The Control Register; Video Control Register Functions - Zenith Data Systems Z-100 Series Installation Manual

Chapter 4
Software Considerations
The Control Register
The control register provides a number of functions for the Z-329 Card.
This register enables the CPU to communicate with the display RAM and
attribute RAM, enables video data to be sent to the monitor, selects the
font which is used, enables the blink attribute, and allows paging of the
display when the optional8K x 8 RAMs are installed.
To control all of these functions, a single 8-bit byte is written into the
control register. The control register is located at I/O port 3B8H. The func-
tion of each bit is shown in Table 4.1.
Table 4.1. Video Control Register Functions
BIT
NUMBER
DBO
DB1
DB2
DB3
SIGNAL
NAME
BRDEN
VIDEN
BIT
VALUE
o
x
x
o
FUNCTION
Prevents communication between the CPU
and the display or attribute RAM. Data stored
in RAM is displayed (if VIDEN is enabled) but
cannot be changed.
Enables communication between the CPU
and the RAM. Data stored in RAM is dis-
played (if VIDEN is enabled). The RAM can
be written to or read by the CPU.
Not used.
Not used.
Disables the display. The data stored in the
display or attribute RAM is not changed but
is not displayed. The RAM can be addressed
by the CPU when the display is in this state
(if the board is enabled).
The display is enabled. Data stored in the dis-
play and attribute RAM is displayed.