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flat assembler > OS Construction > 15 Meg Memory hole?

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Gomer73



Joined: 29 Nov 2003
Posts: 151
Does anybody know about this memory hole between 15-16 meg. I thought MenuetOS had to work around some kind of memory hole. I ran a memory testing program and didn't run into a memory hole on either my 486x66 or my Pentium 466.

Could somebody give me a history on why some systems would have this memory hole.

Attached is the program that checks how much memory you have installed in your system, checks in 1K increments. The top number is the hexadecimal number for the last writeable piece of memory, The bottom number is the decimal equivalent. You might want to clear the screen before running so it doesn't scroll off the screen.

Compile the fos.asm to f.exe and in the subdirectory memory, compile memory.asm to memory(or just run f.exe). Needs to be run in dos. It trashes over some memory about 1 Meg, so you might need to reboot after, but I haven't worked out the code to co-operatively work with himem.sys.


Description:
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Filename: Mem.zip
Filesize: 4.99 KB
Downloaded: 179 Time(s)

Post 09 May 2004, 20:45
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crc



Joined: 21 Jun 2003
Posts: 637
Location: Penndel, PA [USA]
Some old ISA cards require this area of memory for them to work properly. There is usally a BIOS setting for enabling or disabling the 15/16MB memory hole. Enabling this function reserves the memory area for the card's use. In some cases, it may also prevent the system from accessing memory above 15MB.

If you enable this function, 1MB of RAM (the 15th MB) will be reserved and is therefore not available for the OS' use. So, if you have 128MB of RAM, this feature effectively reduces the usable amount of RAM to 127MB.

It's not commonly used, and as ISA is an obsolete technology, it's probably safe to disable it and forget about it. In my work with various OS kernels, and over 20 PC's, I've never encountered even one which needed the memory hole.
Post 10 May 2004, 13:52
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Gomer73



Joined: 29 Nov 2003
Posts: 151
Cool, thank-you.

It is always nice to know the history behind stuff.

I guess the same thing will happen with video memory for machines that have over 4Gig of memory.
Post 10 May 2004, 16:05
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