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 discussion 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:
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