Introduction - Computer Busses - page 8

Intel knew that providing a processor alone
would have very little impact on the market. It
required a development system, which would
allow industrial developers an easy method of
developing hardware and software around the new
processor. Thus, Intel introduced the Intellec 4
development system.
The main competitors to the 8080 were: the
Motorola 6800, the Zilog Z80 and the MOS
Technology 6502. The Z80 had the advantage that
it could run any programs written for the 8080,
and, because it was also pin compatible, it could
be easily swapped with the 8080 processor,
without a change of socket. It also had many other
advantages over the 8080, such as direct memory
access, serial I/O technology, and full use of the
'reserved' op-codes (Intel had used only 246 out
of the 256 available op-codes). The Z80 was also
much cheaper than the 8080 and had a 2.5 MHz
clock speed. After the release of the Z80, Intel
produced a quick response: the 8085. This device
fully used all the op-codes, but it was too late to

stop the tide towards Zilog. Many personal
computers started to appear that were based on
the Z80 processor, including the Radio Shack
TRS-80, Osborne 1 and the Sinclair/Timex
ZX81. The ZX81 caused a great revolution because of
its cheapness, but unfortunately, most home users had
to wait for many months to receive their kit, or for
their prebuilt computer. However, as the computer
was so original and cost effective, users were willing
to wait for their prized system. Another great chal-
lenger was the 6502, which was released in June 1975
and cost $25. This compared well with the 8080,
which cost $150. It was used in many of the great
personal computer systems, such as the Apple II (Fig-
ure 1.3) and Atari 400.

Figure 1.2 Intel 8008 die

Figure 1.3 Apple II computer

For the first
time, home users
could actually build their own computer, and were avail-
able from Altair and Mistral. With the success of the
Z80, many companies were demanding to produce a
second-source supply for the Z80 processors. The
Motorola processor was also more powerful than the
8080. It was simpler in its design and only required a
single 5 V supply, whereas the 8080 required three dif-
ferent power supplies.

Figure 1.4 ZX80

At the end of the 1970s, IBM's virtual monopoly on