How to use overlay hunks?

[Mrs Beanbag]

I first heard of these when using Imploder to crunch some executable, I gather you can use them to load your executable one piece at a time in any order but how does it work? None of my books cover it.

[a4k-oerx]

Maybe this PDF "Amiga Binary File Structure" can help, it explains the format in detail:

http://amiga.rules.no/abfs/abfs.pdf

[Mrs Beanbag]

thanks, but that's going to take me a while to get my head around... maybe a bit too much detail and not enough how to actually use it.

[Toni Wilen]

http://eab.abime.net/showthread.php?t=59407

[Leffmann]

I know you have the Amiga Guru book, and if you look in the last chapter you'll find a few pages with some code examples.

[Samurai_Crow]

According to the documentation with SAS/C, there is a 50 overlay limit. Is that hard-coded into AmigaOS 3.x or is it local to the SAS/C overlay manager?

[hooverphonique]

Originally Posted by Samurai_Crow

According to the documentation with SAS/C, there is a 50 overlay limit. Is that hard-coded into AmigaOS 3.x or is it local to the SAS/C overlay manager?

unless there is a generic 50 hunk limit in amigados, this has to be in sas' overlay manager..

[Toni Wilen]

AmigaDOS does not have any hunk amount limits. (Except available memory)

[Samurai_Crow]

Thanks, guys, I've found another way to do what I wanted without using overlays. And thanks Mrs. Beanbag for bringing up the subject.

[Minuous]

I have a program that compiles and links fine normally, but if I try to use overlays, it gives an error for every global function (except those in the root module) saying it is declared twice, both times in the same file with the same value and type. I have the SAS/C manual but it doesn't have much that is relevant to this error.

[Thomas Richter]

Not sufficient information. What are the compiler options? Care to share a minimum (non-working) example, including compiler and linker options?

Note that SAS/C offers two calling conventions (registerized parameters and stackbased) and the overlay manager needs to fit to this convention. You can also check with the OMD command which symbols a module exports and imports, and thus check whether there are any duplications. Also note how the linker option looks like, potentially you put the same node twice. The syntax for the WITH files requires some careful reading.

[Minuous]

OMD command shows there are no duplications.

Odd that they support two calling conventions but seemingly only provide an overlay manager for one of them :-/

Anyway, I have made and attached a very minimal example. This example is using all default options, therefore there is no SCOPTIONS file to attach. If lines 3..5 of WITHFILE (ie. the overlay part) are removed, it links successfully, otherwise SLink thinks that _hello is defined twice in second.o.

[Minuous]

Does anyone know of an alternative compiler that does have working overlay support?

[Thomas Richter]

SAS/C does have working overlay support, but you probably have to tune the compiler parameters a bit. Maybe ALVs conflict with the trampolines the linker has to generate for overlays, so maybe you should not use the small code model here.

[Minuous]

Actually it does generate a working program despite the linker error messages, however the program is about twice the usual size, and appears to contain all code and data twice.

[Thomas Richter]

Then you did not use the linker properly. It seems you link all modules both in the root node and into overlay nodes. There is an example in the Bantam book how the "WITH" file should look like.

[A10001986]

Yes, indeed, second.o is linked twice.

I use this (with an assembler program, hence no LIB stuff)

ROOT
<all root files>.o
...
OVERLAY
<all overlays>.o
#
TO <bin>
OVLRMGR <name of overlay manager>.o

[Minuous]

Thanks, I have it working now. I thought there was some difference between FROM and ROOT so had both :-(

[oRBIT]

I was interested in this overlay-stuff awhile back, but was pretty terrified how complex it seems to be. Perhaps that's why it's very rarely used (as far as I know) on the Amiga.

[Olaf Barthel]

Originally Posted by oRBIT

I was interested in this overlay-stuff awhile back, but was pretty terrified how complex it seems to be. Perhaps that's why it's very rarely used (as far as I know) on the Amiga.

The overlay feature fell out of favour when Amigas either shipped with more than 512 KBytes of memory, or could be expanded by adding memory plug-in cards (or something like the A590 for the Amiga 500). That was around 1988/1989, if memory (ha!) serves.

The feature itself required preplanning, if you followed the original overlay management design (e.g. Lattice 'C' versions 3 & 4 and SAS/C versions 5 & 6). The documentation which covered how you made it work was decent enough, especially for SAS/C. You had to know in advance which portions of your program could be in memory at a time. Not every program could be conveniently broken down into such units, though.

Manx Aztec 'C' did away with the minute preparations for keeping an exact subset of a program in memory, by virtue of its custom overlay manager. You could break down the program into units at the linker stage and then later load/unload portions of it as required. This was the method preferred by the industry by 1989/1990.

But, as I mentioned, the constraints which required overlay support eventually went away as RAM became cheaper and Amigas became faster with each new iteration of the platform.