> [edit] I was misremembering Mode-X working like 4-bit VGA color mode, which is wrong. Given that the tiles are aligned it should be possible to write an entir

> [edit] I was misremembering Mode-X working like 4-bit VGA color mode, which is wrong. Given that the tiles are aligned it should be possible to write an entire tile without switching segments. You don't have to change segments, but you do have to write to all 4 "planes". However, mode-x planes are interleaved bytes, not bitplanes. The first plane has every 4th (8-bit) pixel, the second plane has every 4th pixel shifted one pixel right, etc. You can select any combination, so if all planes are selected, a single 8 bit write will write same value to a block of 4 pixels. If you have some spare VGA RAM and have your tiles there, you can also use VGA latching to copy 4 pixels (32-bits) with a single 8 bit write. Should be a big win on a 286.