Interlace Basics (Frame Indentification)

Hi all,

I'm currently trying to understand how the Amiga chipsets were/are working better. I understood that bplcon0 & 4 represents the interlace status bit. So because of this bit the chipset knows that the frames should be interlaced. When the frames are drawn, is there any information (a bit in any register or an invisible line in the displayed data) that makes an external/internal device know, if we are currently drawing the first half frame or the second half frame?

Thanks in advance for your help!

[Toni Wilen]

VHPOSR LOF bit tells current field type.

Externally only way to detect it is do same detection as normal televisions do, check timing of sync pulses during vertical blank. I guess you could also count number of lines (313/312 or 263/262).

btw, it is also possible to create normal interlace mode without setting BPLCON0 bit 2, by toggling VHPOSW LOF bit. BPLCON0 bit 2 only means: LOF bit is toggled automatically at the end of each field (which you can also do manually if you really want to..)