Memory Organization: A Comprehensive Guide to Memory Hierarchy and Virtual Memory, Thesis of Computer Networks

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Memory Organization

UNIT 3

Memory Hierarchy registers onchip L cache (SRAM) main memory (DRAM) local secondary storage (local disks) Larger, slower, and cheaper (per byte) storage devices remote secondary storage (distributed file systems, Web servers) Local disks hold files retrieved from disks on remote network servers Main memory holds disk blocks retrieved from local disks offchip L cache (SRAM) L1 cache holds cache lines retrieved from the L2 cache memory CPU registers hold words retrieved from L1 cache L2 cache holds cache lines retrieved from main memory L0: L1: L2: L3: L4: L5: Smaller, faster, and costlier (per byte) storage devices

Memory Hierarchy in computer system

Main Memory

Random Access Memory (RAM)

• (^) The principal technology used for the main memory

is based on semiconductor integrated circuits.

Integrated circuit RAM chips are available in two

possible operating modes, static and dynamic.

• (^) The static RAM consists essentially of internal flip-

flops that store the binary information. The stored

information remains valid as long as power is applied

to the unit.

• (^) The dynamic RAM stores the binary information in

the form of electric charges that are applied to

capacitors.

RAM Chips

• (^) The capacity of the memory is 128 words of eight bits (one byte) per word. This requires a 7-bit address and an 8-bit bidirectional data bus. The read and write inputs specify the memory operation and the two chips select (CS) control inputs are for enabling the chip only when it is selected by the microprocessor. The availability of more than one control input to select the chip facilitates the decoding of the address lines when multiple chips are used in the microcomputer. The read and write inputs are sometimes combined into one line labelled R/W. When the chip is selected, the two binary states in this line specify the two operation of read or write.

Typical ROM chip

Memory Address Map

• (^) The addressing of memory can be

established by means of a table that

specifies the memory address assigned to

each chip. The table, called a memory

address map , is a pictorial

representation of assigned address space

for each chip in the system.

• (^) To demonstrate with a particular example,

assume that a computer system needs

512 bytes of RAM and 512 bytes of ROM.

Auxiliary Memory Magnetic Disks

• (^) A magnetic disk is a circular plate constructed of metal or plastic coated with magnetized material. Often both sides of the disk are used and several disks may be stacked on one spindle with read/write heads available on each surface. All disks rotate together at high speed and are not stopped or started for access purposes. Bits are stored in the magnetized surface in spots along concentric circles called tracks. The tracks are commonly divided into sections called sectors. In most systems, the minimum quantity of information which can be transferred is a sector.

Magnetic Tape A magnetic tape transport consists of the electrical, mechanical, and electronic components to provide the parts and control mechanism for a magnetic-tape unit. The tape itself is a strip of plastic coated with a magnetic recording medium. Bits are recorded as magnetic spots on the tape along several tracks. Usually, seven or nine bits are recorded simultaneously to form a character together with a parity bit. Read/write heads are mounted one in each track so that data can be recorded and read as a sequence of characters.

Associative Memory

• (^) A memory unit accessed by content is called an associative memory or content addressable memory (CAM). This type of memory is accessed simultaneously and in parallel on the basis of data content rather than by specific address or location.

Cache Memory

• (^) Analysis of a large number of typical programs has shown that the references to memory at any given interval of time tend to be confined within a few localized areas in memory. This phenomenon is known as the property of locality of reference locality of reference.
• (^) The performance of cache memory is frequently measured in terms of a quantity called hit ratio. When the CPU refers to memory and finds the word in cache, it is said to produce a hit. If the word is not found in cache, it is in main memory and it counts as a miss. The ratio of the number of hits divided by the total CPU references to memory (hits plus misses) is the hit ratio.