1. Microprocessor and Assembly Language
Lecture-12-Animation with Timer Interrupt
Muhammad Hafeez
Department of Computer Science
GC University Lahore

2. Today’s Agenda User Interrupt Procedures
Animation using Timer Interrupt

3. Moving Objects in a Game
Game’s objects can be moved by displaying an object, erasing it and then displaying it at another new place
CPU speed it fast that we cannot see the movement of object or cannot produce appropriate delays
One way to approach this kind of task is to write loops
Can you write one ?

4. User Interrupt Procedure
A better approach is to write a routine for BIOS INT 1CH that is called by another BIOS INT 08h that is triggered by hardware and ticked by a timer circuit on your IBM machine every 18.2 seconds.
INT 08H always executes INT 1CH instructions before it returns (IRET)
Never overwrite existing ISR of 1CH with your own code, it will crash the system
However, there is a way…

5. User Interrupt Procedure
A better approach is to write a routine for BIOS INT 1CH that is called by another BIOS INT 08h that is triggered by hardware and ticked by a timer circuit on your IBM machine every 18.2 seconds.
INT 08H always executes INT 1CH instructions before it returns (IRET)
Never write code on existing ISR as it will crash the system
However, Interrupt Procedure can be written while saving the existing ISR address from vector table and then load IVT with user interrupt procedure

6. User Interrupt Procedure
How can we store the existing address of interrupt??

7. Get Interrupt Vector Obtain interrupt vector from vector table INPUT:
AH = 35H
AL = INTERRUPT NUMBER
OUTPUT:
ES:BX (INTERRUPT VECTOR)

8. Set Interrupt Vector Store interrupt vector into vector table INPUT:
AH = 25H
AL = INTERRUPT NUMBER
DS:DX INTERRUPT VECTOR
OUTPUT:
NONE

9. Writing user Interrupt Procedure
We can write the code for user interrupt in a separate procedure
This procedure must end with IRET instead of RET instruction
As this interrupt procedure will be called using IVT so it does not know the location of Data Segment (DS)
Hence, if any variable needs to be used in this user interrupt procedure the DS needs to be set before using that variable

10. Call User Interrupt Procedure in User Program
User interrupt procedure that is a function written in user current code segment needs to loaded in IVT
For that we have to have IP and CS of under interrupt procedure to be replaced in IVT

11. Seg and Offset Pseudo-ops
It has been used as to get the offset address of data segment variable, it can also be used to get offset address of procedure where the code is written
SEG:
This pseudo-op can be used to get segment address of user procedure

12. Seg and Offset Pseudo-ops
It has been used as to get the offset address of data segment variable, it can also be used to get offset address of procedure where the code is written
SEG:
This pseudo-op can be used to get segment address of user procedure