SAB8275P

SAB8275P - Video Controller

The 8275 is a very versatile logic chip for generating text displays (*1) so nearly any terminal format of that time, with up to 80 characters and whatever attributes needed, can be handled. It was made to cover the full range from simple 32x20 with 8x8 cells all the way to high end terminals with 80x25 using 16x12 characters. It can generate any display with up to 80 characters per line and 60 character lines per frame with up to 16 (pixel) rows per character and arbitrary number of pixel columns, which allows quite great looking displays. This adds a lot of design freedom about how the display should look.

It's remarkable by having two line buffers to handle character timing independent from memory timing, while all access is done using external feeding. While Intel of course recommends their 8257 DMA controller, it can be done by any other logic as well. By using a 8275, integrated video can be designed without the need to synchronise system clock(s) with video timing (*2), as here video access synchronises with the host.

The other remarkable feature is that it didn't simply output some fixed size character matrix, but worked the data read more like a stream of instructions or a display list. Somewhat like Atari or Sinclair later did. A set high bit denoted a function, which could be either setting attributes for the character(s) following, or controlling memory access. For example a F1h code stopped DMA (and display) for the remaining line. A nice way to reduce memory load. Similarly, a F3h would stop DMA for the remaining screen. So a cleared screen with a prompt would only need to read some 5-6 bytes, reducing memory load greatly.