Re: Assembler: alignment of machine instructions

"Bruce Uttley" <beu@[EMAIL PROTECTED]
> wrote in message 
news:dc8s3i$vk0$1@[EMAIL PROTECTED]
> In article <VPSdnXoSaKoDpnrf4p2dnA@[EMAIL PROTECTED]
>,
> Sven Pran <no.direct@[EMAIL PROTECTED]
> wrote:
>> ...
>>
>>Do you also know and understand the difference between
>>
>>XXX DS 0H
>>
>>and
>>
>>XXX DC 0H'0' ?
>>
>>There is a significant difference (although normally not very
im****tant).
>>
>>regards Sven
>>
>
> The first example merely resets the location counter for the
> next generated output in the object deck.  If the "DS 0H"
> should bump the location counter to a halfword alignment, the
> extra byte is uninitialized data in the object deck.
>
> The second example conditionally generates an extra byte to
> align to the next halfword as if it were a DC.  The object
> deck will contain any forced alignments initialized with needed
> bytes set to X'00'.
>
> The second technique for generating labels in a macro was once
> preferred because it could generate object decks with fewer records
> (because the location counter wasn't being reset) and because
> the extra padding for alignments would be initialized in the
> object deck.  Neither benefit is im****tant today, but it was
> once a [very] small concern.
>

Quite correct.

The "rule" was that if you needed to force alignment immediately
following a DS instruction then you could better use DS 0H, but
if the immediately preceeding instruction was a DC then you ought
to use DC 0H'0' instead.

This "rule" applied to open code as well as in macro coding.

regards Sven