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112 CHAPTER 4 / CACHE MEMORY

Table 4.1 Key Characteristics of Computer Memory Systems

Location Performance
Internal (e.g. processor registers, main Access time
memory, cache) Cycle time
External (e.g. optical disks, magnetic Transfer rate
disks, tapes)
Physical Type
Capacity
Semiconductor
Number of words
Magnetic
Number of bytes
Optical
Unit of Transfer
Magneto-optical
Word
Physical Characteristics
Block
Volatile/nonvolatile
Access Method
Erasable/nonerasable
Sequential
Organization
Direct
Memory modules
Random
Associative
the form of registers (e.g., see Figure 2.3). Further, as we shall see, the control unit
portion of the processor may also require its own internal memory.We will defer dis-
cussion of these latter two types of internal memory to later chapters. 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.
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:
• 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 in-
struction 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.
• Addressable units: In some systems, the addressable unit is the word. How-
ever, many systems allow addressing at the byte level. In any case, the rela-
tionship between the length in bits A of an address and the number N of
addressable units is 2 A = N.
• 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