Example 1.1

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; a program to add three numbers using registers

[org 0x0100]

              mov  ax, 5              ; load first number in ax

              mov  bx, 10             ; load second number in bx

              add  ax, bx             ; accumulate sum in ax

              mov  bx, 15             ; load third number in bx

              add  ax, bx             ; accumulate sum in ax

              mov  ax, 0x4c00         ; terminate program

              int  0x21

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To start a comment a semicolon is used and the assembler ignores everything else on the same line. Comments must be extensively used in assembly language programs to make them readable.

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Leave the org directive for now as it will be discussed later.

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The constant 5 is loaded in one register AX.

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The constant 10 is loaded in another register BX.

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Register BX is added to register AX and the result is stored in register AX. Register AX should contain 15 by now.

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The constant 15 is loaded in the register BX.

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Register BX is again added to register AX now producing 15+15=30 in the AX register. So the program has computed 5+10+15=30.

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Vertical spacing must also be used extensively in assembly language programs to separate logical blocks of code.

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The ending lines are related more to the operating system than to assembly language programming. It is a way to inform DOS that our program has terminated so it can display its command prompt again. The computer may reboot or behave improperly if this termination is not present.