Maximum Overscan

Explains how to setup a maximum overscan screen.

The quick answer

Maximum overscan is 384 x 285 (supporting OCS and up).
This is for Lores mode, no sprites on. Hires and Superhires double and quadruple the horizontal resolution, but you might have to deduct a few pixels at the left edge. If you're using interlaced mode, line $138 might flicker as you're using an odd vertical pixel count. This line might also stop outputting pixels about 2/3 of the way horizontally on some chipsets by the vertical blank trigger. 284 recommended therefore.
Copper list for a 384 x 284 screen:
dc.w $8e,$1b51
dc.w $90,$37d1
dc.w $92,$20
dc.w $94,$d8
dc.w $102,0

For AGA with 64-bit FMODE, you might have to decrease DDFSTOP with up to $36 to make modulo values display correctly.

Display types and visible area

It would be impossible to account for all display types, but experience suggests the following generalizations:
  • CRT monitors will let you display the full overscan, but some might hide the leftmost 13px
  • CRT TVs display ~342 x 268px, sometimes offset to the right by a fixed ~12px
  • Modern flatscreen TVs (2011+) emulate PAL CRT TVs but may reach higher overscan. One older one has followed the description of flatscreen monitors below.
  • Flatscreen monitors I've experienced and owned have had ~360x256, or 320x256, or 320x240 (cropped), all stretched fully to the edges
  • 16:9 TVs and monitors will letterbox horizontally, but less so using an overscan display.

Applications

Often, the point of using overscan is to get rid of the borders around the picture. By using maximum overscan, this does the trick on all display types but 16:9.
For games, there's a small caveat: if your gameplay relies on the player seeing vital graphics such as enemy sprites ahead of time horizontally, or part of the UI vertically (and this is outside the standard 320x256 display window), consider allowing the user to shrink the screen to match his display type, as is done in the game Speedball.