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1993-05-12
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Date: 05-08-93 (09:15) Number: 62411 of 63372 (Refer# 62379)
To: JOE SIEGLER
From: DAVID TERRY
Subj: Overlay Size
Read: 05-10-93 (05:04) Status: PUBLIC MESSAGE
Conf: BETA (6) Read Type: MAIL FROM YOU (R/O) (+)
-> I already tried 16. Seems to speed it up a little bit. However, my
-> original question still stands. Can you tell me what this overlay number
-> thing DOES? I'm a bit unfamiliar with what overlays do, and even more so
-> what this number thing is.
You'll notice that PCBoard's executable is now over 400K in size.
Rather than holding this entire 400K .EXE file in memory, and using up 400K
*just* for the CODE portion (not even counting the DATA requirements of the
program) it is instead using overlays.
This means that only a portion of the 400K .EXE file is in memory at any given
time. How MUCH of the executable can be held in memory depends on two factors:
1) How much of it is "resident" (not in the overlay) and 2) How big the overlay
bufffer is.
A setting of 16 sets up a 64K overlay buffer. That means that PCBoard can swap
pieces of the .EXE file in and out of memory keeping up to 64K of it in the
overlay buffer.
A higher value allows more of the .EXE file to be held in memory. The highest
possible value, of course, would simply load the ENTIRE .EXE file into memory
at "load time" and would never again access the hard disk.
But that would be wasteful of memory. So a smaller value, such as 16, is used.
This lets PCBoard pull in the code that is running, when it is needed, and
leave the rest of the code on disk until it is needed.
Lower values may hurt performance by causing PCBoard to continually swap code
in from disk - creating more disk activity than would otherwise be necessary.
So allowing you to pick the buffer size allows you to "tune" your system to
meet your memory AND performance needs. If you need more memory, go for a
lower overlay buffer size. If you need more performance, go for a higher
buffer size.