Re: LQ instruction question

[David McKenzie <davemck@xxxxxxxxxx>]

Hi Leif,

[Dave McKenzie]
(I'm trying out your new quoting style :-)

I think the tag bit(s) only indicate whether or not there's a pointer
(of _any_ kind) in the quadword, so it's logically just a single bit.

What _kind_ of pointer is there is coded in the leftmost byte of
the quadword itself.  If you compile the following MI snippet:

    CMPPTRT(B) APTR, X'00'/EQ(ATAG);  /* no ptr        */
    CMPPTRT(B) APTR, X'01'/EQ(ATAG);  /* sys ptr       */
    CMPPTRT(B) APTR, X'02'/EQ(ATAG);  /* spc ptr       */
    CMPPTRT(B) APTR, X'03'/EQ(ATAG);  /* data ptr      */
    CMPPTRT(B) APTR, X'04'/EQ(ATAG);  /* instr ptr     */
    CMPPTRT(B) APTR, X'05'/EQ(ATAG);  /* invoc ptr     */
    CMPPTRT(B) APTR, X'06'/EQ(ATAG);  /* proc ptr      */
    CMPPTRT(B) APTR, X'07'/EQ(ATAG);  /* label ptr     */
    CMPPTRT(B) APTR, X'08'/EQ(ATAG);  /* suspend ptr   */
    CMPPTRT(B) APTR, X'09'/EQ(ATAG);  /* sync ptr      */
    CMPPTRT(B) APTR, X'0A'/EQ(ATAG);  /* obj ptr       */
    CMPPTRT(B) APTR, X'0B'/EQ(ATAG);  /* behavior ptr  */
    CMPPTRT(B) APTR, X'0C'/EQ(ATAG);  /* method ptr    */

and then dump & disassemble with SST, you can deduce the
bit combinations (tho I haven't actually slogged thru
this exercise yet myself :-)

--Dave

>  [Leif Svalgaard]
> With all this in mind, how can the LQ instruction know what type
> of pointer it is trying to load? there are only two physical tag bits
> available and they must both be set to have a valid logical tag.
> Maybe Frank is not telling the whole truth or maybe the 4-bit
> field is part of a design that was never actually implemented?
> I've tried various combinations of the 4 bits, and it doesn't seem
> to matter  ??????


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