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Index > OS Construction > E820 map

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spandexyfronts



Joined: 15 Sep 2013
Posts: 6
spandexyfronts
Does anyone know where it's safe to store things when a computer is first turned on?

What i really mean is, where do you store things ( including the E820 map ) before you've used the E820 map to find out where it's safe to store things?!

Any suggestions?
Post 13 Oct 2013, 21:01
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BAiC



Joined: 22 Mar 2011
Posts: 272
Location: California
BAiC
roughly half (640k I think) of the first megabyte is available to you below the 1MB RealMode boundary. whatever is left between the boot sector and the 640k address is usually the largest contiguous block of memory in Real Mode. it's smaller but you can also use memory between 0x500 and 0x7C00 (the boot sector).

once you turn on Protected Mode (32bit) you can safely use almost all addressing space in those 32bits.

Stefan

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byte me.
Post 14 Oct 2013, 05:59
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spandexyfronts



Joined: 15 Sep 2013
Posts: 6
spandexyfronts
I was straying underneath the 0x500 limit you described there in real mode and wondered what was wrong. Thanks for the useful info, i'm able to 'set things in stone' and be a bit more surefooted now.
Post 14 Oct 2013, 16:50
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cod3b453



Joined: 25 Aug 2004
Posts: 619
cod3b453
I normally avoid the following ranges:
Code:
00000-005FF (BIOS IVT & BDA)
07C00-08C00 (boot sector)
9F000-FFFFF (BIOS EBDA, VGA MMIO, BIOS, extension/additional ROMs)    
My current loader uses 0A000+ for the image and 00600-07C00 for stack/temporaries, which has been plenty for long mode and about 0.5MB to spare Smile
Post 14 Oct 2013, 17:14
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