Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!betelgeuse!jamesa
From: jamesa%betelgeuse@Sun.COM (James D. Allen)
Newsgroups: comp.arch
Subject: Re: hardware support of reference and change bits
Message-ID: <50291@sun.uucp>
Date: 21 Apr 88 04:13:38 GMT
References: <1458@hubcap.UUCP>
Sender: news@sun.uucp
Lines: 18
Keywords: page replacement reference bit change bit
Summary: inaccurate reference recording

In article <1458@hubcap.UUCP>, mark@hubcap.UUCP (Mark Smotherman) writes:
> P.S.  The IBM XA Principles of Operation manual gives an unusual disclaimer
> on the reference bit:  "The reference bit may be set to one by fetching data
> or instructions that are neither designated nor used by the program, and,
> under certain conditions, a reference may be made without the reference bit
> being set to one.  Under certain unusual circumstances, a reference bit may
> be set to zero by other than explicit program action." [p. 3-11, March 1983
> edition of SA22-7085-0]  The first case would appear to be caused by
> prefetching that crosses page boundaries (e.g. branch target buffers).  The
> other two cases elude me.

370 Models 165-II, 168, 3032, 3033 (and 308x, 309x ?) do not set the
reference bit on cache hits.

I don't know about "setting the reference bit to zero by other than explicit
program action" but the 168 family stored three copies of each reference
bit and used "majority rule" instead of parity-checking so the clause may
have been intended as a loophole for hardware errors.