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.
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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
