Hardware Reference
In-Depth Information
instruction set. This flexibility was one of the main reasons microprogramming
was introduced in the 360. Wilkes' motivation of reducing tube count no longer
mattered, of course, since the 360 did not have any tubes.
The 360 solved the dilemma of binary-parallel versus serial decimal with a
compromise: the machine had 16 32-bit registers for binary arithmetic, but its
memory was byte-oriented, like that of the 1401. It also had 1401 style serial in-
structions for moving variably sized records around memory.
Another major feature of the 360 was a (for that time) huge address space of
2
24
(16,777,216) bytes. With memory costing several dollars per byte in those
days, this much memory looked very much like infinity. Unfortunately, the 360
series was later followed by the 370, 4300, 3080, 3090, 390 and z series, all using
essentially the same architecture. By the mid 1980s, the memory limit became a
real problem, and IBM had to partially abandon compatibility when it went to
32-bit addresses needed to address the new 2
32
-byte memory.
With hindsight, it can be argued that since they had 32-bit words and registers
anyway, they probably should have had 32-bit addresses as well, but at the time no
one could imagine a machine with 16 million bytes of memory. While the tran-
sition to 32-bit addresses was successful for IBM, it was again only a temporary
solution to the memory-addressing problem, as computing systems would soon re-
quire the ability to address more than 2
32
(4,294,967,296) bytes of memory. In a
few more years computers with 64-bit addresses would appear on the scene.
The minicomputer world also took a big step forward in the third generation
with DEC's introduction of the PDP-11 series, a 16-bit successor to the PDP-8. In
many ways, the PDP-11 series was like a little brother to the 360 series just as the
PDP-1 was like a little brother to the 7094. Both the 360 and PDP-11 had
word-oriented registers and a byte-oriented memory and both came in a range
spanning a considerable price/performance ratio. The PDP-11 was enormously
successful, especially at universities, and continued DEC's lead over the other
minicomputer manufacturers.
By the 1980s,
VLSI
(
Very Large Scale Integration
) had made it possible to
put first tens of thousands, then hundreds of thousands, and finally millions of tran-
sistors on a single chip. This development soon led to smaller and faster com-
puters. Before the PDP-1, computers were so big and expensive that companies
and universities had to have special departments called
computer centers
to run
them. With the advent of the minicomputer, a department could buy its own com-
puter. By 1980, prices had dropped so low that it was feasible for a single individ-
ual to have his or her own computer. The personal computer era had begun.
Personal computers were used in a very different way than large computers.
They were used for word processing, spreadsheets, and numerous highly interac-
tive applications (such as games) that the larger computers could not handle well.
