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Re: The Other Half of the System/360

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According to quadi  <quadibloc@ca.invalid>:
>The System/360 was intended to replace both the 7090 series and the 1401 
>series, as well as other IBM machines.

Right. I would say that it was more important to replace the 1401 and other
commercial machines, since IBM sold a lot more of them than they did of the high
end 709x. All of the microcoded 360 models, 25, 30, 40, 50, 65, and 85, had
emulators for the older machines. IBM only provided emulators for models where
the emulation would run faster than the original machine, so there were 1401
emulators for the /25 and /30 but you needed a /65 to emulate a 7090.

> So the use of SS instructions for BCD arithmetic arose from trying to have
> this portion of the instruction set as similar to the previous commercial
> IBM machines as possible. But the 360 didn't indicate the end of a decimal
> integer by a word mark or by a special character; the SS instructions
> included length fields.

> Numbers were addressed by their most significant digits; the contents of
> the length field could be added to that address if required.

They didn't want word mark bits because they'd have made no sense in the
halfword and fullword scientific instructions.  The field lengths were
hard coded into the second byte of SS instructions, but you could fake
variable length operands with the EX (execute) instruction.  It took
a register operand and OR'ed the low byte of the register with the
second byte of the target instruction before executing, so you could put
a zero length in the instruction and the actual length in the
register.  It was a cute hack.


-- 
Regards,
John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
Please consider the environment before reading this e-mail. https://jl.ly

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The Other Half of the System/360 quadi <quadibloc@ca.invalid> - 2026-05-24 19:00 +0000
  Re: The Other Half of the System/360 Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-05-24 22:49 +0000
    Re: The Other Half of the System/360 quadi <quadibloc@ca.invalid> - 2026-05-25 20:23 +0000
  Re: The Other Half of the System/360 John Levine <johnl@taugh.com> - 2026-05-25 19:11 +0000

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