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Video Overlay Card (VOC)

The Apple IIgs Video Overlay Card sits in slot 3 and can display 640×400 SHR by interlacing the two 32K Super Hi-Res buffers in banks $E0 and $E1.

GSSquared implements that interlace mode as a progressive 60 Hz composite (both fields every frame), matching KEGS. Overlay / genlock / dissolve registers are stored but not rendered. The card has no slot ROM — $C300–$C3FF stays the built-in IIgs 80-column firmware. Control is only via device-select $C0B0–$C0BF.

Second Sight also uses slot 3; pick one or the other in the config.

How to enable it

In the config editor

  1. At System Select, open + or Edit….
  2. Choose an Apple IIgs platform (ROM 01 or ROM 03).
  3. Click slot 3 and pick Video Overlay Card.
  4. Save the .gs2 config, then launch it.

In a .gs2 file

[[cards]]
slot = 3
card = "voc"

Enable 640×400 interlace

From the monitor (CALL -151) or guest software:

C029:C1       ; SHR + host linearization
C0B1:39       ; SHRSource=11, GG Bus enabled
C0B5:80       ; InterlaceEnable

Even lines come from $E12000 (aux); odd lines from $E02000 (main). Each bank has its own scan-line control bytes and palettes. Disable interlace (clear $C0B5 bit 7, or leave $C0B1 5:4 other than 11) to return to normal IIgs video.

Detection reads at $C0B7, $C0B8, and $C0BD return $00 so Uthernet / Second Sight probes do not see those cards.

Further reading