8259A Datasheet, PDF (16/24 Pages) Intel Corporation – PROGRAMMABLE INTERRUPT CONTROLLER

8259A
After Rotate (IR4 was serviced all other priorities
rotated correspondingly)
‘‘IS’’ Status
231468 – 20
Priority Status
231468 – 21
There are two ways to accomplish Automatic Rota-
tion using OCW2 the Rotation on Non-Specific EOI
Command (R ^e 1 SL ^e 0 EOI ^e 1) and the Ro-
tate in Automatic EOI Mode which is set by (R ^e 1
SL ^e 0 EOI ^e 0) and cleared by (R ^e 0 SL ^e 0
EOI ^e 0)
Specific Rotation
(Specific Priority)
The programmer can change priorities by program-
ming the bottom priority and thus fixing all other pri-
orities i e if IR5 is programmed as the bottom prior-
ity device then IR6 will have the highest one
The Set Priority command is issued in OCW2 where
R ^e 1 SL ^e 1 L0 – L2 is the binary priority level
code of the bottom priority device
Observe that in this mode internal status is updated
by software control during OCW2 However it is in-
dependent of the End of Interrupt (EOI) command
(also executed by OCW2) Priority changes can be
executed during an EOI command by using the Ro-
tate on Specific EOI command in OCW2 (R ^e 1 SL
e _{1 EOI} e _{1 and LO – L2} e _{IR level to receive}
bottom priority)
Interrupt Masks
Each Interrupt Request input can bem masked indi-
vidually by the Interrupt Mask Register (IMR) pro-
grammed through OCW1 Each bit in the IMR masks
one interrupt channel if it is set (1) Bit 0 masks IR0
Bit 1 masks IR1 and so forth Masking an IR channel
does not affect the other channels operation
Special Mask Mode
Some applications may require an interrupt service
routine to dynamically alter the system priority struc-
16
ture during its execution under software control For
example the routine may wish to inhibit lower priori-
ty requests for a portion of its execution but enable
some of them for another portion
The difficulty here is that if an Interrupt Request is
acknowledged and an End of Interrupt command did
not reset its IS bit (i e while executing a service
routine) the 8259A would have inhibited all lower
priority requests with no easy way for the routine to
enable them
That is where the Special Mask Mode comes in In
the special Mask Mode when a mask bit is set in
OCW1 it inhibits further interrupts at that level and
enables interrupts from all other levels (lower as well
as higher) that are not masked
Thus any interrupts may be selectively enabled by
loading the mask register
The special Mask Mode is set by OWC3 where
SSMM ^e 1 SMM ^e 1 and cleared where SSMM ^e
1 SMM ^e 0
Poll Command
In Poll mode the INT output functions as it normally
does The microprocessor should ignore this output
This can be accomplished either by not connecting
the INT output or by masking interrupts within the
microprocessor thereby disabling its interrupt input
Service to devices is achieved by software using a
Poll command
The Poll command is issued by setting P ^e ‘1’’ in
OCW3 The 8259A treats the next RD pulse to the
8259A (i e RD ^e 0 CS ^e 0) as an interrupt ac-
knowledge sets the appropriate IS bit if there is a
request and reads the priority level Interrupt is fro-
zen from WR to RD
The word enabled onto the data bus during RD is
D7 D6 D5 D4 D3 D2 D1 D0
I
W2 W1 W0
W0 – W2 Binary code of the highest priority level
requesting service
I Equal to ‘‘1’’ if there is an interrupt
This mode is useful if there is a routine command
common to several levels so that the INTA se-
quence is not needed (saves ROM space) Another
application is to use the poll mode to expand the
number of priority levels to more than 64
Reading the 8259A Status
The input status of several internal registers can be
read to update the user information on the system