Video Modes

A project log for YATAC78 - The WWW TTL Computer

Retro computer built from 1978-era TTL logic chips. Internet capable with built in web browser and server

Alastair HewittAlastair Hewitt 09/25/2019 at 05:120 Comments

An early log talked about 16 possible video modes. This is still the case, but a lot has changed since then. The following should clarify what the current modes are and how they are supported.

The 16 modes are defined by 4 bits with the following states:

Mode0 is a hardware state (bit 4 of the Eo register) and selects whether the GPU executes one (graphics mode) or two (text mode) machine-cycles per process cycle. The two-machine cycle will complete 80 active process cycles per line, representing 80 characters composed of a code point and color byte. The one-machine cycle completes 160 active cycles, either as 160 single color values (low-res graphics) or 160 nibbles (hi-res graphics).

Mode1 is also a hardware state (bit 5 of the Eo register) and selects whether the 8x8 or 8x16 glyphs are selected from the font ROM. This bit is also used to define the high/low resolution setting for the graphics mode.

Mode2 is used to control the number of lines per frame in software. A low value selects a VGA mode (640x480) at a field rate of 75 Hz using 512 lines per field. A high value selects an SVGA mode (800x600) at a field rate of 60 Hz using 640 lines per field.

Mode3 is also used to control the video timing in software. The number of lines are divided down depending on the video mode and there are two different ways to do this: A low value selects a Mod16, allowing the timing to be divided down by 2, 4, 8, or 16. A high selects a Mod15 allowing the timing to be divided down by 3 or 5. Multiples of 2 are also available to divide down by 6 or 10.

The following tables show all the resolutions available by combining the Mode0 and Mode1 bits for the columns and the Mode2 and Mode3 bits for the rows. The value of the modulo is shown in brackets next to the resolution (%n).








VGA%16320x240 (%2)160x120 (%4)80x60 (%8)80x30 (%16)
VGA%15320x160 (%3)160x96 (%5)80x48 (%10)*160x80 (%6)
SVGA%16320x256 (%2)160x150 (%4)80x75 (%8)80x36 (%16)
SVGA%15320x200 (%3)160x120 (%5)80x60 (%10)*160x100 (%6)

*Note: Mod15 is not used for the 8x16 glyph text mode, so an additional lo-res graphics mode is defined using a modulo of 6.