of Microprocessors

136 MUSICAL ApPLICATIONS OF MICROPROCESSORS
Memory Advances
In many ways, the availability and pncmg of memory ICs drive
microcomputer development almost as much as the microprocessor chips do.
The key component is the random-access memory IC, or "RAM chip" for
short. The first RAM chip large enough to do anything significant with was
the 1101. Its capacity was 256 bits, and typical cost was about $4 in 1971.
Shortly thereafter, at about the same time as the 8008 microprocessor was
introduced, the 1103 lK bit RAM became available for $7. This was the first
dynamic RAM, which meant that each row of memory cells had to be read
periodically to refresh and maintain the stored information. Dynamic RAMs
are inherently denser and cheaper to make than static RAMs and thus led the
way in increased RAM chip capacity.
While lK RAM chips were adequate for dedicated applications of
microprocessors, it was expensive and space-consuming to make memories
large enough for a general-purpose microcomputer from them. For example,
an 8K byte memory board required 64 of the 18-pin 1103 RAMs plus a
nontrivial amount of drive circuitry. Microprocessor applications got a good
boost in 1974, however, when 4K bit RAM chips made their appearance.
Most of the industry settled on a 22-pin package with full address decoding
that was known generically as the 2107. Mostek, however, pioneered a 16-pin
version (the MK4096) in which the full 12-bit address was entered into the
IC sequentially as two 6-bit halves on only 6 pins. This allowed packing
densities almost twice as high as the 22-pin version, although many
engineers were reluctant to use them because of greater circuit complexity.
Prices on both versions dropped to the $10 level by 1976 and ultimately to
about $2.50 near the end of their lifespan in 1978. It was now possible to
build a 32K byte memory board for about the same cost as 8K bytes would
have cost 3 years earlier.
The 16K RAM chips made their appearance in 1977 and became really
practical to use in 1979. By then, the wisdom of 16-pin packaging and
address multiplexing was fully realized, and, as a result, only the 16-pin
package style was offered. The availability of these RAM chips is what really
fueled the second wave of CP/M business computers described earlier. One
could actually build the maximum addressable 64K byte memory for a Z-80
microprocessor in a small 3.S-inch X 3.S-inch area in the corner of the
computer's motherboard. Interestingly enough, both the Radio Shack and
the Apple II computers were designed to use the 4096 type of4K RAM with
provisions to also allow use of the 16K chips. When 16K prices fell to the
$12 range in 1979, 16K upgrade kits were offered by dozens of firms for less
than $100. Within a year, Apple and Radio Shack computers with less than
maximum memory (48K bytes) were a rarity indeed.
The 64K RAM chips appeared in 1981 and quickly came down to a
practical price level of about $8 by 1982. Besides the greater bit capacity, a