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From: Anne & Lynn Wheeler <lynn@garlic.com>
Newsgroups: alt.folklore.computers
Subject: Re: IBM system/360 ad
Date: Mon, 25 Nov 2019 14:41:31 -1000
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hancock4@bbs.cpcn.com writes:
> https://archive.org/details/Nations-Business-1964-12/page/n51
>
> Available with up to 8 meg of memory!
>
> (I thought S/360 could handle up to 16 meg?  But maybe in those
> days no one could see needing more than 8 meg.  Indeed, I think
> if you wanted that much you had to get the "LCS" which was
> core but slow core.)

s/360 & s/370 architecture only having 24bit (16mbyte) addressing (real
& virtual) ... except for 360/67 which still had "real" 24bit addressing
but support 32bit virtual. Note that was the architecture ... specific
models might have much smaller number of hardware address lines
... limiting the actual amount of memory that could be attached.

by the time 3033 came around ... 16mbyte was huge constraint because of
the excessively bloated MVS kernel size as well concurrent
multiprogramming needed to try and keep system busy ... lots of page
thrashing. They did a kludge for 64mbyte support. There were two
undefined/unused bits in each page table entry (mapping virtual address
to a hardware address). The used the two unused bits to prepend to the
12bit page number (allowing 64mbytes worth of 4kbyte pages ... rather
than just 16mbytes)

Instructions (virtual and real) could only form a 24bit address ... but
a virtual address (using the page table entry hack) could map to a
64mbyte hardware address ... aka hacked 3033 that had internal 26bit
hardware address lines.

complimenting the 64mbyte page table hack ... in the original 370
architecture was fullword (31bit architecture) channel program IDALs
... which could be used to generate 64mbyte I/O transfer addresses.

-- 
virtualization experience starting Jan1968, online at home since Mar1970