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4.1 / COMPUTER MEMORY SYSTEM OVERVIEW 113

addressable unit. For external memory, data are often transferred in much
larger units than a word, and these are referred to as blocks.
Another distinction among memory types is the method of accessing units of
data.These include the following:
• Sequential access: Memory is organized into units of data, called records. Ac-
cess 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.
• 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.
• Random access: Each addressable location in memory has a unique, physically
wired-in addressing mechanism. The time to access a given location is inde-
pendent 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.
From a user’s point of view, the two most important characteristics of memory
are capacity and performance.Three performance parameters are used:
• 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 ad-
dress 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 mem-
ory and consists of the access time plus any additional time required before a
second access can commence. This additional time may be required for tran-
sients to die out on signal lines or to regenerate data if they are read destruc-
tively. 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).