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The advantage of hardwired control is that it is very fast. The disadvantage is
that the instruction set and the control logic are directly tied together by special
circuits that are complex and difficult to design or modify. If someone designs a
hardwired computer and later decides to extend the instruction set (as we did with
MARIE), the physical components in the computer must be changed. This is prohibitively
expensive, because not only must new chips be fabricated but also the
old ones must e located and replaced.
The other approach, called microprogramming, uses software for control, and
is illustrated in Figure 4.15.
All machine instructions are input into a special program, the microprogram,
to convert the instruction into the appropriate control signals. The microprogram
is essentially an interpreter, written in microcode, that is stored in firmware
(ROM, PROM, or EPROM), which is often referred to as the control store. This
program converts machine instructions of zeros and ones into control signals.
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