Computer architecture memory system
- 2. Outline • Microcomputer Memory -characteristics of memory system -location -capacity -unit of transfer • Access method • Performance • Physical type • Memory Hierarchies • Internal and external memory • RAM-Random access memory • ROM-Read only memory • Memory latency and bandwidth • Memory interleaving • Memory management
- 3. Microcomputer Memory • Memory is an essential component of the microcomputer system. • It stores binary instructions and datum for the microcomputer. • The memory is the place where the computer holds current programs and data that are in use. • None technology is optimal in satisfying the memory requirements for a computer system. • Computer memory exhibits perhaps the widest range of type, technology, organization, performance and cost of any feature of a computer system. • The memory unit that communicates directly with the CPU is called main memory. • Devices that provide backup storage are called auxiliary memory or secondary memory
- 4. Memory Hierarchies • Some fundamental and enduring properties of hardware and software: • Fast storage technologies cost more per byte and have less capacity • Gap between CPU and main memory speed is widening • Well-written programs tend to exhibit good locality • These fundamental properties complement each other beautifully • They suggest an approach for organizing memory and storage systems known as a memory hierarchy
- 6. Characteristics of memory systems • The memory system can be characterised with their Location, Capacity, Unit of transfer, Access method, Performance, Physical type, Physical characteristics, Organisation.
- 7. Location • Processor memory: The memory like registers is included within the processor and termed as processor memory. • • Internal memory: It is often termed as main memory and resides within the CPU. • • External memory: It consists of peripheral storage devices such as disk and magnetic tape that are accessible to processor via i/o controllers Capacity • Word size: Capacity is expressed in terms of words or bytes. — The natural unit of organisation • Number of words: Common word lengths are 8, 16, 32 bits etc. — or Byte
- 8. Unit of Transfer • Internal: For internal memory, the unit of transfer is equal to the number of data lines into and out of the memory module. • External: For external memory, they are transferred in block which is larger than a word. • Addressable unit — Smallest location which can be uniquely addressed — Word internally — Cluster on Magnetic disks
- 9. Access Method • Sequential access: In this access, it must start with beginning and read through a specific linear sequence. This means access time of data unit depends on position of records (unit of data) and previous location. — e.g. tape • Direct Access: Individual blocks of records have unique address based on location. Access is accomplished by jumping (direct access) to general vicinity plus a sequential search to reach the final location. — e.g. disk • Random access: The time to access a given location is independent of the sequence of prior accesses and is constant. Thus any location can be selected out randomly and directly addressed and accessed. — e.g. RAM • Associative access: This is random access type of memory that enables one to make a comparison of desired bit locations within a word for a specified match, and to do this for all words simultaneously. • e.g. cache
- 10. Performance • Access time: For random access memory, access time is the time it takes to perform a read or write operation i.e. time taken to address a memory plus to read / write from addressed memory location. Whereas for non-random access, it is the time needed to position read / write mechanism at desired location. — Time between presenting the address and getting the valid data • Memory Cycle time: It is the total time that is required to store next memory access operation from the previous memory access operation. Memory cycle time = access time plus transient time (any additional time required before a second access can commence). — Time may be required for the memory to “recover” before next access — Cycle time is access + recovery • Transfer Rate: This is the rate at which data can be transferred in and out of a memory unit. — Rate at which data can be moved — For random access, R = 1 / cycle time — For non-random access, Tn = Ta + N / R; where Tn – average time to read or write N bits, Ta – average access time, N – number of bits, R – Transfer rate in bits per second (bps).
- 11. Physical Types • Semiconductor — RAM • Magnetic — Disk & Tape • Optical — CD & DVD • Others — Bubble — Hologram
- 12. Physical Characteristics • Decay: Information decays mean data loss. • Volatility: Information decays when electrical power is switched off. • Erasable: Erasable means permission to erase. • Power consumption: how much power consumes? Organization • Physical arrangement of bits into words • Not always obvious - e.g. interleaved
- 13. Memory latency and bandwidth • The latency (L) of the memory is the delay from when the processor first requests a word from memory until • that word arrives and is available for use by the processor. The latency of a memory system is one attribute of • performance • The other is bandwidth (BW), which is the rate aat which information can be transferred from the • memory system. The bandwidth and the latency are closely related. If R is the number of requests that the memory • can service simultaneously, then: BW=R/L
- 14. Memory latency and bandwidth • From Eq. (1) we see that a decrease in the latency will result in an increase in bandwidth, and vice versa, if R • is unchanged. We can also see that the bandwidth can be increased by increasing R, if L does not increase proportionately. • For example, we can build a memory system that takes 20 ns to service the access of a single 32-bit • word. Its latency is 20 ns per 32-bit word, and its bandwidth is : 32bits/20*10sec
- 15. Internal and External memory • Internal or Main Memory The main memory is the central unit of the computer system. It is relatively large and fast memory to store programs and data during the computer operation. These memories employ semiconductor integrated circuits. The basic element of the semiconductor memory is the memory cell. • The memory cell has three functional terminals which carries the electrical signal. • o The select terminal: It selects the cell. • o The data in terminal: It is used to input data as 0 or 1 and data out or sense terminal is used for the output of the cell's state. • o The control terminal: It controls the function i.e. it indicates read and write. • Most of the main memory in a general purpose computer is made up of RAM integrated circuits chips, but a portion of the memory may be constructed with ROM chips
- 16. External Memory Types • HDD • Magnetic Disk(s) • SDD (Solid State Disk(s)) • Optical • CD-ROM • CD-Recordable (CD-R) • CD-R/W • DVD • Magnetic Tape
- 17. Internal and External memory
- 18. RAM– Random Access memory • Memory cells can be accessed for information transfer from any desired random location. • The process of locating a word in memory is the same and requires of locating a word in memory is the same and requires an equal amount of time no matter where the cells are located physically in memory thus named 'Random access'. • Integrated RAM are available in two possible operating modes, Static and Dynamic
- 19. Types of RAM • Dynamic RAM (DRAM) – are like leaky capacitors; initially data is stored in the DRAM chip, charging its memory cells to maximum values. The charge slowly leaks out and eventually would go to low to represent valid data; before this happens, a refresh circuitry reads the contents of the DRAM and rewrites the data to its original locations, thus restoring the memory cells to their maximum charges • Static RAM (SRAM) – is more like a register; once the data has been written, it will stay valid, it doesn’t have to be refreshed. Static RAM is faster than DRAM, also more expensive. Cache memory in PCs is constructed from SRAM memory
- 20. Dynamic RAM
- 22. Static RAM • Uses flip flop to store information • Needs more space • Faster, digital device • Expensive, big in size • Don't require refreshing circuit • Used in cache memory Dynamic RAM • Uses capacitor to store information • More dense i.e. more cells can be accommodated per unit area • Slower, analog device • Less expensive, small in size • Needs refreshing circuit • Used in main memory, larger memory units
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