Tag Bits was RE: Prototyping printf() -- RPG400-L

"tag bits" and "pointer bits" are the same thing.  They are called "tagged
pointers" and "pointer tag bits".

You may recall that the System/38 main storage page frame was 512 bytes and
that a System/38 disk sector was 528 bytes.  You might ask yourself what was
stored in the other 16 bytes?  Lesse, 1 tag bit per 16-byte aligned quad
word equals 32 bits or 4 bytes.  They must have stuffed the virtual address
in there somewhere or a 3F10 (storage management rebuild) wouldn't have
worked and that would represent another 41 bits or just over 5 bytes.  That
leaves almost 6 bytes for Pac-Man!  Or, maybe some owner information?  Just
imagine if main storage words were actually 65 bits long?  Couldn't you
think of something cool to do what that extra bit?

The System/38 memory layout was different than the RISC boxes but the 38
information was in the Theory of Operation section in the System/38
processor service manuals and in the IMPI manual.

Richard Jackson
mailto:richardjackson@richardjackson.net
http://www.richardjacksonltd.com
Voice: 1 (303) 808-8058
Fax:   1 (303) 663-4325

-|-----Original Message-----
-|From: owner-rpg400-l@midrange.com [mailto:owner-rpg400-l@midrange.com]On
-|Behalf Of Sims, Ken
-|Sent: Friday, October 06, 2000 10:15 AM
-|To: rpg400-l@midrange.com
-|Subject: RE: Prototyping printf()
-|
-|
-|Hi Eric -
-|
-|>The main reason I went through this exercise was to try to simulate
-|>some semblance of object orientation by having externally defined data
-|>Structures for the object and then creating instances of them in
-|>memory with pointers to other objects contained within... I have some
-|>of this working :-) But after Barbara's warning that this is not
-|>supported behavior I will have to figure something else out :-(
-|
-|If I am understanding properly what you are trying to do: there
-|is no way to
-|force your externally described data structures to be aligned on a 16-byte
-|boundary.  Therefore your character fields that you are using for storing
-|pointers may not be aligned on a 16-byte boundary; which means that any
-|pointer that you store in one of those non-aligned fields is immediately
-|invalidated and no longer usable as a pointer.
-|
-|>It would be nice to be able to define a pointer data type externally
-|>:-)
-|
-|That would get around your problem, but ... if pointers could be
-|defined in
-|database files, that would mean that data management would have
-|to actually
-|be able to store pointers in a database file and maintain the tag bits.  I
-|don't recall enough off the top of my head about storing pointers
-|on disk to
-|know if this is feasible.
-|
-|My first thought was to have a "valid pointer" attribute flag like
-|null-capable fields, but it only took about 15 seconds to realize that
-|wouldn't work.  It would be possible to touch either the field without
-|touching the flag or vice versa, allowing an invalid pointer to
-|be marked as
-|valid.  The "valid pointer" flags have to be stored where they are not
-|addressable except directly through the hardware, just as the tag bits for
-|pointers in memory are not directly addressable.
-|
-|Ken
-|Southern Wine and Spirits of Nevada, Inc.
-|Opinions expressed are my own and do not necessarily represent
-|the views of
-|my employer or anyone in their right mind.
-|
-|+---
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