Re: Address bits again, Article on new mainframe use

On Wed, 4 Sep 2024 10:58:38 +0200, Terje Mathisen wrote:

Turbo Pascal had overlay support for a while, I did try it once. Afair
it used a base layer thunking system where all cross-module calls passed
through these thunks which would check that the correct overlay was
currently loaded, swapping it in if needed. This was OK for making a
bunch of calls from the base to a single overlay, but trashed horribly
if/when you had calls between two different overlays.

Sounds similar to the original 68K Macintosh executable format, where the
code was stored in “CODE” resources: the CODE with ID 0 was the table of
cross-segment entry points: CODE 1 contained the mainline, and you could
split off parts of your program into additional higher-numbered segments
as appropriate.

There were two different approaches to this sort of overlay/segmentation
system: the ones where segments containing calls to routines in progress
could be swapped out (so a return to such a routine required it to be
swapped back in), and ones where a segment was locked down as long as any
routine in it had been entered but not yet exited.

As I recall, the Mac used the latter form. This would consume more memory,
but had less potential for trouble. Sounds like Turbo Pascal was trying
the former approach.