Transcript The Greatest Common Denominator

Set 20
Interrupts
INTERRUPTS
The Pentium has a mechanism whereby
external devices can interrupt it.
Devices such as the keyboard, the monitor,
hard disks etc. can cause such interrupts,
when they require service of some kind, such
as to get or receive a byte.
For example, when you press a key on the
keyboard this causes an interrupt.
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WHAT HAPPENS WHEN AN INTERRUPT OCCURS
When the pentium is interrupted, it completes the current
instruction, then pushes onto the stack the flags register plus
the address of the next instruction (the return address).
It then carries out the procedure that services the interrupt
involved.
Then it uses a special return instruction iret which pops the
flags register from the stack, and pops and uses the return
address to resume doing whatever it was doing before the
interrupt occurred.
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EXAMPLES OF INTERRUPT ROUTINES
When you press a key on the keyboard, the interrupt routine
that the Pentium carries out is to input the key and store it in a
buffer in memory.
Another example is provided by a separate timer chip which
interrupts the Pentium typically every 10 milliseconds. This
interrupt is called a timer tick. Here the Pentium’s interrupt
routine updates it’s time of day (it actually stores the number
of timer ticks since midnight).
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HOW DOES THE PENTIUM KNOW WHAT
CAUSED THE INTERRUPT?
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from the timer chip
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identification
of the interrupt
from the keyboard
data bus
from the monitor
PENTIUM
3
interrupt
acknowledged
2
interrupt
requested
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INTERRUPT
CONTROLLER
1.
The device signals its request, for an interrupt service, to the interrupt
controller.
2.
The interrupt controller sends a signal to the Pentium requesting an interrupt.
3.
The Pentium is hard-wired to respond by finishing its current instruction,
pushing the flags register, and the address of the next instruction (the return
address) onto the stack, and then sending an acknowledgment signal back to
the interrupt controller.
4.
The interrupt controller then puts onto the data bus a code that identifies
which which of its input lines (and hence which device) is requesting this
interrupt.
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HOW THE PENTIUM FINDS THE CORRECT
INTERRUPT ROUTINE
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All Pentiums maintain an area of memory at the
same fixed location called a jump table.
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This consists of a sequence of 4 byte entries,
where each entry consists of an (offset value,
segment register value).
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These entries (called interrupt vectors) specify
the locations of the various interrupt routines.
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The code that the interrupt routine sends the
Pentium, to identify the kind of interrupt, is used
to index this jump table and so locate the
appropriate interrupt7 routine.
NOTES
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From the instant the device signals its request for an
interrupt routine, up to when the Pentium starts executing
the routine required, everything described on the
proceeding slides takes place automatically entirely by
wired-in hardware.
•
The jump table makes it simple for new versions of the
interrupt routines to be written and placed at different
locations in memory when new versions of the operating
system are produced. All that needs to be changed is the
corresponding entries in the jump table.
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INTERRUPT VECTOR NUMBERS
•
The 4 byte entries (interrupt vectors) in the jump
table are numbered.
•
For example interrupt vector 9 is for the
keyboard, and interrupt vector 3Ch is for the
timer tick.
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RETRIEVING & CHANGING INTERRUPT VECTORS
The code to retrieve an interrupt vector (so you can store & later restore it)
is of the form:
MOV AL,… ; the interrupt vector no., e.g. 9 for the keyboard
MOV AH, 35h
INT 21h
This puts the offset value part of the interrupt vector into BX, and the
segment register part into ES
The code to change an interrupt vector is of the form:
MOV AL,…
; the interrupt vector no.
LEA DX,…
; the offset of the new interrupt routine
; DS should contain the segment value concerned
MOV AH, 25H
INT
21H
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