The normal horizontal pixel resolution in TV mode is 8MHz and 16MHz in VGA mode - this means there can be approximately (a maximum of) 368 pixels visible on any one scan line. As there is spare bandwidth in TV mode, there is the option to increase the pixel rate to 16MHz, though this sacrifices colour depth (reduced to 16 colours max) – see bit 3 of vreg_ext_vidctrl - this is referred to as Hi-Res Mode. In this mode, the colour index of each normal pixel is split into two 4-bit indices (7:4 = Leftside hi-res pixel, 3:0 = Rightside hi-res pixel).

• The reason why the colour depth is sacrificed is to do with OSCA's line buffers. They are 512 bytes in size so can normally only hold enough pixels for a standard resolution scanline. A true hi-res mode (and not a "filter" such as this) would've required too much reworking of the logic.

Limitations:

This process is applied at the final pixel output stage (sprites included) so can be thought of as a mere "filter”. As such the rest of OSCA is unaware of it, EG: the sprite priority interleave mode system still processes priorities based on full 8-bit pixel values and the hardware line draw system doesn't take account of the higher output resolution. Care should be taken using these systems with hi-res mode.