2. CONTENTS
Interrupt
Interrupts in 8086
CPU’s ‘Fetch-Execute’ cycle
The operation of an interrupt sequence on 8086
Mouse features
Pixel
Mouse pointer
Mickey
Interrupt Vector Table(IVT)
IVT Format
Mouse functions
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3. INTERRUPT
In computing an interrupt is an asynchronous signal indicating the
need for attention.
Interrupts are a commonly used technique for computer multitasking,
Such a system is said to be interrupt driven.
An act of interrupting is referred to as an interrupt request (IRQ).
The part of a program that deals with the interrupt is referred to as an
service routine (ISR) or interrupt handler.
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4. Interrupts in 8086
An 8086 Interrupt can come from any of three sources.
Hardware Interrupt
• External interrupt applied to non-maskable interrupt NMI.
Interrupt response cannot be disabled ( masked) by
any program instruction.
• External interrupt applied to maskable interrupt INTR.
Software Interrupt
• Execution of INT instruction.
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5. CPU’s ‘Fetch-Execute’ Cycle
Fetch instruction at IP
Decode the fetched instruction Save context
Get INTR ID
Execute the decoded instruction
Lookup ISR
Advance IP to next instruction
Execute ISR
Interrupt? yes IRET
no
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6. The Operation of an Interrupt sequence on
the 8086 Microprocessor:
1. External interface sends an interrupt signal, to the Interrupt Request
(INTR) pin, or an internal interrupt occurs.
2. The CPU finishes the present instruction (for a hardware interrupt) and
sends Interrupt Acknowledge (INTA) to hardware interface.
3. The interrupt type N is sent to the Central Processor Unit (CPU) via the
Data bus from the hardware interface.
4. The contents of the flag registers are pushed onto the stack.
5. Both the interrupt (IF) and (TF) flags are cleared. This disables the INTR
pin and the trap or single-step feature.
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7. Interrupt sequence contd…
6. The contents of the code segment register (CS) are pushed onto the
Stack.
7. The contents of the instruction pointer (IP) are pushed onto the Stack.
8. The interrupt vector contents are fetched, from (4 x N) and then placed
into the IP and from (4 x N +2) into the CS so that the next instruction
executes at the interrupt service procedure addressed by the interrupt
vector.
9. While returning from the interrupt-service routine by the Interrupt
Return(IRET) instruction, the IP, CS and Flag registers are popped from
the Stack and return to their state prior to the interrupt.
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8. Interfacing with mouse
Second primary input devices used is the mouse.
It has a greater flexibility and movement.
Mouse system consist of the mouse and the mouse driver.
Mouse driver is the memory resident program that provides
communication between the mouse and the computer.
Mouse driver maintains the cursor position of the mouse and the status
of the mouse buttons.
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9. Mouse Features
Pixel:
A pixel or pel (picture element) is a single point in a raster image, or
the smallest addressable screen element in a display device.
It is the smallest unit of picture that can be represented or controlled.
Each pixel has its own address.
The address of a pixel corresponds to its coordinates.
Pixels are normally arranged in a two-dimensional grid and are often
represented using dots or squares.
Each pixel is a sample of an original image. More samples typically
provide more accurate representations of the original image.
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10. Mouse Features
Mouse pointer:
A mouse pointer, or a cursor, is a visible indicator displayed on a
computer screen.
By moving the mouse, the computer's user can move the mouse
pointer around the screen.
The mouse pointer located on the screen can determine how and
where the user can press a button on the mouse to input text or execute
a command.
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11. Mouse Features
Mickey:
A unit of measure for movement of the mouse, approximately 1/200 of an
inch.
A mickey is a unit of measurement for the speed and
movement direction of a computer mouse.
The speed of the mouse is the ratio between how many pixels the cursor
moves on the screen and how many centimeters you move the mouse on
the mouse pad.
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12. Interrupt Vector Table – IVT (in memory)
x86 has 256 interrupts, specified by Type Number or Vector.
1 byte of data must accompany each interrupt(specifies Type).
Vector is a pointer (address) into Interrupt Vector Table (IVT).
- IVT is stored in memory from 0000:0000 to 0000:03ffh
- IVT contains 256 far pointer values (addresses)
- Far pointer is CS:IP values
Each far pointer is address of Interrupt Service Routine (ISR).
- Also referred to as Interrupt Handler.
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13. IVT Format
0000:0000
0000:0001
Offset
0000:0002
Interrupt 0 IP LSB
0000:0003
Segment IP MSB
0000:0004 CS LSB
0000:0005
Offset CS MSB
Interrupt 1
0000:0006
0000:0007
Segment
Given a Vector, where is the
ISR address stored in memory ?
Offset Type 4
0000:03fc
0000:03fd
Offset Example:int 36h
0000:03fe
Interrupt 255
Segment Offset = (54 4) = 216
0000:03ff
= 00d8h
14. Mouse functions
We can access the cursor position and the button status with interrupt
33h.
The mouse driver has several functions by specifying the function
number in the AX register when calling interrupt 33h.
Some of the functions are
0- Resets the mouse and retrieves the mouse status.
1- Displays the mouse cursor
2- Hides the mouse cursor.
3- Retrieves the mouse cursor positioned the status of
the mouse buttons.
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15. Mouse functions contd…
Initializing the mouse:
To be able to use the mouse it must be first tested whether it is present
or not.
int 33h
mov ax,00
int 33h
mov result,ax
Using int 33h is the necessary first step toward using the mouse.
If this service returns a non-zero value, the mouse is initialized.
Otherwise, the mouse cannot be used.
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16. Mouse functions contd…
Display mouse cursor:
Initializing the mouse does not display the mouse cursor.
A function that initialize the mouse and returns the result.
Int 33h service 1 – display mouse cursor.
mov ax,01h
int 33h
At this point ,the mouse system is active and the cursor has appeared
on the screen.
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17. Mouse functions contd…
Hide mouse cursor:
If the cursor is already off, it stays off (int 33h service 2).
The very important reason for hiding the mouse cursor , as the mouse
cursor moves over the screen,the mouse driver software reads the
character at the preset position before it displays the mouse cursor.
mov ax,02h
int 33h
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18. Mouse functions contd…
Mouse buttons:
It is also possible to read right and left button information from the mouse int
33h service 3.
Getting mouse information:
Int 33h service 3 set AX=3 .This will return
BX meaning
0 No button down
1 Left button down
2 Right button down
3 Both the buttons down
CX = current mouse cursor column.
DX= current mouse cursor row.
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19. Mouse functions contd…
Mov ax, 03h
Int 33h
Mov button, bx
Mov row, dx
Mov col, cx
This service 3, returns information in bx, dx, and cx.
bx indicates which button(s) are down.
This service also returns the current row and column of the mouse
cursor in dx and cx, respectively.
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