IBM RT Series Hardware Reference Manual page 153

Bit 1
Bit 2-Bit 3
Bit 4
Bit 5
Bit 6
Bit 7
addresses to 3DX and Input Status Register l's
address to 3DA for Color/Graphics Monitor
Adapter emulation.
Enable RAM-A logical 0 disables RAM from
the processor; a logical 1 enables RAM to
respond at addresses designated by the Control
Data Select value programmed into the Graphics
Controllers.
Clock Select-These two bits select the clock
source according to the following table:
Bits
3 2
o
0- Selects 14 MHz clock from the processor
I/O channel
o
1- Selects 16 MHz clock on-board oscillator
1 0- Selects external clock source from the
feature connector.
1 1- Not used
Disable Internal Video Drivers-A logical 0
activates internal video drivers; a logical 1
disables internal video drivers. When the internal
video drivers are disabled, the source of the direct
drive color output becomes the feature connector
direct drive outputs.
Page Bit For Odd/Even-Selects between two
64K pages of memory when in the Odd/Even
modes (0,1,2,3,7). A logical 0 selects the low
page of memory; a logical 1 selects the high page
of memory.
Horizontal Retrace Polarity-A logical 0 selects
positive horizontal retrace; a logical 1 selects
negative horizontal retrace.
Vertical Retrace Polarity-A logical 0 selects
positive vertical retrace; a logical 1 selects
IBM Enhanced Graphics Adapter 13