[LinuxBIOS] Question about protect mode?

[Juergen Beisert]

On Wednesday 30 May 2007 12:20, Feng, Libo wrote:
> I am also confused a little. The propriety BIOS runs in the real mode, how
> does it test the memory beyond 1MB?

It switches also into the Linear Flat Mode. After the test is done it switches 
back to realmode.

> Another question is BIOS ROM can attach to XBUS, LPC, someone told me, even
> PCI, how dose the address forward to the location?

At what time it accesses this devices? At system start? Or when something like 
DOS runs?
At system start see above.

When DOS runs? Hmmm. There are ways to setup all segment registers to real 
mode limits except for one, the ES. This one you could setup for baseaddress 
0 and limit 4GiB. With this setting you could run real mode code, and 
whenever you like access addresses beyond 1MiB you simply use the segment 
override es:<blablub> for this mnemonic. But this only works within 
assembler. Its a solution, but an ugly one.
Another way is to switch into pm for every access beyond 1MiB and switch back 
to real mode when its done. This is the way some BIOSs goes to copy chunks of 
memory content from below 1MiB to above 1MiB.

Juergen