ELEC VIDEO BIOS ROUTINES

Transcription

1 It is less confusing to the user if we remove unnecessary messages from the display before giving information or instructions. At the command prompt, we do this with the DOS command CLS. However, this doesn t work when we are running a program, so we must do this in our program. We will now explore how to accomplish a clear screen routine using a Video BIOS routine. INT 10H Video BIOS Routines Function 6 Screen Scroll Up In MS/DOS, the display consists of 80 columns across, and 25 rows. There are also 16 different colors that can be assigned. Each character on the display is 8 pixels wide and 8 pixels high. We make characters by illuminating or darkening each pixel of the 64 pixels that makes each character. In the CGA mode, the screen resolution is 640 pixels (80 columns of characters by 8 pixels per character) by 200 (25 rows x 8 pixels per character). Armed with the knowledge of the row and column position where we wish to print, we may place the cursor anywhere you like on the display. Once positioned, the cursor moves one character to the right for each additional character. When we clear the screen in MS/DOS, the screen is cleared and cursor is homed to the top left corner of the screen. As characters are sent to the CRT the cursor moves from left to right and top to bottom. When the display is filled, the screen scrolls up. Clear Screen Using Screen Scroll Up Function 6: Screen Scroll Up Entry Parameters: Register AH: 06H Register AL: number of rows to scroll, 0 for all Register BH: see explanation below Register CH: Top Row number Register CL: Top Column Number Register DH: End Row Number Register DL: End Column Number Exit Parameter: Display is scrolled Uses INT 10H With appropriate values, this function clears the screen just like the DOS command CLS. Requirements: If the 7 registers contain data that must be used after the interrupt (before initialization) they should be pushed on the Stack, or saved in storage. Register AH <= 06H the Video BIOS Function Register AL <= Number of rows to scroll (0 for all) Register BH <= Controls foreground and background (see the Video Display below) Register CH <= Row number at top of the region Register CL <= Column number at top left of the region Register DH <= Row number at bottom of the region Register DL <= Column number at bottom right of the region Andrew H. Andersen 1

2 VIDEO DISPLAY NUMBER D7 D6 D5 D4 D3 D2 D1 D0 I/Blink Red Green Blue Intensity Red Green Blue Background Color Foreground Color In a color monitor with three guns, Red-Green-Blue (RGB) colors are obtained by turning a gun on or off at each pixel position. Each gun has a corresponding bit in the Video Display Number for the Foreground Color and the Background Color. Foreground color is usually the text, and the background is the rest of the display. For Black RGB = 000 (all guns off). For White, RGB = 111 (all guns on). In Hex, White text on a Black background has a value of 07H. A dark Blue background with bright Yellow text is 1EH. This is not intuitive for many colors. Depending on the mode of operation, the MSB of the background is either for intensity (if blink is not enabled) or Blink (if blink is enabled). The default mode is intensity. We can put his routine anywhere. However, it seems like something we may wish to do from various locations in a program, so we will discuss how to do this as a procedure Clear Screen Algorithm AH <= 6 ; The Video BIOS function for scroll up AH <= 7 ; The Video BIOS function for scroll down (use one or the other) AL <= Number of lines to scroll, 0 = all CX <= Start row column info (0 for top left) We may enter the Row in CH and the column in CL separately DX <= End row column data. The data in Hex is For the bottom, the 25 th row is 19H and placed in DH For the right, the 80 th column is 50H and placed in DL So DX <= 1950h BH <= foreground/background information 1EH for a dark blue screen with bright yellow text As a procedure, we might wish to define the Video BIOS data as words and bytes, and just load the registers as needed: vbios dw 600h row0 dw 0 row25 dw 1950h color db 1Eh In the code area at the appropriate location, we would write the clear screen code CLR: AX <= vbios CX <= row0 DX <= row25 BH <= color Andrew H. Andersen 2

3 Function 2 - Direct Cursor Positioning Often, we wish to place the cursor in a specific location on the display and begin writing data to the display from that point on. Once we place the cursor at a particular position on the display, writing to the display functions normally. We identify the row and column where we wish to place the cursor as an offset from the top left position on the display. Positioning the cursor function does not erase the display from the top left to the new cursor position so any current messages above and to the left of the new cursor position is retained on the display. However, any screen contents from the current position will be overwritten one character at a time as data are sent to the display at the current cursor position. Function 2: Set Cursor Position Entry Parameters: Register AH: 02H Register DH: Row position (0 24) Register DL: Column position (0 79) Register BH: Video Page number Exit Parameter: Cursor positioned on display Uses INT 10H This interrupt is used to position the cursor at a desired row and column. When used with BIOS INT 21H Function 9, the output string begins at the current cursor location and continues from there normally. Requirements: If the registers contain data that must be used after the interrupt (before initialization) they should be pushed on the Stack, or saved in storage. The pseudo code to initialize this function is: Register AX <= 02H Register DH <= Row data from 0 to 24 Register DL <= Column data from 0 to 79 Register BH <= Video Page (always 0 in this course) Here is a typical setup modifying the previous example. Assume we wish to start on Row 7 Column 20. We could load DX (061F) or load DH (06) and DL (1F) Keep in mind, the computer counts from 0 so you must subtract 1. Here is a simple example. Position1 dw 61Fh There may be other declarations. In the code section, there may be other statements. This routine might be coded directly where needed or as a procedure. More than likely, this will be written and a procedure, so that is how it will be demonstrated Andrew H. Andersen 3

4 Here is a typical setup modifying the previous example. Assume we wish to start on Row 7 Column 20. We could load DX (061F) or load DH (06) and DL (1F) Keep in mind, the computer counts from 0 so you must subtract 1. Here is a simple example. Position1 dw 61Fh There may be other declarations. In the code section, there may be other statements. This routine might be coded directly where needed or as a procedure. More than likely, this will be written and a procedure, so that is how it will be demonstrated. We will first look at the basic procedure where we will always place the cursor in the same position. SetCur proc DX <= Position1 ;the row and column data AH <= 2 BH <= 0 ;video page no. 0 SetCur endp However, this has limited value. Now let us look at an example that positions the cursor at different locations, and prints the different messages. First consider the following: 1. Initialize the cursor position in calling module 2. Call SetCur 3. Initialize the message pointer for function 9 4. Call Function9 5. Repeat steps 1 4 as many times as needed Let us assume the data area looks similar to the following: line1 equ 0515h line2 equ 0615h line3 equ 0715h line4 equ 0915h line5 equ 0a15h line6 equ 0b15h line25 equ 1914h msg1 db To exit: enter <CR> instead db of entering the answer.$ prmpt1 db This program will ask you to answer two $ prmpt2 db questions as Y for yes or N for no.$ prmpt3 db Are you familiar with computers?$ prmpt4 db Enter 1st reply -----> ans1 db? db $ prmpt5 db Do you know how to use ?$ prmpt6 db Enter 2nd reply -----> Andrew H. Andersen 4

5 ans2 db? db $ The following is located in the code area. There may be other code but that is not defined. Note.code call Clr mov dx, line25 lea dx,msg1 mov dx, line1 lea dx,prmpt1 mov dx, line2 lea dx,prmpt2 ;and so on to mov dx, line6 lea dx,ans2 Clr proc ; same as before ret Clr endp SetCur proc ; This time, DX is passed in from calling module ;so delete from here AH <= 2 BH <= 0 ;video page no. 0 SetCur endp Function9 proc AH <= 9 INT 21h Function9 endp Andrew H. Andersen 5