System Programming In C

To do system programming its very important to have a deep knowledge how we can manipulate register values directly . Every thing on computer like colorful GUI,rock music,etc ends up in 10101010 stored in some register,that is what computer world is all about.
So lets have look on some of important data structure which help us dealing with register values directly.
In the header file dos.h there are two important structures and union (Remember this structure)

1. struct BYTEREGS {
unsigned char al, ah, bl, bh;
unsigned char cl, ch, dl, dh;
};

2. struct WORDREGS {
unsigned int ax, bx, cx, dx;
unsigned int si, di, cflag, flags;
};

3. union REGS {
struct WORDREGS x;
struct BYTEREGS h;
};

There is function int86() which has been defined in dos.h header file. It is general 8086 software interrupt interface. It will better to explain it by an example.



 c program to display mouse pointer

#include<stdio.h>
#include<dos.h>
void main()
{
union REGS i,o;
i.x.ax=1;
int86(0x33,&i,&o);
getch();
}


Interrupt table for c programming language

Interrupt table

List of interrupt numbers and its use

Input	Output	Service No	Use
Interrupt No: 0X33 Use: Mouse
ax		1	Show mouse pointer
ax		2	Hide mouse pointer
ax		0	Initialize mouse
	ax	0
ax		7	X co-ordinate restriction
cx		X1 co-ordinate
dx		Y1 co-ordinate
ax		8	Y co-ordinate restriction
cx		X2 co-ordinate
dx		Y2 co-ordinate
ax		3	Get mouse position
	bx	Button
	cx	X position
	dx	Y position
ax		4	Set mouse position
cx		X co-ordinate
dx		Y co-ordinate
ax		5
bx		B=0 left
		B=1 right
		B=2 center
	ax	Button status
	bx	Button press counter
	cx	X co-ordinate
	dx	Y co-ordinate
ax		9	Set graphics pointer shape
bx		Hot spot offset from left
cx		Hot spot offset from right
es:dx		Segment: offset
ax		0XA	Set text pointer type
bx		Pointer type
		0-soft type
		1-Hardware
cx		Starting line number
dx		Ending line number
Interrupt no: 0X10 Use: Monitor
ah		2	Positioning cursor
dh		row no
dl		Column no
bh		Page no
ah		6	Clear screen
al		0 to clear	N line to scroll
ch		St.row
cl		St column
dh		End row
dl		End column
bh		Col
ah		6	Scroll window l line up
al		1
bh		Color
ch		St row
cl		St column
dh		End row
dl		End column
ah		8	Read a character from screen
bh		Page
		Ah
ah		9	Write a char on a screen
bh		Page
		Ah
ah		0	Set video mode
al		0x13	Switch to 320 X 200 and 256 color graphics mode
	Ch	0x12	Switch to 640X480 and 16 color graphics mode
	Cl	0x11	Switch to 640X480 and 2 color graphics mode
	Dh	0x3	Switch to 25X80 and 16 color test mode
	dl	0x1	Switch to 40X25 and 16 color text mode
ah		0x2	Switch to 80X25 and 16 color text mode
bh		3	Get cursor position
		Page number
		Starting line for cursor
		Ending line for cursor
		Row position
		Col position
		Page number
ah
bh
Interrupt No: 0X1A Use: Time
		2	Get Time
Ah	Ch	Hours
	Cl	Minutes
	Dh	Second
	Dl	Daylight-saving
		Time code
Ah		3	Set time
Ch		Hours
Cl		Minutes
Dh		Second
Dl		Daylight-time
ah		4	Get date
	Ch	Century
	Cl	Years
	Dh	Month
	dl	day
Ah		5	Set date
Ch		Century
Cl		Year
Dh		Month
dl		day
Interrupt No: 0X16 Use: Key Board
Ah		0	Get the scan code
	Ah
	al		Get ascii code
Interrupt No: 0X21 Use: Miscellaneous
Ah		1	Echo input character on screen
	al	Data
Ah		2	Character output
Dl		Character
Ah		5	Pinter output
Dl		Character
Ah		9	Display string
Ds:dx		String
Ah		0XE	Select disk
Dl		Drive no
Ah		0XF	Open file
Ds:dx		File control
Ah		0X10	Close file
Ds:dx		File control
Ah		0X11	Find first file
Ds:dx		File control
Ah		0X12	Find next file
Ds:dx		File control
Ah		0x13	Delete file
Ds:dx		File control
Ah		Ox16	Create file
Ds:dx		File control
Ah		0x17	Renaming file
Ds:sx		Special file control
Ah		0x19	Get current disk
	Al
Ah		Drive number
Ds:dx		0x23	Get file size
		File control
Al	ah	0x25	Set interrupt
Ds:dx		New function
Ah		0x35	Get interrupt vector
Al		Interrupt no
		Interrupt handler
Ah		0x39	Create a directory
Ds:dx		Directory name
Ah		0x3A	Delete directory
Ds:dx		Directory name
Ah		0X3B	Set current directory
Ds:dx		Directory name

in the WORDREGS

struct WORDREGS {
unsigned int ax, bx, cx, dx;
unsigned int si, di, cflag, flags;
};

And WORDRGS is define in the union REGS

union REGS {
struct WORDREGS x;
struct BYTEREGS h;
};

So to access the ax first declare a variable of REGS that is

REGS i,o;

To access the ax write i.x.ax (We are using structure variable i because ax is input, see interrupt table).
So, to display mouse pointer assign the value of service number:

i.x.ax=1;

To pass the information to microprocessor we use int86 function. It has three parameters

1. Interrupt number i.e. 0x33
2. union REGS *inputregiste i.e. &i
3. union REGS *outputregiste i.e. &o;

So write: int86 (0x33, &i, &o);



Now lets see how we can write value to a specific port.Dealing with port become very important when we want to interface some new device with our regular device.
Its similar to saying we have to give new interface card order  in language which the new device understands.The ports here are medium through which we communicate .We give orders to the ports (generally predefined) and they follow.

The following set of functions will do the task pretty well,

inport reads a word from a hardware port.
inportb reads a byte from a hardware port.
outport outputs a word to a hardware port.
outportb outputs a byte to a hardware port.


INPORT is used for word by word reception. The syntax is as follows:

inport (portid);

here portid is the address of the port. For parallel port the address is 0x378. For example

int result;
result=inport( 0x378 );

this will read the word of data from the parallel port and will be stored in the variable result.

INPORTB is used for byte by byte reception. The syntax is as follows:

inportb (portid)

For example

unsigned char result;
result = inportb(0x378);

this will read a byte from the port and will be saved in the variable result.

OUTPORT is used to output a word to the port. The syntax is as follows:

outport (portid,value);

here value is the data for output. For example

int value =450;
outport(0x378,value);

this will output the data 450 to the parallel port.

OUTPORTB is used to output a single byte to a port. The sysntax is as follows:

outportb (portid, value);

For example

outportb (0x378,255);


List of VGA ports and register name

Register name	port	index	mode 3h (80x25 text mode)	mode 12h (640x480 planar 16-bit color mode)	mode 13h (320x200 linear 256-color mode)	mode X (320x240 planar 256 color mode)
Mode Control	0x3C0	0x10	0x0C	0x01	0x41	0x41
Overscan Register	0x3C0	0x11	0x00	0x00	0x00	0x00
Color Plane Enable	0x3C0	0x12	0x0F	0x0F	0x0F	0x0F
Horizontal Panning	0x3C0	0x13	0x08	0x00	0x00	0x00
Color Select	0x3C0	0x14	0x00	0x00	0x00	0x00
Miscellaneous Output Register	0x3C2	N/A	0x67	0xE3	0x63	0xE3
Clock Mode Register	0x3C4	0x01	0x00	0x01	0x01	0x01
Character select	0x3C4	0x03	0x00	0x00	0x00	0x00
Memory Mode Register	0x3C4	0x04	0x07	0x02	0x0E	0x06
Mode Register	0x3CE	0x05	0x10	0x00	0x40	0x40
Miscellaneous Register	0x3CE	0x06	0x0E	0x05	0x05	0x05
Horizontal Total	0x3D4	0x00	0x5F	0x5F	0x5F	0x5F
Horizontal Display Enable End	0x3D4	0x01	0x4F	0x4F	0x4F	0x4F
Horizontal Blank Start	0x3D4	0x02	0x50	0x50	0x50	0x50
Horizontal Blank End	0x3D4	0x03	0x82	0x82	0x82	0x82
Horizontal Retrace Start	0x3D4	0x04	0x55	0x54	0x54	0x54
Horizontal Retrace End	0x3D4	0x05	0x81	0x80	0x80	0x80
Vertical Total	0x3D4	0x06	0xBF	0x0B	0xBF	0x0D
Overflow Register	0x3D4	0x07	0x1F	0x3E	0x1F	0x3E
Preset row scan	0x3D4	0x08	0x00	0x00	0x00	0x00
Maximum Scan Line	0x3D4	0x09	0x4F	0x40	0x41	0x41
Vertical Retrace Start	0x3D4	0x10	0x9C	0xEA	0x9C	0xEA
Vertical Retrace End	0x3D4	0x11	0x8E	0x8C	0x8E	0xAC
Vertical Display Enable End	0x3D4	0x12	0x8F	0xDF	0x8F	0xDF
Logical Width	0x3D4	0x13	0x28	0x28	0x28	0x28
Underline Location	0x3D4	0x14	0x1F	0x00	0x40	0x00
Vertical Blank Start	0x3D4	0x15	0x96	0xE7	0x96	0xE7
Vertical Blank End	0x3D4	0x16	0xB9	0x04	0xB9	0x06
Mode Control	0x3D4	0x17	0xA3	0xE3	0xA3	0xE3

Reference:c-pointer
                 osdev