These are the memory address register (MAR) and the memory data register (MDR). When writing to
memory, the CPU first transfers the value to be written to the MDR, and the address of the location to be used
to the MAR. At the next memory access cycle, the value in MDR will be copied into the location identified by
the contents of the MAR.
When retrieving from memory, the CPU first stores the address to read in the MAR. When the read occurs
on the next memory access cycle, the value in that location is copied into the MDR. From the MDR in the
memory controller, the data value can be transferred to one of the CPU registers or elsewhere.
Main computer memory, such as we have in our PCs, is referred to as random access memory (RAM). That
means we can access any element of memory at will, and with roughly the same speed, regardless of address.
By contrast, consider information and data stored on a magnetic tape. Magnetic tape is a kind of memory
(we can store data on a magnetic tape), but magnetic tape is definitely not random access. Magnetic tape is
serial access. We can read the contents of memory location 4000 only after having read and passed over all those
locations that come before.
CHAP. 3] COMPUTER ORGANIZATION 39
In addition to main memory, which has been the focus of our discussion so far, computer designers also
usually provide small, high-performance memories, called cache memories, that are located close to the CPU.
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