Video Card Input/Output Devices - Zenith Z-100 Reference Manual

Video Card Input/Output Devices

Table 7.8 defines the I/O registers contained on the video interface and the port
assignments.
Table 7.8. Video Card Input/Output Port Assignments
PORT ADDRESS R E GISTER
To address a given port, the address lines are programmed as
7.9.
Q
Table 7.9. Video Input/Output Port Selection
P ORT
3DO
3DI
3D8
3D9
3DA
3DB
3DC
X = Don't care
The 6845 CRT controller occupies two I/O port addresses. The RS (register select)
line is connected to the least-significant address bit, AO. When AO is reset, the
address register is accessed to load it with the register address of the desired
parameter which would then be accessed by setting AO (RS).
A typical register access operation would then consist of two consecutive device
write cycles, or a write cyde followed by a read cyde. These cydes would be
directed to consecutive memory or I/O locations.
3DO 6845
3DI 6845
3D8 Mode Control
3D9 Color Select
3DA Status/Mode Input/Output
3DB Light Pen Latch CLEAR
3DC Light Pen Latch PRESET
A9 A8 A7
I I I
I I I
I I I
I
I I
I
I I
I
I I
I
I I
Video Graphics Programming
A6 A5 A4 A3
I 0 I 0
I 0 I 0
I 0 I I
I 0 I I
I 0 I I
I 0 I I
I 0 I I
page 7.27
defined in Table
A2 AI AO
X
X 0
X
X I
0
0 0
0 0 1
0 I 0
0 I I
I 0 0