Path: utzoo!mnetor!uunet!husc6!uwvax!rutgers!mtune!codas!novavax!hcx1!hcx2!daver
From: daver@hcx2.SSD.HARRIS.COM
Newsgroups: comp.arch
Subject: 68020 - Indirect Post-indexed
Message-ID: <93900009@hcx2>
Date: 11 Jan 88 20:59:00 GMT
Lines: 16
Nf-ID: #N:hcx2:93900009:000:839
Nf-From: hcx2.SSD.HARRIS.COM!daver    Jan 11 15:59:00 1988


The MC68020 provides a Memory Indirect Post-indexed addressing mode.

Referring to the 2nd edition of the User's Manual, the last paragraph of 
page 2-10 indicates that "all four user-specified values are optional."
Does this mean that an addressing mode (described on the next page) could
be written as:    ([bd,An], , )    ?
How would the extension word for this be encoded?  According to pages 2-19
and 2-20, it would seem reasonable to set the Index Suppress (IS) bit to '1'
and the Index/Indirect Selection (I/IS) bits to '101'.  But Table 2-2 says 
that such a combination is 'Reserved.'

Are all four user-specified values really optional?  What does the hardware
do when it sees this particular reserved value?  (The preliminary version of
the MC68030 User's Manual seems to say the same thing as the MC68020 manual
in this area.)