Post by Dave Froble
I'm going to confess to curiosity. Why are R0, R1, SP, and PC passed
to an AST routine?
Technically, because VAX.
VAX never saved R0 and R1 across CALLS/CALLG calls as those registers
were the return value path, and PC and PS/PSL are the rest of restoring
the context around the AST. If those registers didn't get preserved
(somewhere) across the AST, then the AST delivery would corrupt the
Pragmatically, because the programming languages that the OpenVMS devs
largely utilized (then and now) could ignore those arguments, and "the
right thing" would happen. (There's a side-discussion in argument
mismatching to be had here, too.)
In aggregate, it's a dumb-arsed API design that exposes the hardware,
and one that will probably only change as part of overhauling BASIC for
64-bit and for more modern expectations. And possibly of overhauling
the OpenVMS APIs.
Minimally, better BASIC doc would be nice, as the AST API has
eventually bitten most BASIC developers, this stuff is ~not documented.
Searching for "asynchronous" in the current BASIC user and reference
manuals gets nada, and searching for "AST" in the user guide has
nothing useful for writing an AST routine in BASIC and nothing in the
reference manual. This would be where a discussion of AST and thread
reentrancy would likely be placed, too.
Smallest and most isolated VSI code change would be an added AST
function keyword that would allow those arguments to be preserved by
the BASIC run-time, but suppressed in the BASIC app source code.
Syntactic sugar in the BASIC compiler.
Moderate to larger changes would involve moving OpenVMS and BASIC to
object support, and with a hypothetical object AST routine would get an
object containing everything, and the app source code would never need
to reference those bits of hardware data. Or as ASTs are a predecessor
to threading, replace ASTs for KP threading or GCD/libdispatch-style
threading save for preserving the existing old-style-API AST source
code. But last I checked, BASIC was AST-safe and not thread-safe, so
this threading overhaul and object-overhaul would be a
comparatively large investment in the future of BASIC.
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