of Microprocessors

MICROPROCESSORS 159
right will set it. Since indexed addressing is available with the shifts, very
efficient bit testing and control loops are possible.
Besides logic replacement, the 6502 can be used quite successfully for
general-purpose applications. The main factor inhibiting this besides the
course of history is its reliance on a base page and its limited indexed
addressing range. The base page complicates a relocating linking loader
considerably because both base page and regular memory relocation are required.
Although the 6502 is a model of efficiency when processing tables
less than 256 bytes in length, bigger lists cannot be handled using
straightforward indexed addressing. Instead, pointers must be formed on the
base page and address calculation done with normal arithmetic instructions.
Direct synthesis can be done significantly faster with the 6502 than
with other 8-bit processors. The addressing modes are quite efficient in
handling the waveform and other tables involved provided they do not exceed
256 bytes in length. Properly programmed, translation of a byte via table
lookup may add as little as 4 f.Lsec to execution time. An 8 X 8 unsigned
multiply can be performed by software in as little as 92 clock cycles average
for a full 16-bit product. These figures make the idea of using a 6502
microprocessor for each voice in a direct synthesis system at least worth
considering.
The 68000 for General Purpose and High Performance
Of the existing 16-bit microprocessors available for use by anyone in
1984, the Motorola 68000 is clearly the best performer in musical
applications. It is inherently fast with speeds as high as 3 million register-toregister
instructions per second. It has a rich register complement of 16 32­
bit general-purpose registers, which makes that speed claim meaningful.
And it can handle big applications using very large data arrays because of its
unsegmented addressing range of 16M bytes. These factors together allow an
instruction set that is very easy to program in assembly language and has the
power necessary for efficient code generation by high-level language
compilers.
Whereas an 8-bit microprocessor can be thoroughly documented in a
16- or 24-page brochure, Motorola needs a 200-page paperback book to do
the same for the 68000. Much of this bulk concerns usage in very complex,
multiuser, time-sharing computer systems for which it has been highly
promoted. In fact, the performance of the 68000 is comparable to that of
large minicomputers typically used in time-sharing systems and beats the
pants off the room-sized IBM 360/40 the author used in 1970. However, it is
possible, even easy, to use the 68000 in small single-user systems or even
logic-replacement-dedicated applications as well, which is the case for most
musical applications. These are the usage modes that will be covered here
with emphasis on those features that make the 68000 unusually effective in
high-performance single-user applications.