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programming BIOS Memory access

Posted on 1998-06-01
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Last Modified: 2010-04-27
How can I access physical memory above 1meg in an adapter BIOS that I am writing?  I need to access physical memory address like 0xE0000000.
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Question by:eby
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by:rbr
ID: 1018660
Which OS and in which language?
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by:jhance
ID: 1018661
Under MSDOS, you need to get the processor into protected mode to be able to access memory above 1MEG.  The usual method is to use a DOS extender that supports DPMI or VCPI.  

Under Win95, you run into the protected memory problem.  You need to map ask Windows to map the physical address space to your process.  Another approach is to use a VxD virtual device driver which runs as a protected process and has free access to any memory location.

Under NT, you probably need to write a device driver to do wht you want.


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by:eby
ID: 1018662
The question does not involve any OS.  The question was specifice to BIOS usage.  More clearly BIOS ROM.  More specific; PCI BIOS.
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jhance earned 140 total points
ID: 1018663
The BIOS runs in real-mode.  As such, you will have no way to access a ROM which has been located above 1MEG.  This is why the space between 640K and 1MEG is so precious.  It's the only place where you can locate a ROM and get it to run at power on.  If you have to have more space, a common practice is to place a "bootstrap" in the < 1MEG region for you adapter to catch the POST (Power on Self Test) search for an adapter BIOS and then have your BIOS extension switch to protected mode and map the ROM BIOS to a 32-bit address.  Since you're running outside the context of a 32-bit environment, you have to build everything yourself to support protected mode.
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by:zealsoft
ID: 5898036
Under Windows 9x/NT/2000, you can use MemAccess Library(http://www.zealsoftstudio.com/memaccess/) to access the momory and PCI BIOS.
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