer wants a total of 16 Mbit (= 2 MB) of memory, with a word size of 16 bit This could be organized in two extreme ways: • The word-per-chip system: One chip, containing 1 Million (220) 16-bit words • The bit-per-chip system: 16 chips, each containing 1 Million 1-bit words —Accessing a word means accessing (in parallel!) its 16 bits, one from each chip Memory Organisation vs. Architecture ―Compromise‖ organisation: 16Mbit chip organised as a 2048 x 2048 x 4bit array Important ―trick‖: reduce number of address pins by multiplexing —Multiplex row address and column address —11 pins to address (211=2048) —What happens if a twelfth address pin is added? → memory chip capacities generally grow by factors of 4 Simplified DRAM Read Timing (p.181 of our text) DRAM Read timing diagram Typical 16 Mb DRAM (4M x 4) Refreshing Modern chips are ―self-refresh‖, i.e. all refresh circuits are included on chip. • Older chips depend (partially) on memory controller There are several refresh methods. Simplest is RAS-only: • Disable chip • Count through rows and activate RAS • Read & Write back (entire row at once!) • In any case, refresh takes time → Slows down the external accesses Explain how to connect Packaging CE for 2 and 4 chips How is the function of WE and OE performed on the EPROM chip? Putting the memory chips together in a module Example: 256 kByte Module made of 256 k x 1 chips One level higher: 1MByte super-module Each column is a module from prev. slide Interleaved Memory A collection of DRAM chips can be: • A memory module (as on prev. slides) • A memory bank • In a bank, consecutive words are stored on different chips • K banks can service k requests (of consecutive words!) simultaneously —As in the case of caches, locality of reference makes requests of consecutive words worthwhile —What is being read may well be a block, going in a line of cache! This concludes section 5.1 This is the material required for the midterm To solve in notebook for next time: End of chapter problems 5.2 and 5.3. Next time: • Review in class • Exam in the lab • Lost one lecture due to Dr. appt. 5.2 Error Correction Story from the early days • Hard Failure of the Univac … —Permanent physical defect —Bit is ―stuck‖ at 0 or 1, or switches randomly • Soft Error —Random, non-destructive —No permanent damage to memory • Two types of codes: —Only error detection Easy to implement using XOR gates! – E.g. even or odd parity bit – E.g. ―Excess nine‖ —Error detection and correction – E.g. Hamming code Error Detection and Correction Syndrome word Redundant bits Generates an even-parity bit! XOR gate Draw the symbol and the truth table for a 3-input XOR 4-bit example, using Venn diagram Original 4 bits Adding K=3 An error occurs. parity bits How can we detect (even parity) and correct it? Source: http://www.apl.ucl.ac.uk/lectures/3c42/3c42-13.html Go to the link for more examples and a nice discussion about possible sources of errors! 8-bit example • Problem: Venn diagrams with 4 or more bits become progressively harder to visualize • Idea: Insert the parity bits in the word itself, in those positions which are powers of two —Unlike normal bit numbering, start with position 1 Bit Position 12 11 10 9 8 7 6 5 4 3 2 1 Position 1100 1011 1010 1001 1000 0111 0110 0101 0100 0011 0010 0001 Number Data Bit D8 D7 D6 D5 D4 D3 D2 D1 Check Bit C8 C4 C2 C1 Figure 5.9 Layout of Data Bits and Check Bits • FYI: on pp. 170-171 it is explained how to figure out the exact number of parity bits for a code that corrects 1 error. • However, the method of construction of the Hamming code explained on the prev. slide already has this number built-in automatically! —We simply stop inserting bits when we’ve run out of original bits Back to our 8-bit example … Bit 12 11 10 9 8 7 6 5 4 3 2 1 position Position 1100 1011 1010 1001 1000 0111 0110 0101 0100 0011 0010 0001 number Data bit D8 D7 D6 D5 D4 D3 D2 D1 Check C8 C4 C2 C1 bit Word 0 0 1 1 0 1 0 0 1 1 1 1 stored as Word fetched 0 0 1 1 0 1 1 0 1 1 1 1 as Position 1100 1011 1010 1001 1000 0111 0110 0101 0100 0011 0010 0001 Number Check 0 0 0 1 Bit Figure 5.10 Check Bit Calculation Each check bit is calculated as an even-parity bit over those data bits whose position numbers contain the respective power of 2 (it’s a lot easier in binary!) 8-bit example (continued) Compare new and old check bits to create syndrome • Syndrome word is 0110 • This is the binary code of position 6 where the error has occurred! SEC vs. SEC-DED The price to pay: capacity overhead Single-Error Correction/ Single-Error Correction Double-Error Detection Data Bits Check Bits % Increase Check Bits % Increase 8 4 50 5 62.5 16 5 31.25 6 37.5 32 6 18.75 7 21.875 64 7 10.94 8 12.5 128 8 6.25 9 7.03 256 9 3.52 10 3.91 Table 5.2 Increase in Word Length with Error Correction/Detection SEC-DED is considered adequate for memory organizations with 1 bit per chip Can you think of a reason why it wouldn’t be adequate if there are more bits per chip? (Hint: Errors generally come in localized bursts) 3.3 Advanced DRAM Organization • Basic DRAM same since first RAM chips • New ideas: —Synchronous DRAM —Enhanced DRAM – Contains small SRAM used as a prefetch buffer —Cache DRAM – Larger SRAM component, used as cache or serial buffer Synchronous DRAM (SDRAM) • Access is synchronized with an external clock • Address is presented to RAM • RAM finds data (CPU waits in conventional DRAM) • Since SDRAM moves data in time with system clock, CPU knows when data will be ready • CPU does not have to wait, it can do something else • Burst mode allows SDRAM to set up stream of data and fire it out in block • DDR-SDRAM sends data twice per clock cycle (leading & trailing edge) Table 5.4 SDRAM Pin Assignments (The ones with bar are active low) A0 to A13 Address inputs How large is this CLK Clock input memory chip? CKE Clock enable CS Chip select Row address strobe RAS Column address strobe CAS WE Write enable DQ0 to DQ7 Data input/output DQM Data mask SDRAM Read Timing IBM 64 Mb SDRAM We have covered pp.169-176. Read and take notes! Solve problem 5.10! Lab week 7 End of chapter problems • 2 • 3 • 9 • 10 • 11 Quiz Why does the text say that the latency here is 2 cycles? Doesn’t it look more like 3? Check the explanation at the bottom of p.176.
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