Hardware Reference
In-Depth Information
market for computers was about four or five units. Instead, DEC mostly sold small
circuit boards to companies to integrate into their products.
When the PDP-1 finally appeared in 1961, it had 4096 18-bit words of core
memory and could execute 200,000 instructions/sec. This performance was half
that of the IBM 7090, the transistorized successor to the 709, and the fastest com-
puter in the world at the time. The PDP-1 cost $120,000; the 7090 cost millions.
DEC sold dozens of PDP-1s, and the minicomputer industry was born.
One of the first PDP-1s was given to M.I.T., where it quickly attracted the
attention of some of the budding young geniuses so common at M.I.T. One of the
PDP-1's many innovations was a visual display and the ability to plot points any-
where on its 512-by-512 pixel screen. Before long, the students had programmed
the PDP-1 to play Spacewar, and the world had its first video game.
A few years later DEC introduced the PDP-8, which was a 12-bit machine, but
much cheaper than the PDP-1 ($16,000). The PDP-8 had a major innovation: a
single bus, the omnibus, as shown in Fig. 1-6. A
bus
is a collection of parallel
wires used to connect the components of a computer. This architecture was a
major departure from the memory-centered IAS machine and has been adopted by
nearly all small computers since. DEC eventually sold 50,000 PDP-8s, which es-
tablished it as the leader in the minicomputer business.
Console
terminal
Paper
tape I/O
Other
I/O
CPU
Memory
Omnibus
Figure 1-6.
The PDP-8 omnibus.
Meanwhile, IBM's reaction to the transistor was to build a transistorized ver-
sion of the 709, the 7090, as mentioned above, and later the 7094. The 7094 had a
cycle time of 2 microsec and a core memory consisting of 32,768 words of 36 bits
each. The 7090 and 7094 marked the end of the ENIAC-type machines, but they
dominated scientific computing for years in the 1960s.
At the same time that IBM had become a major force in scientific computing
with the 7094, it was making a huge amount of money selling a little business-ori-
ented machine called the 1401. This machine could read and write magnetic tapes,
read and punch cards, and print output almost as fast as the 7094, and at a fraction
of the price. It was terrible at scientific computing, but for business record keeping
it was perfect.
The 1401 was unusual in that it did not have any registers, or even a fixed word
length. Its memory was 4000 8-bit bytes, although later models supported up to a
then-astounding 16,000 bytes. Each byte contained a 6-bit character, an adminis-
trative bit, and a bit used to indicate end-of-word. A
MOVE
instruction, for ex-
ample, had a source and a destination address and began moving bytes from the
source to the destination until it hit one with the end-of-word bit set to 1.
