How to use overlay hunks?

[Thomas Richter]

Well, if you look for example at the evolution of DPaint, then it had used some form of "overlay management" in (almost) every version. The first versions used the MANX overlay manager. The last version used external libraries, one library for CPU blitting, another library for custom-chip blitting. The net effect of these two designs is identical: You only load the program code needed, and leave the unused code just sitting on disk "in case".

However, the library solution has a drawback: You can no longer just move the program around on your harddisk, you need to copy it along with the libraries. Or store the libraries in LIBS: However, as the API of these libraries is private and undocumented, they are of no use for any other program - it is somehow the wrong answer for the problem EA wanted to solve (despite falling RAM prices), namely to use system RAM wisely.

With overlays, the entire program code is kept nicely together in a single place, and there is no need to clutter the application directory with private libraries nobody else is able to use anyhow.

[CFou!]

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.

EOB2 main game file use OVERLAY structure (which caused me some problems resourcing the game code)