On 2 October 2015 at 13:57, Scott Ford <[email protected]> wrote:
> I am building a branch table like this:
>
> FUNC DS CL10
>
> FUNCTBL DS 0H
> DC 'INIT 'A(INIT_DSP)
> ......etc
> TBLENT DS F'5'
>
> ------------------------------------------------------------------------------
>
> LA R4,FUNCTBL
> L R5,TBLENT
> LOOPTBL DS 0H
> CLC FUNC,0(R4)
> BE FOUND_IT
>
> FOUND_IT DS 0H
> LA R4,10(R4)
> L R14,R4
> BALR R14,R15
>
>
> Is my logic correct ? or did i miss something ?
It's on the right track, but is a little confused...
Your CLC is using the implict length of FUNC, which is 10, so you need
to be sure all your table entries like DC 'INIT ' are also (at
least) 10 bytes long. This is a lot easier to see in a fixed-pitch
font; yours is correct, but best to specify them as CL10'INIT' to get
trailing blank padding. Even better would be to define a symbol,
either an ordinary symbol with an EQU or a variable symbol via SETA,
to use in all places. Then you can easily change the length of your
function keywords globally. Something like:
&FunLen SETA 10
FUNC DS CL&FunLen
...
CL&FunLen'INIT '
...
LA R4,&FunLen(,R4)
I appreciate that it's just a snippet, but your code falls into the
"found" case. You show no code for the case where the CLC fails. You
need to increment the table pointer over both the text and the adcon.
But... FUNCTBL starts on a halfword boundary. The assembler will force
the adcon following the 10 bytes of text to a fullword boundary, which
may or may not be correct for the first table entry, but will
certainly be incorrect for each subsequent entry. So when you
increment by 10 as you search, you will not be pointing at the text in
the next entry. Either make your text field (&FunLen) a multiple of 4
(perhaps 8 or 12), or change your adcon to not boundary-align, i.e.
AL4 rather than just A. Or put the adcon before the text; there are
several approaches.
TBLENT is a DS rather than a DC. It won't be set to any predictable
value at run time.
> L R14,R4
This sets R14 to the address of the adcon in the entry you just found
in the table (or it would if the above-mentioned boundary alignment
problem wasn't there). You need
L Rn,0(,R4)
But in any case you don't want R14 since the immediately following
BALR sets R14 and branches to the unset R15.
Tony H.
