29 Jun
2020
29 Jun
'20
6:01 p.m.
On 6/29/20 12:41 PM, Rich Alderson via cctalk wrote:
From: Chuck Guzis
Sent: Sunday, June 28, 2020 4:51 PM
My point being that implicit "byte" addressability isn't essential if
there are no instructions that can take advantage of it. The PDP-10
does have halfword instructions and although they're called "byte"
instructions, are what I'd call "bit field" instructions (LDB, DPB,
etc.)--but nothing that would implicitly be called a "byte".
Whereas, on say, S/360, an 8-bit byte is intrinsic to the hardware and
so defines addressing granularity.
Character sets on other systems can be wildly variable; consider WPS-8,
for example.
You do what works--and the lack of byte granularity is no particular
hindrance.
--Chuck
It's noteworthy that on the Univac 1100
series, a "byte" could be 6, 9
or 12 bits, but not 8. (36 bit words). The PDP-10 had similar issues,
such as the "packed" string format of 5 7-bit characters per word, with
one bit unused.
Of course, on the PDP-10, bytes can be anywhere from 1 to 36 bits long;
the size is defined in the pointer, not the hardware.
And in the 7-bit ASCII text format, bit 35 (the word is big-endian) *is*
used by the default editor: In order to allow line numbering in source
files for languages which do not allow it, the line numbers are ASCII
strings with bit 35 set, and the monitor (=kernel=operating system) strips
them out before handing them to compilers' input streams.