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MEMORY

MEMORY. Combinatorial Logic vs. Sequential Logic. A circuit is said to be “combinatorial” if its output is determined solely by its inputs Such a circuit can be realized in terms of logical gates (ANDs, ORs and NOTs)

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MEMORY

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  1. MEMORY

  2. Combinatorial Logic vs. Sequential Logic • A circuit is said to be “combinatorial” if its output is determined solely by its inputs • Such a circuit can be realized in terms of logical gates (ANDs, ORs and NOTs) • A circuit is said to be “sequential” if its output is in part determined by its history • History dependence here means bits (high or low states) that were previously stored by the circuit • If the sole purpose of the circuit is to store values, it is called memory

  3. Reading and Writing • The action of placing a value in a memory circuit (a.k.a. a cell) is called writing • Putting a high value into a cell is referred to as “setting” • Putting a low value into a cell is referred to as “resetting” • Accessing the value in a memory cell without affecting it is called reading • The CPU is the subject; the memory is the object • The CPU writes to memory • The CPU reads from memory

  4. ROM • Read Only Memory • as the name suggests the user only reads this type of memory; the writing is typically done by the manufacturer • ROM is nonvolatile, that is, a supply of energy is not required to keep the data intact • ROM is not lost when the computer is turned off.

  5. Using ROM • Most personal computers contain a small amount of ROM that stores critical programs such as the program that boots the computer. In addition, ROMs are used extensively in calculators and peripheral devices such as laser printers, whose fonts are often stored in ROMs.

  6. Types of ROM • ROM:data is written by manufacturer • PROM:programmable read-only memory, data is written by user • PROMs are manufactured as blank chips on which data can be written with a special device called a PROM programmer (a.k.a. burner) • “Burning a CD”

  7. Types of PROM • PROM usually implies “write once” • EPROM erasable programmable read only memory data can be erased and rewritten • Some use ultraviolet (UV) light, requires removing it from the computer • EEPROM, electrically erasable programmable read-only memory, is erased by electronic means • Does not have to be removed from comuter

  8. Uses Of ROM • Microcode portion of CPU • The lowest-level instructions that directly control a microprocessor. • The “plus” instruction will be stored in a program in RAM; the code that tells the hardware what to do with a plus is in ROM • Basic Input/Output System. • Code burned into motherboard’s ROM; it handles interfacing between the operating system and peripherals (IO)

  9. RAM • RAM Random Access Memory • Data can be accessed (read) • Sequentially: you start at the beginning and go through the sequence of locations • Like a cassette or video tape • Randomly: you can jump to any location without proceeding through all previous locations • Like a CD or DVD (no need for fast-forwarding, rewinding, etc.) • ROM is random access, but the term RAM refers to volatile memory, memory requiring an energy source

  10. Types of RAM • SRAM: Static RAM • The objective of memory is to hold a value, so being static is good. The value is held without the need for “refreshing” • Fast but expensive • DRAM: Dynamic RAM • The value must be “refreshed” or it will become lost or corrupted • Slow but cheap

  11. Types of DRAM • Synchronous DRAM actually synchronizes itself with the CPU's bus • Deals with data in “bursts” so it doesn’t have to go through the addressing process of successive data locations • EDO (extended data out) • VRAM (video) a special form of memory that allows simultaneous reads and writes. • It provides a serial read interface and a parallel write interface. The advantage of using VRAM is that it’s much faster and doesn’t require as much detection code on the part of the application or device driver

  12. Types of DRAM (cont.) • WRAM (windows) • FPM (fast paging mode) • Rambus DRAM A type of memory that uses fast-and-thin bus interface to provide bandwidth that is double that of SDRAM.

  13. Cache • A generic term used to describe guessing what data will be used next and placing it where it can be accessed more quickly • A cache hit is a correct guess • A cache miss is an incorrect guess • Memory Cache uses SRAM which is faster than DRAM • L1: SRAM that is right on the microprocessor chip • L2: not on the chip

  14. Cache • When writing to cache, there are different scenarios • Write back: updates cache first and memory later (faster) • Write through: updates cache and memory together (slower but maintains consistency)

  15. Error Detection • Parity • Odd parity: add an extra bit to a word and insist that the total number of 1’s be odd • Even parity, ditto but number of 1’s even • Example of odd parity

  16. Vertical parity

  17. Error Correction Code • If a parity check reveals a parity error, one can request data be resent • Error Correction Code: in addition to detecting a mistake can help fix it • Hamming code • CRC (cyclic redundancy check) • checksum

  18. Virtual memory • What if there’s not enough memory for a program? • The operating system has a book-keeping scheme called virtual memory • It assigns the program virtual addresses • At any given time, only a part of the program is really in memory • The operating system keeps track of the correspondence between virtual addresses and real addresses

  19. Virtual memory (cont.) • As program runs different parts of the program need to be in the real memory • The operating systems breaks the code into pieces called pages • Putting the new part of the code into real memory is called swapping or paging • Swapping is slow; it’s why adding memory might improve speed

  20. Shadowing • Putting parts of the ROM BIOS which can be slow into RAM which can improve speed

  21. Memory Packaging • SIPP: single inline chip packages • Little circuit board with memory chip • Has pins, hard to install • SIMM single inline memory module • Like SIPP but no pins, easier to install • a 32-bit path to the memory • DIMM dual in-line memory module, • a 64-bit path • Because Pentium requires a 64-bit path to memory, you install SIMMs two at a time, but DIMMs one at a time.

  22. Bank • The block of system memory that can be read by the CPU in a single clock cycle. The data width of a bank of memory must match that of the motherboard bus. For example, on a 64-bit Pentium motherboard, two 32-bit SIMMs compose a single bank

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