Programming Sequence And Register Addressing - Intel 80C188EC User Manual

8.4.2

Programming Sequence and Register Addressing

All of the 8259A module registers reside within an address window of two bytes. Write access to
individual registers is controlled by a combination of the following:
•
the address of the register (state of the A0 address line on the 8259A module)
•
the data written to the register
•
the sequence in which the data is written
Registers are read from the 8259A module by first sending a "read command" and then immedi-
ately reading from the module. The Interrupt Mask Register is an exception to this rule; it can be
read directly.
Each 8259A module occupies two locations in the memory map. For the 80C186EC/C188EC,
each module occupies two consecutive words in the Peripheral Control Block. These access ports
are named MPICP0, MPICP1, SPICP0 and SPICP1 (the M and S refer to master and slave). It is
through these access ports that the Initialization and Operation Command Words are sent.
8.4.3 Initializing the 8259A Module
The 8259A module must be initialized before it can be used. After reset, the states of all the
8259A registers are undefined. The 8259A modules must be initialized before the Interrupt En-
able flag in the Processor Status Word is set (enabling interrupts).
8.4.3.1 8259A Initialization Sequence
The 8259A module initialization sequence is usually performed as a part of the boot code for the
system. The Initialization Command Words are written to the 8259A module following the se-
quence shown in Figure 8-11. The exact sequence must be followed. The 8259A module has a
state machine that controls access to the individual registers. If the sequence is not followed cor-
rectly, the state machine will get "lost" and cause improper initialization. Should the initialization
sequence be interrupted, the state machine can be reinitialized by re-starting the initialization pro-
cess.
INTERRUPT CONTROL UNIT
8-21