"IBM Mainframe Assembler List" <[email protected]> wrote on
03/03/2022 05:22:52 PM:
> Not counting any branch following the determination, I had
> previously managed to trim this down to three instructions. For
> example, to determine if a prospective insertion crosses a 4 K-byte
boundary:
> 1. LAY S,-1(L,P) where "S" is a scratch register.
> 2. XR S,P determine the difference
> 3. NILF S,X'FFFFF000' Turn off insignificant bits.
The second operand of the first instruction in this sequence
caught my eye -- having not seen negative displacements -- before but
immediately appreciating its applications. So, I first thought I would
ask what are the benefits of doing it that way as opposed to the following
more traditional approach (other than the obvious of simply "one less
instruction")?
LA S,0(L,P)
AHI S,-1
Secondly... I missed the Y the first time and tried testing the
following. That, of course, generated a syntax error (invalid
displacement).
LA S,-1(L,P)
So, I looked at the original email again and did some research
into the LAY instruction. That is when I read about the Long Displacement
Facility (LDF) having to be installed for this instruction to work
(otherwise: operation exception). So, other than just trying it, how
would I determine if the box we are on (2818) has the LDF? Further, if
developing software for distribution, is it best to avoid using the
"newer," fancier instructions and just stick with the more traditional
approaches?
Sincerely,
Dave Clark
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