Hardware Reference
In-Depth Information
were as incompatible as two machines could be. One was a high-speed number
cruncher using parallel binary arithmetic on 36-bit registers, and the other was a
glorified input/output processor using serial decimal arithmetic on variable-length
words in memory. Many of its corporate customers had both and did not like the
idea of having two separate programming departments with nothing in common.
When the time came to replace these two series, IBM took a radical step. It in-
troduced a single product line, the System/360, based on integrated circuits, that
was designed for both scientific and commercial computing. The System/360 con-
tained many innovations, the most important of which was that it was a family of
about a half-dozen machines with the same assembly language, and increasing size
and power. A company could replace its 1401 with a 360 Model 30 and its 7094
with a 360 Model 75. The Model 75 was bigger and faster (and more expensive),
but software written for one of them could, in principle, run on the other. In prac-
tice, a program written for a small model would run on a large model without prob-
lems. However, the reverse was not true. When moving a program written for a
large model to a smaller machine, the program might not fit in memory. Still, this
was a major improvement over the situation with the 7094 and 1401. The idea of
machine families caught on instantly, and within a few years most computer manu-
facturers had a family of common machines spanning a wide range of price and
performance. Some characteristics of the initial 360 family are shown in Fig. 1-7.
Other models were introduced later.
Property
Model 30 Model 40 Model 50 Model 65
Relative performance
1
3.5
10
21
Cycle time (in billionths of a sec)
1000
625
500
250
Maximum memory (bytes)
65,536
262,144
262,144
524,288
Bytes fetched per cycle
1
2
4
16
Maximum number of data channels
3
3
4
6
Figure 1-7.
The initial offering of the IBM 360 product line.
Another major innovation in the 360 was
multiprogramming
, having several
programs in memory at once, so that when one was waiting for input/output to
complete, another could compute. This resulted in a higher CPU utilization.
The 360 also was the first machine that could emulate (simulate) other com-
puters. The smaller models could emulate the 1401, and the larger ones could
emulate the 7094, so that customers could continue to run their old unmodified bi-
nary programs while converting to the 360. Some models ran 1401 programs so
much faster than the 1401 itself that many customers never converted their pro-
grams.
Emulation was easy on the 360 because all the initial models and most of the
later models were microprogrammed. All IBM had to do was write three micro-
programs, for the native 360 instruction set, the 1401 instruction set, and the 7094
