Status Register - Zenith Z-100 Reference Manual

Page 7.32
Video Graphics Programming

Status Register

The status register, an 8-bit read only buffer, resides at input port address 3DA.
Table 7.16 summarizes the logical
Table 7.16. Status Port 3DA Logic
DESCRIPTION
BIT
Logical I indicates that a horizontal or vertical retrace is in progress.
Used to update the video memory during screen refresh intervals.
Logical I indicates that a positive going light pen signal has set the
light pen trigger.
This trigger is reset at powerup and may also be deared by issuing
an I/O OUT command to port address 3DB. The reset is address
activated; no specific data needs to be output.
Logical 0 indicates that the light pen switch is on.
Switch signal is not latched nor debounced.
3 Logical I indicates that a vertical retrace is in progress.
4 LSB of the character font number.
5 MSB of the character font number.
Bits 6 and 7 are not used
operation
o f this register.