DISCOVERY ZONE ASSEMBLY PROGRAMMING AND OS DESIGN

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Another great reference

Assembly Language Programs and Organizations of the IBM PC

ASSEMBLY PROGRAMMING AND OS DESIGN

2. Intro to Assembly
2.1 Data Types
2.2 Registers
2.3 DOS Int Calls
2.4 Basic Commands
2.4.1 DOS Hello World
2.5 BIOS Int Calls
2.5.1 BIOS Hello World
2.6 String Routines

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CH2.5 BIOS Int Calls

The first step towards doing some real programming is to begin removing our dependence on the Operating System and learning how to do things for ourselves. This jump cannot be made in one step however, so we begin by dropping to the BIOS for some of our needs.

The first Hello World program you wrote used DOS to output the words one character at a time to the display. BIOS can do the same thing for you with int 10h. To keep things brief, we will only output one character.

Ex 10

              org    100h

Main:         mov    AH,0Eh ;function 0Eh outputs one char and
                            ; updates the cursor
              mov    AL,'H' ;AL holds letter to output
              mov    BH,0   ;BH holds text page to output to
              int    10h

              mov    ax,4ch
              int    21h

Function 0Eh outputs single characters, but function 13h will output a whole string. The following version of Hello World will demonstrate this in better detail.

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Email any questions to the author, Nathan Daniels.