of Microprocessors

160 MUSICAL ApPLICATIONS OF MICROPROCESSORS
The 68000 family is actually available in four different configurations
and three different clock speeds. The "standard" 68000, upon which this
discussion is based, comes in a very large 64-lead dual-in-line package that
measures about 1 inch X 3.5 inches. This version has a separate 16-bit data
bus and 24-bit address bus that together account for 40 of those pins. The
68008 is an 8-bit data bus version with 20 address lines (1M byte addressing
range) that fits into a more petite 40-lead package. It is absolutely
instruction-set-compatible with the standard 68000 but costs substantially
less (under $20) and runs about 60% as fast. The 68010 is essentially the
same as the 68000 but has hardware enhancements that are valuable in
multiuser applications. Due to more efficient internal operation, it is slightly
faster for most programming as well but costs twice as I?uch as a 68000. The
standard clock speed for these three versions is 8 MHz, although 10 MHz and
12.5 MHz are available at extra cost and 16 MHz may ultimately become
available.
The 68020 is the newest family member with a full 32-bit data bus and
separate 32-bit address bus. That gives it a linear, unsegmented addressing
range of more than 4 gigabytes! It is packaged in a small, difficult-to-use,
1l2-lead "pin grid array" that is less than 1.5 inch square. Because of the 32­
bit data bus and incredibly efficient internal operation, it is 3 to 10 times
faster than the 68000, particularly for short loops, common in most direct
synthesis algorithms. The standard clock speed is projected to be 16 MHz. It
is interesting to note that the initial price of $487 is actually less than the
8080's initial price of $360 in 1974 if inflation is taken into account.
Bus Structure and Timing
The 68000 is really a 32-bit microprocessor with a 16-bit data bus.
Only 24 of the potential 32 address bits are available externally, and 32-bit
operands are transferred 16 bits at a time. All addresses are byte addresses,
which makes byte manipulation software easy. Since the registers and data
bus are more than 1 byte wide, a property known as "byte sex" becomes
important. In essence, this specifies how multiple byte operands such as 16­
and 32-bit numbers are stored in memory. The 68000 uses positive byte sex in
which the most significant bytes (leftmost in registers) are stored at lower
byte addresses. Thus, reading values from a memory dump listing is easy
because they are listed in the same order one would write them on paper or
visualize them in registers. The address of a 2- or 4-byte operand is the
address of its most significant byte, which in the 68000 must always be even.
In order to simplify hardware handling of individual 8-bit bytes on a
16-bit data bus, the lowest address line on the 68000 is replaced by a lower
data strobe line and an upper data srrobe line. For 16-bit (word) transfers,
both strobes are activated, whereas for byte transfers, only one is pulsed:
upper for even addresses and lower for odd addresses.