Re: Fat references (Anton Ertl)
Sun, 03 Jan 2010 20:09:09 GMT

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From: (Anton Ertl)
Newsgroups: comp.compilers,comp.arch
Followup-To: comp.arch
Date: Sun, 03 Jan 2010 20:09:09 GMT
Organization: Institut fuer Computersprachen, Technische Universitaet Wien
References: 09-12-045 09-12-055 10-01-003 10-01-008 10-01-009
Keywords: architecture, history
Posted-Date: 03 Jan 2010 16:16:48 EST

glen herrmannsfeldt <> writes:
>Certainly the PDP-11 was described as having a 16 bit word, but I
>don't remember double word, or especially quad word being used in
>context with the PDP-11.

Not "double word", but "longword". And if that was not used for the
original PDP-11, it was used for its successors, the Virtual Address
eXtension (VAX) of the PDP-11 and the extended VAX (EV, aka Alpha).
[The PDP-11's optional floating point units did address doublewords,
which were different from VAX longwords because they goofed and stored
the high 16 bits at the lower addresses. -John]

The PDP-11 was not a compatible successor of another DEC machine, so
the PDP-11's natural word size (16-bits) was used for the "word"-named

>The 8008, 8080, 6800, 6502, and others at that time were considered
>'eight bit processors' even though the 8080 could do some 16 bit
>operations. One might have called 16 bits a word, but it wasn't
>commonly needed.

Yes, 16 bits were called a word on such 8-bit processors (there was
another name for 8-bit units: byte), and it was commonly needed,
because these machines used 16 bits for addressing their 64KB address

>Already being used to a 32 bit word for S/360, I remember being
>disappointed at the time that VAX called 16 bits a word. Especially
>if one believed that VAX was supposed to be in the 32 bit processor

But it also was marketed as a successor to the PDP-11, with various
compatibility features. And apparently it's important to keep "word"
the same size when doing such successions, and so 16 bits are still
called a word even in the Alpha architecture.

And you have the same story for the 6800 and the 68000.

And the same story with 8008, 8086 (ok, there it's natural), 80386
(aka IA-32) and AMD64 architectures (and also IA-64), except that here
the 32-bit units are called "double words".

And the analogous story with the originally 32-bit IBM/360, PowerPC,
MIPS, and SPARC architectures and their current 64-bit successors.
They all kept what they called a word the same even though the natural
word size grew.

Crossposted to comp.compilers,comp.arch, followups set to comp.arch.

- anton
M. Anton Ertl

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