COMPUTER MEMORY SYSTEM OVERVIEW

Characteristics of Memory Systems

Key Characteristics of Computer Memory Systems

Location

• Processor
• Internal (main)
• External (secondary)

    Internal memory is often equated with main memory. But there are other forms of internal memory. The processor requires its own local memory, in the form of registers. Cache is another form of internal memory. External memory consists of peripheral storage devices, such as disk and tape, that are accessible to the processor via I/O controllers.

Figure 1.1 Processor

Capacity

• Word size
• Number of words

    For internal memory, the capacity 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.

Unit of Transfer

• Word
• Block

    For internal memory, the unit of transfer is equal to the number of data lines into and out of the memory module. This may be equal to the word length, but is often larger, such as 64, 128 or 256 bits.

    Word: The “natural” unit of organization of memory. The size of word is typically equal to the number of bits used to represent a number and to the instruction length. Unfortunately, there are many exceptions.

    Addressable units: In some systems, the addressable unit is the word. However, many systems allow addressing at the byte level. In any case, the relationship between the length in bits A of an address and the number N of addressable units is 2A = N

    For main memory, the unit of transfer is the number of bits read out of or written into memory at time. It need not equal a word or an addressable unit. For external memory, data are often transferred in much larger units than a word and these are referred to as blocks.

Access Method

• Sequential
• Direct
• Random
• Associative

    Sequential access: 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 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. Thus, the time to access an arbitrary record is highly variable. Tape units are sequential access.

    Direct access: 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, counting or waiting to reach the final location. Access time is variable. Disk units are direct access.

Figure 1.2 Disk

    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 associative access.

Performance

• Access time
• Cycle time
• Transfer rate

    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, access time 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 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. For random-access memory, it is equal to 1/(cycle time). For non-random-access memory, the following relationship holds: 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)

Physical Type

• Semiconductor
• Magnetic
• Optical
• Magneto-optical

    The most common physical types of memory today are semiconductor memory, magnetic surface memory, used for disk and tape and optical and magnetic-optical.

Physical Characteristics

• Volatile/nonvolatile
• Erasable/nonerasable

    In a volatile memory, information decays naturally or is lost when electrical power is switched off. In a 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 may be either volatile or nonvolatile. Nonerasable memory cannot be altered, except by destroying the storage unit. Semiconductor memory of this type is known as read only memory (ROM). Of necessity, a practical nonerasable memory must also be nonvolatile.

Figure 1.3 Read Only Memory (ROM)

Organization

    For random-access memory, the organization is a key design issue. By organization is meant the physical arrangement of bits to form words.

The Memory Hierarchy

Figure 1.4 The Memory Hierarchy

    As one goes down in the hierarchy, the following occur:
        a) Decreasing cost per bit
        b) Increasing capacity
        c) Increasing access time
        d) Decreasing frequency of access of the memory by the                  processor

    The fastest, smallest and most expensive type of memory consists of the registers internal to the processor. Typically, a processor will contain a few dozen such registers, although some machines contain hundreds of registers. Main memory is the principal internal memory system of the computer. Each location in main memory has a unique address. Main memory is usually extended with a higher-speed, smaller cache.

Reference: William Stallings. (2003). Computer Organization & Architecture DESIGN FOR PERFORMANCE(6th ed.):                    Computer Memory System Overview(pp 96-101). Upper Saddle River, NJ: Pearson