[SetBank]

Specifies the commands necessary in order to switch banks on a frame 
buffer device. The function is called as part of the saving and restoring 
of the frame buffer for any mode with an aperture smaller than the 
PageLength. The bank number is assumed to be in r0. If the aperture is 
large enough to accommodate direct access to VRAM needed in any mode, the 
section may be omitted. 

Here is a sample .PMI file for a 9GXE S3 card: 

[SetBank]
                         //r0 contains the bank number
inb(r1, 0x3d4);          //save index
outb(0x3d4, 0x35);
r2   = r0   &  0x0000000f;
outb(0x3d4, 0x51);
inb(r2, 0x3d5);
r2   & = 0x00000030;
r0   >>= 0x00000002;
r2   & = 0x000000f3;
r0   |= r2  ;
outb(0x3d5, r0);         //set bank
outb(0x3d4, r1);         //restore index