of Microprocessors

MUSIC SYNTHESIS SOFTWARE 645
operating system. The latter defect in turn begat a variety of incompatible
solutions to the problem. Although Modula-2 has addressed this in the
language specification, it is probably too late to help.
C is another structured language born in the commercial world and is
probably the most used in large microcomputer software projects. It was
developed at Bell Laboratories and is often seen in conjunction with UNIX, a
very large time-sharing operating system (UNIX is in fact written in C). It is
available as a stand-alone language for most microcomputer systems from the
Z-80 and 6502 up to the 68000. In fact, a large and comprehensive delayed
playback music synthesis system written in C (the CARL project at UCSD) is
readily available and will be described in the next chapter. As mentioned in
Chapter 5, C has structural features that allow for highly efficient object code
generation-if the compiler takes advantage of them. In particular, the
68000 is a good match for the requirements of C because of its unsegmented
memory addressing and numerous registers capable of holding 32-bit
"double" values. The sos6 family (SOSS, SOlS6, S02S6) is not as good
because of the lack of those same features. In particular, their segmented
addressing will either impose a 64K limit on the size of arrays, which could
be a problem in musical applications, or will exact a severe space and time
penalty if the compiler attempts to overcome it. A big advantage of C is that
I/O and system call conventions are written into the language specification
and thus differ little from one implementation to the next. A potential
drawback of C is its often cryptic appearance, which makes adequate
comments in programs essential.
One programming language property that is taking on increasing
significance is portability. At the current rate of microprocessor evolution, it
is quite likely that the computer being used will become obsolete in the
course of music system development (this has happened three times to the
author). Parts of the system implemented in a high-level language should be
easily transportable to a new system if required. C promises to be very good
in this respect. Although the low-level assembly language routines are not
portable, they can usually be kept small, thus minimizing the effort needed
to reimplement them on different hardware.
Low-Level Programming Techniques
Because they are executed so much, low-level sample computation
routines completely dominate the speed characteristics of a direct computer
synthesis system. Arithmetic operations in turn dominate the execution time
of these low-level routines, particularly on a microcomputer. This is in stark
contrast with most general-purpose assembly level programming in which
arithmetic instructions are among the least used. Unless the reader has access
to a mainframe or supermicrocomputer with floating-point hardware, it is a
sure bet that these arithmetic operations will be of the fixed-point variety.