UEFI Programming

It's not simple. First, you need an UEFI ABI wrapper. With that, you can call UEFI services. Then you get a handle for the GOP Protocol. Using that handle, you query a list of supported video modes. You pick one, and you use it's "mode code" to call SetMode method of the GOP protocol. After that you'll have a standard framebuffer, same as with VESA.

Here's some code that queries the list of video modes, sorts that and returns info in an array. The array consists of 32 bytes entries, and one looks like this:

Code:

;  8 bytes handle
;  8 bytes interface
;  2 bytes horizontal resolution
;  2 bytes vertical resolution
;  2 bytes scanline
;  2 bytes pixelformat
;  8 bytes mode number

Using that last "mode number" you can set the resolution "vertical" x "horizontal".
To figure out the framebuffer's address and size, get the second field in an entry, then do

Code:

mov     rax, qword [gopinterface]
mov     rax, qword [rax + EFI_GRAPHICS_OUTPUT_PROTOCOL.Mode]
mov     rbx, qword [rax + EFI_GRAPHICS_OUTPUT_PROTOCOL_MODE.FrameBufferBase]

which returns it in rbx.

As said, once you got the framebuffer's address, you can forget UEFI, and you can draw on that by simply using mov. Each line contains "scanline" bytes, so to draw in the 3rd row on screen for example, add 3 * scanline to the framebuffer's address.

Cheers,
bzt