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Simple Echo Program for Character Input and Display Using DOS Interrupts

This guide provides a straightforward implementation of an echo program using assembly language for microprocessors. It demonstrates how to read a character from the keyboard and display it on the next line using DOS interrupts. The program uses BIOS function calls to facilitate input/output operations, including displaying a question mark as a prompt and processing carriage return and line feed for proper formatting. Steps include assembling the source code, linking object files, and creating an executable file.

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Simple Echo Program for Character Input and Display Using DOS Interrupts

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  1. Microprocessor

  2. 1st prog! Q: Read a char – from a keyboard & display it at the beginning of the next line! ====== A A

  3. Display a question mark: MOV AH, 2 ;display character fn. – p.456  DOS Interrupts – Function 2h  display output to standard output device I/P: AH = 02h o/p: DL = character

  4. MOV DL, ‘?’ ; char is ‘?’ • It moves 3Fh [‘?’] into DL INT 21h ; display character •  Next read a char. MOV AH, 1 ; read char fn • Fn 1h  Keyboard input INT 21h ; char in AL

  5. Now display the char on the next line: [the char MUST be saved in another register] MOV BL, AL ; save it in BL Why? We move the input character from AL to BL – as the INT 21h, function 2  changes AL.

  6. To move the cursor to the beginning of the next line, we must execute a carriage return and line feed. So, put the ASCII codes for them in DL and execute INT 21h MOV AH, 2 ; display char fn [আগের] MOV DL, 0Dh ; for carriage return INT 21h ; execute carr ret. MOV DL, 0Ah ; for line feed INT 21h ; execute line feed

  7. So full prog TITLE Echo prog .MODEL SMALL .STACK 100h .CODE MAIN PROC ; make a comment for a block MOV AH, 2 ; display char func … … ; another block… … … ; return to DOS MOV AH, 4Ch ; DOS exit func INT 21h ; exit to DOS MAINENDP END MAIN

  8. RUN! • Create the source program file – in a word proc/text/… [*.asm file] • Assemble the prog using an Assembler [*.obj file] • Link object program – to link one or more obj files to create a run file [*.exe file]

  9. Do/Read 4.11 Displaying a string INT 21h , Function 1  to read a single char Function 2  to display a single char Function 9  to display a string Input: DX = offset address of string The string must end with a ‘$’ char

  10. LEA • LEA – Load Effective Address - puts a copy of the source offset address into the dest. LEA DX, MSG - Puts the offset address of the variable MSG into DX

  11. Read 4.12 A case conversion prog. lowercase to UPPER UPPER to lover

  12. Chapter 4 – finish the exercises Chapter 5 – next

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