Zenith Z-100 PC series Service Manual page 267

Page
3.20
Circuit Description
")
The register at 3DA functions according to the logic of Table 3.9.
Table 3.9. Mode Select Port 3DA Logic
Bit 0 A logical 1 overrides the VIDEO ENABLE signal from 3D8; will eliminate
monitor display flicker when in text mode.
Bit 1 Character font selection. Least-significant bit (LSB).
Bit 2 A logical 1 enables underline in text mode when Attribute bit 6 is a
logical 1.
Bit 3 Not defined.
Bit 4 A logical 1 selects Z -319 video.
Bit 5 A logical 1 enables fast hardware scrolling in the 80 x 25 submode
only.
Bits6and 7
Used to select one of four pages of display text from the 16K video
RAM in text mode, according to the following logic:
BIT7 BIT6 SCREEN PAGE SELECT
o
o
First page
o
1 Second page
o
Third page
Fourth page