Ref: Your note of Thu, 6 Nov 2014 11:01:48 -0700 Thanks for the comments!
Paul Gilmartin wrote: > On 2014-11-06, at 04:24, Jonathan Scott wrote: > > > > 1. Dependent USING support for long displacements > > ... > I would not impose the 20-bit limit. HLASM should immunize itself > against potential hardware extensions. ... If it can assume 20-bit displacements when OPTABLE includes the support, it could assume something different if anything new came along. Keeping the base value within range means that we can at least avoid one potential cause of wrap/overflow problems. > > 2. USING support for long displacement limits > > ... > > USING (base,limit,lower,upper),... > > ... > > The above change for USING limits also allows the same base > > registers to be used to address multiple discontiguous parts > > of the same section, ... > > > I question the value of this. It leads to the paradoxical > situation that a certain address may be addressable with > short displacement but not with long. A 12-bit displacement has to satisfy both pairs of limits for the location to be addressable, so this problem cannot occur. The idea is to keep the existing 12-bit limit independent, as it is sometimes used to ensure a safe margin of addressability for future expansion, which could still be useful even when some instructions use 20-bit displacements. > > 3. DROP by address > > ... > > The suggested solution is that if an operand to DROP is a > > relocatable expression rather than a register, it drops the > > USING statement that would be used to resolve the relevant > > address. > > > Are dependent USINGs referring to the same base register > allowed to overlap? There should be no objection to this > provided that they resolve to identical base and displacement > regardless which USING is used. But this introduces an > ambiguity in "DROP by address" when that address can be > resolved, albeit equivalently, by either of two dependent > USINGs. When all else is equal a new dependent USING for a given location should supersede any older one for the same location and base USING. Jonathan Scott IBM Hursley, UK
