Publication No. HRMCR11 Rev. B.0
About This Manual 47
Interrupt Controller
Timer
compatible with the Intel 8
2
3
7
A
.
The D
M
A Controller I (DMACI) i
s
used for
byte-wide transfers whi
l
e the DMAC2 is used for word-wide tran
s
fer
s
.
The Intem1pt controller on a standard PC consists of two 82C59A devices with
eight i11tem1pt request lines each. The two controllers are cascaded so that 14
external and two intemal interrupt sources are avai
l
ab
l
e
.
The master
i
11tem
1
pt
controller provides IRQ (7
...
I
]
,
the s
l
a
v
e interrupt con
t
roller provides IRQ
[15 .
.
.
8). IRQ2 is used to cascade the two controllers, IRQO is used as a system
tinier interrupt and is tied to interval tinter I, counte
r
0
.
The remaining 14
interrupt lines are mapped to various onboard device
s
.
Each 82C59A provides
several internal registers. The intem1pts at the IRQ input lines are handled by
two reg
i
s
t
ers
,
the inte
m
1pt request register IRR and the in-service register ISR
.
For programming detai
l
s see the 82C59A data sheet. The CR! I supports also
the Interrupt haudliug
s
with APIC (Advanced Interrupt Controller
)
.
This
handling of the APIC interrupt services must be supported by the operating
system. The I/0 APIC handles intem1pts very differently than the 8259.
Standard PCs like the CR! I are equipped with an 8254 compatible tinter
.
This
tinier contains tl1ree counters
.
Each counter output provides a key system
fw1ctio11
.
Counter
O
is connected to intem1pt controller input IRQO and
provides a system tinter i11tem1pt for time-of-day, floppy disk tinteout and ot
h
er
system timing fw
1
ctions
.
Cow1ter I generates a refresh
r
eq
u
est si
gn
al and
Counter 2 generates the sow1d for the speaker.
The following table gives au overview ove
r
tl1e 8254 ftmctions.
Table 23: Inte .. val time .. functions
Interval Tinier Fw1c1iio11s
Function
Gate
Clock In
Out
Function
Gate
Clock In
Out
Function
Gate
Clock In
Out
Counter
O
(System Tinter)
Always on
1.193 MHz
(
OSC/12
)
IRQO (INTI
)
Counter I (Refresh Req
u
est
)
Always on
1.193 MHz (OSC/12)
Refresh Request
Counter 2 (Speaker Tone)
Programmable via Port $061
1.193 MHz (OSC/12)
S eaker
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