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System ProgrammingBSCS-7th Semester

Dr. Wajeeha Khalil

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TSR programs must be compiled with .com in

extension .com doesnt exceed 64kB size.

TSR programs should be run by making its com fileand exit the compiler, then run it.

Save the program using with .c extension.

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Running a TSR Program

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#include#includechar st[80] ={"Hello World$"};char st1[80] ={"Hello Students!$"};void interrupt (*oldint65)( );void interrupt newint65( );void main()

{ oldint65 = getvect(0x65);setvect(0x65, newint65);keep(0, 1000);\\made program resident

}3

Another Example

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void interrupt newint65( ){if (( _AH ) = = 0){

_AH = 0x09;_DX = (unsigned int) st;geninterrupt (0x21);

}else{

if (( _AH ) = = 1)

{_AH = 0x09;_DX = (unsigned int) st1;geninterrupt (0x21);

}

}}

Continued

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#include#include

void main(){

_AH = 1;

geninterrupt (0x65);_AH = 0;geninterrupt (0x65);

}5

2nd Program

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Different ISR provides different services. Each

interrupt invoked points to a number of services forexample Interrupt 21h has many services, numbered0, 1, 2, 3

Sw interrupts have number of services.

Service number is stored in AH register For example, Int 21h,

A case statement, checks the service number andexectues the corresponding service

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Interrupt Interception,

Hooks/Stealing

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Execution Interrupted

ISR Perform I/O

Normal Execution of Interrupt

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Normal execution reviewed

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Interrupt Interception

New Routine

Original Routine

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Interrupt Interception Reviewed

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Other form of Interrupt Interception

New Routine

Original Routine

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Interrupt Interception Reviewed:continued

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void Interrupt newint();void Interrupt (*old)(); \\ declare an interrupt pointer

void main(){

old=getvect(0x08); \\ saving the original ISR addressSetvect(0x08,newint);\\ setting the vectorKeep(0,100);\\ make program resident in memory

}void interrupt newint ()

{. . . . . . . . . . . . . . \\ perform your operations(*old)();\\ call the original ISR

}*You can also call the (*old)() in the start of newint().

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Program (ISR Interception)

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Hardware Interrupt

Invoked by means of hardware

Occurs 18.2 times per second(one 18th of a second)

Generated by Programmable Interrupt Timer (PIT)

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TIMER interrupt

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BIOS Data Area (0040:0000)

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A description of bios data area is as follows

Cant write values in this area, just reading.

A certain area in RAM, used by BIOS for storage of data

etc, while BIOS routines are processed. Segment 40 and offset 0. there are various variables

placed here.

On this starting address, a variable is placed that shows

us the status of the keyboard. Lets c.

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Keyboard Status Variable

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7 6 5 4 3 2 1 0

Right ShiftkeyLeft Shift Key

Ctrl Key

Alt Key

InsertkeyCaps Lock Key

Num Lock key

Scroll lock key

40:17H

Keyboard Status Word

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Interrupt Interception

New Routine

Original Routine

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A device may be interfaced with the processor toperform memory mapped or isolated I/O.

Main memory and I/O ports both are physically a kindof memory device.

In case of Isolated I/O, I/O ports are used to hold datatemporary while sending/receiving the data to/from

the I/O device. If the similar function is performed using a dedicated

part of main memory then the I/O operation ismemory mapped.

Memory isolated and mapped I/O

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Isolated I/O

MP

I/O ports

IN

OUT

The device or controller uses certain ports to

communicate with processor Control bus signals

Signal M means to read/write frommemory..(Isolated I/O)

Si nal I/O means to read write from I O .

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Memory Mapped I/O

MP

I/O Ports

MOV

MOV

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Memory map Some portion of memory is reserved, so place decreased

In isolated we used ports instead

Isolated i/o has complicated architecture, differentnumber of signals. Complicated hardware design.

Simple hardware memory map design instead

As number of control increases

Monitor uses memory mapped I/O

MonitorVGAMemory

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The output on the monitor is controller by acontroller called video controller within the PC.

One of the reason for adopting memory mapped I/O

for the monitor is that a large amount of data isneeded to be conveyed to the video controller inorder to describe the text or that graphics that is tobe displayed.

Such large amount of data being output throughisolated I/O does not form into a feasible idea as thenumber of port in PCs is limited to 65536.

Memory Mapped I/O on Monitor

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In text mode 80 columns, 25 rows (characters)80*25 characters

Which characters to show on monitor? Two bytes of storage required for each character

Low byte = ASCII Code

High byte = attribute byte

Character to be displayed are stored in VGA cards memory.

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Memory Maped I/O on Monitor

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B8OO:0000

B8OO:0001

B8OO:0002B8OO:0003

Low Byte = ASCII CODE

High Byte =Attribute Byte

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Overview of Attribute Byte

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X X X X X X X X

Back Color ColorBold

Blink

Low Byte = Ascii CodeHigh Byte = Attribute Byte

000 Black

100 Red

010 Green

001 Blue

111 White

fore color

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Program

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unsigned int far *scr=0xb8000000;

void main(){

(*scr)=0x0756; //56 = B(*(scr+1))=0x7055; //55 = U

}

This example will generate the output UV

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The far pointer scr is assigned 0xb800H in the high word (segment address)

0x0000H in the low word (offset address).

The word at this address is loaded with the value

0x0756H

next word is loaded by the value 0x7055H,

0x07 is the attribute byte meaning black back color

and white fore color and

the byte 0x70h means white back color and black forecolor. ).

0x56 and 0x55 are the ASCII value of V and Urespectively.

Explaination

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