When using PAL or NTSC TV modes, you can interlace the display to double the apparent vertical resolution. To use this mode you need to set bit 2 of vreg_ext_vidctrl and then alternate the display between two "fields". The first field contains all the odd lines of a given frame and the second field contains all the even lines.

When Interlace Mode is set, the frame timing is altered causing the TV to offset the display every other frame by half the thickness of a scan line. You can find out which field is being displayed by reading bit 7 of vreg_read.

The field switching can be achieved automatically without CPU intervention by using the bitplane modulo register bitplane1a_loc and LineCop, though more manual methods are fine too.