Chapter 4_The Memory System and Hierarchy

location

refers to whether memory is internal or external to the computer.

Internal

• E.g., processor registers, cache, main memory

• often equated with main memory

Cache

another form of internal memory

External

• E.g., optical disks, magnetic disks, tapes

• Consists of peripheral storage devices, such as disk and tape, that are accessible to the processor via I/O controller

Capacity

• For internal memory, this is typically expressed in terms of bytes (1 byte = 8 bits) or words.

• Common word lengths are 8, 16, and 32 bits.

• External memory capacity is typically expressed in terms

• of bytes.

• Number of words

• Number of bytes

unit of transfer

• For internal memory, this is equal to the number of electrical lines into and out of the memory module.

• For main memory, this is the number of bits read out of or written into memory at a time.

Word

• The “natural” unit of organization of memory

• is typically equal to the number of bits used to represent an integer and to the instruction length.

Addressable Units

In some systems, it is the word. However, many systems allow addressing at the byte level.

blocks

For external memory, data are often transferred in much larger units than a word, and these are referred to as _____

Records

Units of data where memory is organized.

Sequential Access

• Access must be made in a specific linear sequence.

• Stored addressing information is used to separate records and assist in the retrieval process.

• 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.

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 this to reach a general vicinity plus sequential searching, counting, or waiting to reach the final location.

Random access

• 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.

• Main memory and some cache systems are random access.

Associative

• This is a 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.

• Thus, a word is retrieved based on a portion of its contents rather than its address.

• As with ordinary random-access memory, each location has its own addressing mechanism, and retrieval time is constant independent of location or prior access patterns.

• Cache memories may employ this.

Access time (latency):

• For random-access memory, this is the time it takes to perform a read or write operation, that is, the time from the instant that an address is presented to the memory to the instant that data have been stored or made available for use.

• For non-random-access memory, this is the time it takes to position the read–write mechanism at the desired location.

Memory 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.

• This additional time may be required for transients to die out on signal lines or to regenerate data if they are read destructively.

• Note that it 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.

• For random-access memory, it is equal to 1/(cycle time).

Volatile Memory

information decays naturally or is lost when electrical power is switched off.

Nonvolatile Memory

information once recorded remains without deterioration until deliberately changed; no electrical power is needed to retain information.

Magnetic-surface memories

are nonvolatile

Semiconductor memory

(memory on integrated circuits) may be either volatile or non-volatile.

Nonerasable memory

cannot be altered, except by destroying the storage unit.

read-only memory (ROM)

Semiconductor memory of this type (non-erasable) is known as _____

memory modules

organization (1)

organization

• For random-access memory, it is a key design issue.

• refers to the physical arrangement of bits to form words.

Cache memory

is designed to combine the memory access time of expensive, high- speed memory combined with the large memory size of less expensive, lower-speed memory.

Memory Cell

The basic element of semiconductor memory

Semiconductor memory cell

share certain properties:

• They exhibit two stable (or semi-stable) states

• They are capable of being written

• They are capable of being read

lines

The cache consists of m blocks, called _____

line size

The length of a line, not including tag and control bits

Random-Access Memory (RAM)

• most common type of semiconductor memory

• The other distinguishing characteristic is that it is volatile.

• can be used only as temporary storage.

Dynamic RAM (DRAM)

is made with cells that store data as charge on capacitors

dynamic

refers to this tendency of the stored charge to leak away, even with power continuously applied.

Static RAM (SRAM)

• a digital device that uses the same logic elements used in the processor.

• binary values are stored using traditional flip-flop logic-gate configurations.

• Will hold its data as long as power is supplied to it.

Read-only Memory (ROM)

• contains a permanent pattern of data that cannot be changed

• Nonvolatile

• no power source is required to maintain the bit values in memory

• important application is microprogramming

• Other potential applications include:

  • Library subroutines

  • System programs

  • Function tables

Programmable ROM (PROM)

nonvolatile and may be written into only once

Read-Mostly Memory

useful for applications in which read operations are far more frequent than write operations

Erasable Programmable Read-Only Memory (EPROM)

read and written electrically

Electrically Erasable Programmable Read-Only Memory (EEPROM)

a read-mostly memory that can be written into at any time without erasing prior contents

Flash Memory

• form of semiconductor memory

• gets its name because the microchip is organized so that a section of memory cells are erased in a single action or “flash.”

• introduced in the mid-1980s

• intermediate between EPROM and EEPROM uses an electrical erasing technology

• can be erased in one or a few seconds

• It is used both for internal memory and external memory applications.

• Its important characteristic is that it is persistent memory, which means that it retains data when there is no power applied to the memory.

• It is useful for secondary (external) storage, and as an alternative to random access memory in computers.

Synchronous DRAM

• exchanges data with the processor synchronized to an external clock signal and running at the full speed of the processor/memory bus without imposing wait states.

• One of the most widely used forms of DRAM.

Double Date-Rate DRAM (DDR SDRAM)

• provides several features that dramatically increase the data rate

• developed by the JEDEC Solid State Technology Association

• achieves higher data rates in three ways:

  • the data transfer is synchronized

  • DDR uses higher clock rate on the bus

  • a buffering scheme is used

NOR Flash Memory

• Cells are connected in parallel to the bit lines so that each cell can be read/write/erased individually.

• If any memory cell of the device is turned on by the corresponding word line, the bit line goes low.

• This is similar in function to a NOR logic gate.

• Provides high-speed random access.

• It can read and write data to specific locations and can reference and retrieve a single byte.

NAND Flash Memory

• Organized in transistor arrays with 16 or 32 transistors in series.

• The bit line goes low only if all the transistors in the corresponding word lines are turned on.

• This is similar in function to a NAND logic gate.

• reads and writes in small blocks.

• Provides higher bit density than NOR and greater write speed.

• Does not provide a random-access external address bus so the data must be read on a block-wise basis (also known as page access), where each block holds hundreds to thousands of bits.