AMP program no.3

   Write an assignment on screen functions 06H (Scroll screen up), 07H (Scroll screen down) of BIOS INT 10H and program to simulate CLS (Clear Screen) command.

About screen (video display):

 The display supports a text mode containing 25 lines with 80 characters per line.

 A block of memory is reserved by display adapter and is referred as video RAM.

 Each character on the screen actually occupies 2 bytes of video RAM.

 The first is the ASCII codes for character to be displayed on the screen, along with it second is the attribute byte.

 Attribute byte: It is a one byte information, about the foreground and background

 The 8 bits of attribute byte are assigned as follows:

Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
Blink	Red	Green	Blue	Intensity	Red	Green	Blue

 Bits 6 – 4 are used for background, and Bits 2 – 0 are used for foreground

 Therefore there are 4000 bytes associated with one video page.

BIOS INT 10H:

►   Function 00h: set video mode

 When the BIOS interrupt 10h along with the above function value is executed, the video mode i.e. the display size changes.

 There are 2 modes of our concern mode1 and mode 3.

 Mode3 is the default mode in which the CGA adapter is able to display 25 rows and 80 columns on the screen.

 But If  mode1 is selected, then the CGA will display 25 rows and 40 columns on the screen.

 To select the mode of display, AL register must be loaded with either 1 or 3.

►   Function 02H : set cursor position

This interrupt is used to place the cursor (dos prompt) at any desired position on the text screen.

 For its occurrence AH must be loaded with the function value 02H

 The cursors new row and column numbers must be loaded in DH and DL registers respectively.

Function 0AH: write character to screen

►     This interrupt allows us to write individual characters or group of them to the screen at the current cursor position.

►     Register AH must be loaded with 0Ah.

►     The ascii code for the screen character must be placed in AL

►     The number of characters written to the screen must be placed in CX register.

Function 09h: write character/attribute to screen

►     This interrupt also writes a character on the display screen. The added feature is to control the attribute of the character.

►     AH must be loaded with 09h

►     AL must be loaded with the ASCII value of the character to be displayed on the display screen.

►     BL must be loaded with characters attribute.

►     CX must be loaded with number of characters to write

Function 06h/07H: scroll current page up/ scroll current page down

►     Whenever we press enter the screen moves up by one line, but dos prompt remains at the bottom of the screen.

►     This is done by copying data within the video RAM from one group of locations to another.

►     This process is performed automatically for us by function 06h

►     Seven registers are used to control the operation of this interrupt

AH = 06h/07h

AL = No. of rows to scroll up/down

BH = Attribute used

CH = row number at top of region

CL = column number at top of left region

DH = row number at bottom of region

DL = column number at bottom right of region