Re: Fat references

glen herrmannsfeldt <>
Sun, 3 Jan 2010 21:33:39 +0000 (UTC)

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From: glen herrmannsfeldt <>
Newsgroups: comp.compilers,comp.arch
Date: Sun, 3 Jan 2010 21:33:39 +0000 (UTC)
Organization: California Institute of Technology, Pasadena
References: 09-12-045 09-12-055 10-01-003 10-01-008 10-01-009 10-01-016
Keywords: architecture, history
Posted-Date: 04 Jan 2010 11:19:33 EST

In comp.compilers Anton Ertl <> wrote:
(snip discussion of word, doubleword, and quadword)

> 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).

Yes. I believe that quadword and octoword for 64 and 128 bits came
from the time of VAX.

> [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]

and continued that goof into the VAX and Alpha.

> 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
> unit.

> 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
> space.

But not quadword and octoword.

(snip regarding VAX and 16 bit words

> 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.

It is also important to show the technology advances. VAX was
supposed to be DEC's entry to the 32 bit world. Keeping the word size
at 16 bits dilutes the effect.

I do remember that in early Alpha C compilers a C (long) was 64 bits,
but later changed to 32 bits with (long long) as the 64 bit type.

-- glen

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