of Microprocessors

152 MUSICAL ApPLICATIONS OF MICROPROCESSORS
Since there is now almost total overlap between microprocessor types
and typical applications, the real distinctions have become performance
(speed and ease of assembly language programming) and cost (total hardware
cost of a typical system). Accordingly, only two microprocessors, one at each
end of the spectrum, will be selected for detailed description here.
In the comparative discussions to follow, two areas of difference among
microprocessors will be concentrated upon. These are its machine language
instruction set, since that directly influences assembly language programming,
and its bus structure and timing, which influences system hardware
design, particularly in the logic replacement application. Only the most
important points that distinguish the subject microprocessor from others will
be covered. More detail can be found in the wealth of manufacturer's literature
and microprocessor design books available.
The 6502 for Logic Replacement and Low Cost
The original notion behind development of microprocessors was to
provide a standard LSI chip that, when combined with standard memories,
could perform the functions of a custom LSI chip. The reasoning was that,
since semiconductor cost is nearly inversely proportional to production
volume, the lower cost ofstandard parts would outweigh any cost advantages
of an optimized custom chip. Although things did not quite work out like
that, quite a lot of microprocessors are used for logic replacement instead of
building microcomputers.
Of all the 8-bit microprocessors now on the market, the 6502 comes
closest to filling all logic replacement needs. Its price, although not the
absolute lowest on the market, is low enough to replace even relatively small
logic systems. Its raw speed, which is the highest of all 8-bit MOS
microprocessors, is high enough to replace all but the most speed-sensitive
logic systems. Its instruction set, although not a programmer's dream, is
powerful and straightforward, unlike the obscure and convoluted sets of
many other logic replacement microprocessors. Its bus structure, which is a
model of simplicity, is very easily interfaced while at the same time allowing
some very sophisticated direct memory access and multiprocessor schemes to
be implemented with a minimum of effort. Although not nearly as popular
as the 2-80, there is a sizable core of users who swear by it for generalpurpose
computer applications also.
In music, there are numerous jobs that might normally be done with
conventional logic that can also be done by a 6502 cheaper, simpler, faster,
smaller, or just plain better. For example, a digitally scanned music
keyboard with velocity sensing on both press and release is one possibility.
Another is a universal multichannel envelope generator with the envelope
shapes programmable by the user. A supersequencer is another obvious application.
The 6502 is fast enough to even generate tones with program-