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From: robin51@dodo.com.au
Newsgroups: comp.compilers
Subject: Re: C compiler pointer management on DSPs
Date: Fri, 28 Feb 2020 10:26:48 +1100
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On 2020-02-28 09:23, gah4@u.washington.edu wrote:

> Machines not so well designed require masking off the appropriate
> bits before operating with them, though many machines ignore high
> bits on shift operations.  (The 8086 allows shifts up to 255 bits.)

Who can say that the CDC machines (7600; 70 series, etc) were not
well designed?

They were intended to be fast, and to carry out operations on
words (of 60 bits).

To be sure, it was necessary to mask bits (usually characters),
but there was a simple instruction(s) to generate a mask of
n bits (better than loading a word containing bits to be used
as a mask).

On the other hand, the IBM S/360 was designed from the beginning to
handle bytes of 8 bits, half-words of 16 bits, and words of 32 bits.

Instructions could load and store a byte into/from the low-order
bits of a register, without affecting the other bits.  Later models
could load/store one or more bytes into/from a register without
affecting the other bits.

For more general work, masking operations were available in the
32-bit registers.