HOME Acknowledgement Chapter 1 - Basics of Computer Organization Chapter 2 - Introduction to NASM Chapter 3 - Basic I/O in NASM Chapter 4 - Introduction to Programming in NASM Chapter 5 - Integer Handling Chapter 6 - Subprograms Chapter 7 - Arrays and Strings Chapter 8 - Floating Point Operations
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NASM Manual NITC

Chapter 3 - Basic I/O in NASM

In this chapter, we will learn how to obtain user input and how to return the out- put to the output device. The input from the standard input device (Keyboard) and Output to the standard output device (monitor) in a NASM Program is im- plemented using the Operating Systems read and write system call. Interrupt no: 80h is given to the software generated interrupt in Linux Systems. Applications implement the System Calls using this interrupt. When an application triggers int 80h, then OS will understand that it is a request for a system call and it will refer the general purpose registers to find out and execute the exact Interrupt Service Routine (ie. System Call here).The standard convention to use the software 80h interrupt is, we will put the system call no: in eax register and other parameters needed to implement the system calls in the other general purpose registers. Then we will trigger the 80h interrupt using the instruction INT 80h. Then OS will implement the system call.

  1. Exit System Call

    • This system call is used to exit from the program
    • System call number for exit is 1, so it is copied to eax reg.
    • Output of a program if the exit is successful is 0 and it is being passed as a parameter for exit( ) system call. We need to copy 0 to ebx reg.
    • Then we will trigger INT 80h
  2. Read System Call

    • This system call is used to exit from the program
    • System Call Number for Read is 3. It is copied to eax.
    • The standard Input device(keyboard) is having the reference number 0 and it must be copied to ebx reg.
    • We need to copy the pointer in memory, to which we need to store the input string to ecx reg.
    • We need to copy the number of characters in the string to edx reg.
    • Then we will trigger INT 80h.
    • We will get the string to the location which we copied to ecx reg.
    • This method is also used for reading integers and it is a bit tricky. If we need to read a single digit, we will read it as a single character and then subtract 30h from it(ASCII of 0 = 30h). Then we will get the actual value of that number in that variable.
  3. Write System Call

    • This system call can be used to print the output to the monitor
    • Using this we could write only string / character.
    • System Call Number for Write is 4. It is copied to eax.
    • The standard Output device(Monitor) is having the reference number 1 and it must be copied to ebx reg.
    • We need to copy the pointer in memory, where the output sting resides to ecx reg.
    • We need to copy the number of characters in the string to edx reg.
    • Then we will trigger INT 80h.
      • See the below given example which is equivalent to the C statement printf()
    • This method is even used to output numbers. If we have a number we will break that into digits. Then we keep each of that digit in a variable of size 1 byte. Then we add 30h (ASCII of 0) to each, doing so we will get the ASCII of character to be print.