COMPUTER MEMORY SYSTEM OVERVIEW

The complex subject of computer memory is made more manageable if we classify memory systems according to their key characteristics.The most important of these are listed in Table 4.1. The term location in Table 4.1 refers to whether memory is internal and external to the computer. 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 (e.g., see Figure 2.3). Further, as we shall see, the control unitportion of the processor may also require its own internal memory.We will defer discussion of these latter two types of internal memory to later chapters.


Pengertian Cache , Apa itu Cache?
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.

An obvious characteristic of memory is its 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.

Pengertian A related concept , Apa itu A related concept
A related concept is the unit of transfer. For internal memory, the unit of
transfer is equal to the number of electrical 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.

To clarify this point, consider three related concepts for internal memory:

Pengertian Word , Apa itu Word ?
• Word : The “natural” unit of organization of memory. The size of the word is
typically equal to the number of bits used to represent an integer and to the instruction
length. Unfortunately, there are many exceptions. For example, the
CRAY C90 (an older model CRAY supercomputer) has a 64-bit word length
but uses a 46-bit integer representation. The Intel x86 architecture has a wide
variety of instruction lengths, expressed as multiples of bytes, and a word size
of 32 bits.

Pengertian Addressable units , Apa itu Addressable units
 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.

Pengertian Unit of transfer , Apa itu Unit of transfer
• 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. For external memory, data are often transferred in much larger units than a word, and these are referred to as blocks. Another distinctionamong memory types is the method of accessing units of data.

These include the following:

 Pengertian Sequential access , Apa itu  Sequential access ?
• 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, discussed in
Chapter 6, are sequential access.

Pengertian Direct access , Apa itu Direct 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. Again, access time is variable. Disk units, discussed in
Chapter 6, are direct access.

Pengertian Random access , Apa itu Random access?
• 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.

Apa itu Associative ? Pengertian Associative  
• 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.

From a user’s point of view, the two most important characteristics of memory
are capacity and performance.Three performance parameters are used:

Apa itu Access time (latency)? Pengertian Access time (latency)
• 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.

 Apa itu Memory cycle time? Pengertian Memory cycle time
• 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.

Apa itu Transfer rate? Pengertian Transfer rate
• 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).
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