Friday, 11 February 2011

Video Mode Interrogation via BIOS calls.

While investigating the video modes on my laptop to do some sanity checking I thought I'd have a go at using libx86 to allow me to call into the BIOS directly from userspace.  After short consultation with the trusty Ralph Brown Interrupt List, I hacked up some code to get the list of all the video modes and interrogate each one to get the resolution.

I was getting random crashes before I realised that clearing the LRMI_regs struct before populating it with the desired x86 register settings fixed the problem.

Anyhow, the code is available in my git repository here - and it requires one to install libx86-dev and run with root privileges.   I'm still amazed that the code can jump from userspace into the BIOS, gather data and return without totally messing up the system.

Interestingly, a lot of UEFI systems still provide a legacy mode BIOS, so this code still works on these newer systems.

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