John Savard
2006-07-04 13:52:09 UTC
On April 7, 1964, International Business Machines announced System/360.
The initial announcement comprised models 30, 40, 50, 60, 62, and 70.
The last three models were replaced by models 65 and 75 prior to
delivery; the front panels looked pretty much the same, but the
performance was improved over what had been announced originally.
IBM extended the series upwards later, with the model 91, followed by
the 85 and the 195 (not to mention the two model 95s sold to NASA, IBM's
one and only thin-film computer).
There was a special-purpose machine in the middle of the series, the
model 44, which had particularly high floating-point performance.
And, of course, there was the model 67, which offered Dynamic Address
Translation in what was otherwise a model 65.
They also extended the series at the low end, with the models 22 and 25.
Then there was the Model 20.
Not only couldn't you get floating-point for it, but it didn't even have
all the standard System/360 instructions: it wouldn't do arithmetic on
32-bit integers.
The popularity of the IBM 360 inspired imitations... and
near-imitations.
The closest of the near-imitations were the RCA Spectra 70 computers.
Only those portions of the instruction set that required privileged mode
to use were different. Presumably, this was intended to ensure they
didn't need to fear a lawsuit by encouraging people to pirate IBM's
operating system software.
Usually not even counted as an imitation at all were the Sigma
computers. These computers had instructions that were all 32 bits in
length; register-to-register operations were provided by means of
assigning memory addresses to the registers. But the features offered
were intentionally matched to those of the System/360, and the data
formats were the same - with the exception of floating-point numbers, if
they were negative.
In data formats, the PDP-6 had the same sort of relation to the IBM 7090
and its relatives, but even the fact that the KA-10 version of the
PDP-10 had a front panel with a sloped front, and a near-horizontal
extension with the rows of rocker switches on it doesn't really lead me
to think of the PDP-6 and PDP-10 as imitations of the 7090. Had it not
used two's complement arithmetic for integers, I might have reconsidered
that.
Then there were the minis from Interdata.
But back to the System/360 Model 20. I listed what it didn't have. But I
didn't list what it _did_ have. Because it didn't _just_ do halfword
arithmetic. It also came with the full decimal arithmetic instruction
set.
That meant that it wasn't just a minicomputer at mainframe prices. It
was a practical and useful machine, avoiding un-needed features, which
was very attractive to a certain market.
It doesn't seem to have offered, unlike the Model 30 (or the Model 25,
which came later) the option of 1401 emulation. But it was still very
useful to meet the data processing needs of 1401 customers.
And, since IBM *did* provide a FORTRAN compiler for the 1401, infamous
for requiring a record number of passes, and FORTRAN ran on the IBM
1620, another decimal machine, it would have been possible, at least in
theory, for someone to write a FORTRAN compiler for Model 20 owners.
Floating-point numbers could have been represented by a 16-bit binary
exponent accompanying a BCD mantissa of up to 16 digits ... the number
of digits being susceptible, as with 1401 FORTRAN, to arbitrary
specification. I doubt, of course, that anyone *did* that, but it was
certainly possible.
John Savard
http://www.quadibloc.com/index.html
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The initial announcement comprised models 30, 40, 50, 60, 62, and 70.
The last three models were replaced by models 65 and 75 prior to
delivery; the front panels looked pretty much the same, but the
performance was improved over what had been announced originally.
IBM extended the series upwards later, with the model 91, followed by
the 85 and the 195 (not to mention the two model 95s sold to NASA, IBM's
one and only thin-film computer).
There was a special-purpose machine in the middle of the series, the
model 44, which had particularly high floating-point performance.
And, of course, there was the model 67, which offered Dynamic Address
Translation in what was otherwise a model 65.
They also extended the series at the low end, with the models 22 and 25.
Then there was the Model 20.
Not only couldn't you get floating-point for it, but it didn't even have
all the standard System/360 instructions: it wouldn't do arithmetic on
32-bit integers.
The popularity of the IBM 360 inspired imitations... and
near-imitations.
The closest of the near-imitations were the RCA Spectra 70 computers.
Only those portions of the instruction set that required privileged mode
to use were different. Presumably, this was intended to ensure they
didn't need to fear a lawsuit by encouraging people to pirate IBM's
operating system software.
Usually not even counted as an imitation at all were the Sigma
computers. These computers had instructions that were all 32 bits in
length; register-to-register operations were provided by means of
assigning memory addresses to the registers. But the features offered
were intentionally matched to those of the System/360, and the data
formats were the same - with the exception of floating-point numbers, if
they were negative.
In data formats, the PDP-6 had the same sort of relation to the IBM 7090
and its relatives, but even the fact that the KA-10 version of the
PDP-10 had a front panel with a sloped front, and a near-horizontal
extension with the rows of rocker switches on it doesn't really lead me
to think of the PDP-6 and PDP-10 as imitations of the 7090. Had it not
used two's complement arithmetic for integers, I might have reconsidered
that.
Then there were the minis from Interdata.
But back to the System/360 Model 20. I listed what it didn't have. But I
didn't list what it _did_ have. Because it didn't _just_ do halfword
arithmetic. It also came with the full decimal arithmetic instruction
set.
That meant that it wasn't just a minicomputer at mainframe prices. It
was a practical and useful machine, avoiding un-needed features, which
was very attractive to a certain market.
It doesn't seem to have offered, unlike the Model 30 (or the Model 25,
which came later) the option of 1401 emulation. But it was still very
useful to meet the data processing needs of 1401 customers.
And, since IBM *did* provide a FORTRAN compiler for the 1401, infamous
for requiring a record number of passes, and FORTRAN ran on the IBM
1620, another decimal machine, it would have been possible, at least in
theory, for someone to write a FORTRAN compiler for Model 20 owners.
Floating-point numbers could have been represented by a 16-bit binary
exponent accompanying a BCD mantissa of up to 16 digits ... the number
of digits being susceptible, as with 1401 FORTRAN, to arbitrary
specification. I doubt, of course, that anyone *did* that, but it was
certainly possible.
John Savard
http://www.quadibloc.com/index.html
_________________________________________
Usenet Zone Free Binaries Usenet Server
More than 140,000 groups
Unlimited download
http://www.usenetzone.com to open account
