Mode Select Port 3Da Logic - Zenith Z-100 Reference Manual

Page 7.30
Video Graphics Programming
The output register at 3DA functions according to the logic of Table 7.14.
Table 7.14. Mode Select Port 3DA Logic
DESCRIPTION
BIT
A logical I overrides the VIDEO ENABLE signal from 3D8.
Eliminates monitor display flicker when in text mode.
Character font selection (see Table 7.15).
Character font selection (see Table 7.15).
Reserved for video diagnostics.
Used to light the LED located on the video card.
A logical 1 disables the color video interface and connects video output
from an alternate video card located within your central computing
unit to output connectors on the resident video card.
A logical 1 enables fast hardware scrolling in the 80 x 25 submode
only.
6%7 Used to select one of four pages of display text from the 16K video
RAM, when bit 5 is set, according to the following logic:
7 6 S cre e n Page Select
0 0 1 st p a g e
0 1 2nd pa g e
I 0 3rd pa g e
I I 4th pa g e