Actually the 24-bit architecture wasn't the legacy problem as much as
not restricting the leading byte of a fullword (whether in memory or
registers) that the hardware would interpret as an address. That led to
the use of the high-order bit to denote a 31-bit address, which led to
the missing 2G in the 64-bit architecture.
On 2021-11-30 11:57 a.m., Charles Mills wrote:
I remember well. It was very much about saving storage. Storage was precious.
Main memory sizes were measured in kilobytes, not megabytes or gigabytes. There
was no virtual storage, so main memory size was very much a limiting factor --
not just to performance, as it is now, but an absolute limit on what program
and how many programs one could run at once.
Charles
Gary Weinhold
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DATAKINETICS | Data Performance & Optimization
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-----Original Message-----
From: IBM Mainframe Assembler List [mailto:ASSEMBLER-LIST@LISTSERV.UGA.EDU] On
Behalf Of Martin Ward
Sent: Tuesday, November 30, 2021 8:52 AM
To:ASSEMBLER-LIST@LISTSERV.UGA.EDU
Subject: Re: Base-less macros
On 30/11/2021 16:36, Paul Gilmartin wrote:
Didn't IBM do that?
Doesn't mean that it was a good idea :-)
"The 64-bit IBM zSeries (2001) still supports 24-bit mode,
40-plus years later. Why did 24-bit happen? I’m told that it was
all for the sake of one low-cost early model, the 360/30,
where 32 bits would have run slower because it had 8-bit data paths.
These were last shipped more than 30 years ago. Were they worth
the decades of extra headaches?" John R. Mashey, "The Long Road
to 64 Bits" ACM Queue, 4, 8, October 2006
https://queue.acm.org/detail.cfm?id=1165766
