VIDEO PROCESSOR BOARD

The screen of the Aster CT-80 is managed by an MC6845 VDU IC. Video memory on board is 2 kByte, which allows for 25 lines of 80 characters in CP/M mode. In TRS-80 mode only 1 kByte is accessible, for 16 lines of 64 characters. 80 character lines are possible, but only 12.5 lines are accessible.

The 6845 VDU is accessible on ports FCh and FDh. Port FCh is the address register and FDh the data register. As the 6845 VDU only uses the lower nibble of the address register, some extra Aster functionality is present at the higher nibble:

FCh,1xh enable TRS80 mode
FCh,2xh enable CP/M mode

FCh,4xh enable deglitcher
FCh,8xh disable deglitcher

Note the TRS80 and CP/M mode only controls the pixel-clock of the VDU and the selected character set (for newer VDU-boards). Not the memory map.

The original character generator ROM is 4 kByte large, containing 8x12 characters.
Later VDU boards have an 8 kByte character generator ROM, containing two different character sets (for TRS-80 and CP/M?).

The deglitcher works by detecting CPU-access when the VDU scans video-memory and asserts the CPU /WAIT until the VDU makes the blanking signal valid. With a disabled deglitcher the program runs faster, but the screen shows some 'snow'.