On 12/29/15, Paul Gilmartin posted one of his favorites:
000000 00000 00002 1 LOC1 CSECT
000000 00000 00002 2 LOC2 LOCTR
000000 3 LOC2C DS 0C
000000 4 LOC2H DS H
000000 00000 00002 5 LOC1 LOCTR
000000 6 DS (LOC2H-LOC2C)C
** ASMA080E Statement is unresolvable
000000 7 DS 0H
8 END
and commented first:
"It's ironic that every expression depends only on symbols which appear
earlier in the source, but may be unresolvable."
This is because a LOCTR group is a set of statements to which values are
not assigned until the complete sequence of LOCTRs and CSECT statements
have been processed -- effectively, they are "suspended" with unknown
values at the time statement 6 is encountered. In this specific case,
it's not known yet whether the symbol LOC2H will require a padding byte to
follow the LOC2C symbol.
He then commented:
"I wonder whether HLASM's lookahead is documented sufficiently completely
that a determined programmer with the Reference can decide the correctness
of any program without performing a test?"
First, the problen in the little program example above has nothing to do
with lookahead mode, which is specific to conditional assembly. When the
attributes of a symbol are required in a conditional assembly expression,
and the symbol is unknown, the assembler makes a forward scan over the
SYSIN file searching for a first occurrence of the symbol. It doesn't
matter whether or not that occurrence will eventually appear in the final
assembled version of the program.
Second, a "determined" programmer can indeed decide on the correctness of
a such program; but it's much easier to let HLASM do the work for you.
I hope this helps clarify both situations.
Regards... John Ehrman
