Programming Considerations; Programming The Registers - IBM RT Series Hardware Reference Manual

Programming Considerations
Programming the Registers
Each of the LSI devices has an address register and a number of
data registers. The address register serves as a pointer to the
other registers on the LSI device. It is a write-only register that is
loaded by the processor by executing an 'OUT' instruction to its
I/O address with the index of the selected data register.
The data registers on each LSI device are accessed through a
common I/O address. They are distinguished by the pointer
(index) in the address register. To write to a data register, the
address register is loaded with the index of the appropriate data
register, then the selected data register is loaded by executing an
'OUT' instruction to the common I/O address.
The external registers that are not part of an LSI device and the
Graphics I and II registers are not accessed through an address
register; they are written to directly.
The following tables define the values that are loaded into the
registers by BIOS to support the different modes of operation
supported by this adapter.
62 IBM Enhanced Graphics Adapter