Chapter 8 computer memory system overview
- 1. Chapter 8 COMPUTER MEMORY SYSTEM OVERVIEW
- 2. COMPUTER MEMORY SYSTEM OVERVIEW • Characteristics of Memory Systems – Location : refers to whether memory is internal or external to the computer. • Internal (e.g. processor registers, cache, main memory) • External (e.g. optical disks, magnetic Disks, tapes) any memory accessible to the processor via I/O controllers – Capacity • Internal memory Common word lengths are 8, 16, and 32 bits. (Bytes/MB) • External memory capacity is typically expressed in terms of bytes. (GB)
- 3. Cont.. • Unit of Transfer – Word: • The size of the word is typically equal to the number of bits used to represent an integer and to the instruction length. – Addressable units: • In some systems, the addressable unit is the word. However, many systems allow addressing at the byte level. – Unit of transfer: • For main memory, this is the number of bits read out of or written into memory at a time. The unit of transfer need not equal a word or an addressable unit – Block (For external memory, data are often transferred in much larger units than a word, and these are referred to as blocks.) – Word (For internal memory, the unit of transfer is equal to the number of electrical lines into and out of the memory module.) 3
- 4. Cont.. – Access Method • Sequential – Memory is organized into units of data, called records. – Access must be made in a specific linear sequence. – Stored addressing information is used to separate records . – A shared read–write mechanism is used, and this must be moved from its current location to the desired location, passing and rejecting each intermediate record. – Thus, the time to access an arbitrary record is highly variable. » E.g tape units 4
- 5. Cont.. • Direct – As with sequential access, direct access involves a shared read–write mechanism. – However, individual blocks or records have a unique address based on physical location. – Access is accomplished by direct access to reach a general vicinity plus sequential searching to reach the final location. – Again, access time is variable. » Disk units 5
- 6. Cont.. • Random – Each addressable location in memory has a unique, physically wired-in addressing mechanism. – The time to access a given location is independent of the sequence of prior accesses and is constant. – Thus, any location can be selected at random and directly addressed and accessed. » E.g: Main memory and some cache systems are random access. • Associative – A word is retrieved based on a portion of its contents rather than its address. – Cache memories may employ associative access. 6
- 7. Cont.. – Performance ( 3 parameters used) • Access time: – For random-access memory, this is the time it takes to perform a read or write operation. – For non-random-access memory, access time is thetime it takes to position the read–write mechanism at the desired location. • Cycle time: – This concept is primarily applied to random-access memory and consists of the access time plus any additional time required before a second access can commence. – Note that memory cycle time is concerned with the system bus, not the processor. • Transfer rate: – This is the rate at which data can be transferred into or out of a memory unit.
- 8. Cont.. – Physical Type • Semiconductor (the most common) • Magnetic • Optical • Magneto-optical – Physical Characteristics • Volatile/non-volatile • Erasable/non-erasable – Organization • By organization is meant the physical arrangement of bits to form words. 8
- 10. Semiconductor Memory • RAM – Misnamed as all semiconductor memory is random access – Read/Write – Volatile – Temporary storage – Static or dynamic
- 12. 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
- 13. 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
- 14. 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
- 15. Read Only Memory (ROM) • Permanent storage – Nonvolatile • Microprogramming • Library subroutines • Systems programs (BIOS) • Function tables
- 16. Types of ROM • Written during manufacture – Very expensive for small runs • Programmable (once) – PROM – Needs special equipment to program • Read “mostly” – Erasable Programmable (EPROM) • Erased by UV – Electrically Erasable (EEPROM) • Takes much longer to write than read – Flash memory • Erase whole memory electrically
- 17. 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)
- 18. RAMBUS • Adopted by Intel for Pentium & Itanium • Main competitor to SDRAM • Vertical package – all pins on one side • Data exchange over 28 wires < 12 cm long • Bus addresses up to 320 RDRAM chips at 1.6Gbps • Asynchronous block protocol – 480ns access time – Then 1.6 Gbps
- 19. DDR SDRAM • SDRAM can only send data once per clock • Double-data-rate SDRAM can send data twice per clock cycle – Rising edge and falling edge
- 20. QUIZ • Classify memory by – Location – Access method – Capacity – Unit transfer • And give examples • Define – Access time – Cycle time – Transfer rate • Which of the following memory types are more fast and which are more capacity – Main memory – Registers – Cache – Hard disk