[Info-vax] BASIC and AST routines

[Stephen Hoffman]

On 2021-11-19 05:35:20 +0000, Dave Froble said:

> 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 
mainline.

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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