On 2020-09-05, at 09:31:51, Ed Jaffe wrote: > > On 9/4/2020 10:46 AM, Seymour J Metz wrote: >> VM uses a token of 8X'FF' at the end of the (R1) parameter list. I don't >> recall what the convention is for the (R0) extended parameter list >> introduced by VM/SP. > > We've adopted "eight bytes of foxes" as an end marker for variable-length > parameter lists that use 64-bit pointers. Of course, we realize it means we > have a permanent restriction that the singular byte address of "sixteen > exabytes minus one" cannot be passed unambiguously as a parameter in such > parameter lists. We really don't see that as posing any practical problem for > our software even if that byte is available for acquisition. > > We're also sort of assuming RSM will never hand out the last 1M or perhaps > even 2G virtual in a manner similar to what's done today for the 4K page at > x'7FFFF000'. > C also makes a similar assumption, namely that an address greater than that of any object can be used as a list terminator. No practical limitation except perhaps for certain memory segmentation architectures.
-- gil
