🚀 go-pugleaf

> correct opcodes for 386 bit string insert and extract instructions

These instructions, which were documented in early 386 data sheets, are
not supported.  According to Intel's errata sheet:

"Since the 80386 has unique and powerful 64-bit Double Shift instructions,
and fast multi-bit shift and rotate instructions, the "Bit String Insert"
and "Bit String Extract" instructions were removed.  The insert/extract
complex instructions did not provide an additional benefit that fully
justified including them in 80386 silicon and all future compatible
processors.  A review concluded that the 80386 user obtains full performance
in bit string manipulations using other powerful instructions such as 64-bit
Double Shift, and other multi-bit shift/rotate instructions.  These
instructions support extremely fast manipulation of general unaligned bit
strings of any length, by processing them in 32-bit chunks."

The question that naturally arises is, so why were these instructions
included in the architecture in the first place?  I suppose we could be
generous and take Intel's explanation at face value, and assume that
the inclusion of these instructions was an overzealous action on the
part of some architect that Intel later saw fit to rectify.

On the other hand, I tend to be more cynical, and assume that these
instructions didn't work, and that Intel decided to remove them from
the spec rather than wait for another silicon pass.

Intel has never gone out of their way to let people know that these
instructions were dropped.  Supposedly, everyone that purchased 386
chips got the errata sheet, and Intel notified the complier writers.
And no one programs in assembly language any more, right?

Michael Slater     Editor and Publisher    Microprocessor Report
550 California Ave., Suite 320, Palo Alto, CA 94306  415/494-2677
mslater@cup.portal.com     sun!portal!cup.portal.com!mslater