HP Vectra Reference Manual page 113

Light Pen
Any access to I/O address 3DB, read or write, will clear the light
pen latch. No data is transferred, if a read is performed the data
returned is meaningless.
Any access to I/O address 3DC, read or write, will set the light pen
latch. No data is transferred, if a read is performed, the data
returned is meaningless.
Extension Control Register
The extension control register is a 6-bit write-only register.
The programmer has control over the font style, but not the
height. The type of monitor determines the cell size of the
alphanumeric character font.
.~
......
Bit
Data
Definition
0 0 Vertical Resolution, emulate 200 line display
1 Enable 400 line display
0 Underline Enable, normal
1 Enable underline if bits 2-0 are 001.
2 0
Font Select, STD on Display Graphics page 1
1 HP, Roman 8 font on Display Graphics page 2
3 0 Memory Disable, normal
1 Disable memory from responding to RIW access.
4 0 16/32K select, wrap first 16K into 32K
1 Use all 32K
.~
5 0 Page, use first 16K
1 Use second 16K (graphics)
96 Multi-Mode Video Adapter Card