Arthur T. wrote:
On 13 Jul 2005 16:01:06 -0700, in bit.listserv.ibm-main
(Message-ID:<[EMAIL PROTECTED]>)
[EMAIL PROTECTED] (Edward E. Jaffe) wrote:
In open code, people often coded
LABEL EQU *
but, if there was an odd-length constant prior to it, it would not
be halfword aligned, so people started using the DS 0H technique,
instead. (Also, IIRC, the TEST instruction has more information if
you code as DS 0H.)
Of course, with modern HLASM it's preferable to code 'DC 0H' rather
than the 'DS 0H' as required by older assembler implementations.
Would you please elucidate? IIRC, DS 0H will fill with zeros
just as will DC 0H. Why is the latter preferable?
No. 'DS 0H' does *not* fill with zeros! Only 'DC 0H' does that!
The HLASM Language Reference is quite explicit about this. Read the
description of the DS instruction here:
http://publibz.boulder.ibm.com/cgi-bin/bookmgr_OS390/BOOKS/ASMR1010/5.19
It contains the following admonishment:
<Admonishment>
Bytes Skipped for Alignment: Unlike the DC instruction, bytes skipped for
alignment are not set to zero. Also, nothing is assembled into the
storage area reserved by a DS instruction. No assumption should be made
as to the contents of the skipped bytes or the reserved area.
</Admonishment>
'DS 0H' simply updates the offset within the emitted object deck.
If/when the slack byte is needed, this causes a new control record to be
generated in the load module, which uses more disk space and slows down
the fetch process. (At least this used to be true for the linkage
editor. Admittedly, I haven't checked lately to see if the z/OS V1.6
binder continues this treatment.)
In any case, the doc is pretty clear about this substantial difference
between DC and DS.
--
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