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Index > OS Construction > Can't understand the DOS HMA stuff. Help.

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sid123



Joined: 30 Jul 2013
Posts: 340
Location: Asia, Singapore
sid123
The OSDev Wiki Says
Quote:

If you then use that segment register as a base, with an offset of 0x10 to 0xFFFF, you can access physical memory addresses from 0x100000 to 0x10FFEF

Alright I set DS to 0xFFFF, then enable the A20 Line, but I am not getting this,
DS as a base? The Segment:Offset is as follows (in my opinion)
0xFFFF:0x10, but how am I supposed to use DS as a base with that segment offset?

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Post 29 Oct 2013, 09:43
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nop



Joined: 01 Sep 2008
Posts: 165
Location: right here left there
nop
this is just 65520 bytes of memory above the 1meg limit which any real mode os can acess using 16bit segmented memory regs including cs so code can be run from there also Smile
Post 30 Oct 2013, 08:31
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cod3b453



Joined: 25 Aug 2004
Posts: 619
cod3b453
The segment represents bits 19:4 as a "base" and the offset is added on for bits 15:0; hence the linear address is "(base*16)+offset". Setting any segment to FFFF will mean you start at linear address FFFF0 and can add an offset of up to FFFF to reach 10FFEF if the A20 (i.e bit 20) line is enabled. (If A20 is disabled, this will truncate to 0FFEF.)
Post 01 Nov 2013, 22:35
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