ZiLOG Z80 Handbook page 13

tion repertoire, the Intel 8080, introduced in late 1973, included
the instruction set of the 8008 and supplemented it with 30 more
instructions. Users of the 8008 could now change to a faster, more
versatile microprocessor while not discarding 8008 software pro-
grams, since all 8008 software would presumably execute on the
8080. The 8080 was an NMOS (N-channel metal-oxide semiconduc-
tor) microprocessor that allowed faster clock rates. Additions of two
8-bit operands could now be carried out at rates of 500,000 per sec-
ond. In addition, all other instruction times were much shorter than
the 8008 as the 8080 was built around a 40-pin chip, requiring the
CPU to do much less time sharing of the data bus between data
transfers and instruction implementation.
The 8080 supplemented the hardware features of the 8008. In
place of 16,384 (16K) memory addresses, the 8080 could address
65,536 (64K). Rather than a limited 7-level memory stack, the 8080
offered a memory stack in external memory itself instead of the CPU.
A binary-coded decimal or bcd capability was built into the arith-
metic and logic unit in the CPU; additions of two bed operands
could now be implemented. Expanded addressing modes to permit
direct addressing of external memory was offered. Although the 78
instructions of the 8080 still seemed strange to many programmers,
the instruction set decidedly had moved away from one for pri-
marily control applications to one that was more general purpose in
nature.
In 1976, Intel brought out several variations on the 8080. The
Intel 8085 included a serial input/output capability on the micro-
processor chip itself. In addition, the 8085 had a requirement of
only a single-phase clock (the 8008 and 8080 were two-phase clocks)
and a single 5-volt power supply (the 8008 and 8080 required two
and three voltages, respectively). As the number of supporting
packages had grown impressively (such chips as a programmable
peripheral interface, interrupt controller, and crt controller) Intel
provided very powerful computing capability at faster and faster
speeds (770,000 8-bit adds per second), while still retaining com-
patability with existing software written for the 8008 and 8080.
Although the 8085 was an improvement over the 8080 in many
features, the instruction set remained very similar to the 8080. Only
two new instructions were added, one to read serial and interrupt
data, and one to write serial and interrupt data. Many of the inherent
inadequacies of the 8008 and 8080 remained.
The Zilog, Inc. Z-80 microprocessor chip has provided another
level of sophistication for the widely used 8008/8080 base. Bearing
in mind that the super computer of today is the surplus bargain of
tomorrow, the Z-80 has supplemented the instruction set and capa-
bilities of the 8080 in the same fashion as the 8080 increased the
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