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5.7: Return Values with PromptInt Subprogram

  • Page ID
    28510
  • The next subprogram in this chapter, PromptInt, shows how to return a value from a subprogram. The registers $a0..$a3 are used to pass values into a subprogram, and the registers $v0..$v1 are used to return values. The program from Chapter 5.6 has been changed to include the PromptInt subprogram, and the changes are highlighted in yellow.

    # File:        Program5-2.asm
    # Author:      Charles Kann
    # Purpose:     To illustrate implementing and calling a subprogram named PrintNewLine.
    .text
    main:
        # read an input value from the user
        la $a0, prompt
        jal PromptInt
        move $s0, $v0
        
        # print the value back to the user
        jal PrintNewLine
        la $a0, result
        move $a1, $s0
        jal PrintInt
        
        # call the Exit subprogram to exit
        jal Exit
    
    .data
        prompt: .asciiz "Please enter an integer: "
        result: .asciiz "You entered: "
     
    # subprogram:    PrintNewLine
    # author:        Charles Kann
    # purpose:       to output a new line to the user console
    # input:         None
    # output:        None
    # side effects:  A new line character is printed to the user's console
    .text
    PrintNewLine:
        li $v0, 4
        la $a0, __PNL_newline
        syscall
        jr $ra
    .data
        __PNL_newline: .asciiz "\n"
    
    # subprogram:     PrintInt
    # author:         Charles W. Kann
    # purpose:        To print a string to the console
    # input:          $a0 - The address of the string to print.
    #                 $a1 - The value of the int to print
    # returns:        None
    # side effects:   The String is printed followed by the integer value.
    .text
    PrintInt:
        # Print string. The string address is already in $a0 
        li $v0, 4
        syscall
        
        # Print integer. The integer value is in $a1, and must # be first moved to $a0.
        move $a0, $a1
        li $v0, 1
        syscall
        
        #return
        jr $ra
    
    # subprogram:    PromptInt
    # author:        Charles W. Kann
    # purpose:       To print the user for an integer input, and to return that input value to the caller.
    # input:         $a0 - The address of the string to print.
    # returns:       $v0 - The value the user entered    
    # side effects:  The String is printed followed by the integer value.
    .text
    PromptInt:
        # Print the prompt, which is already in $a0
        li $v0, 4
        syscall
        
        # Read the integer value. Note that at the end of the
        # syscall the value is already in $v0, so there is no
        # need to move it anywhere.
        move $a0, $a1
        li $v0, 5
        syscall
        
        #return
        jr $ra
    
    # subprogram:    PrintString
    # author:        Charles W. Kann
    # purpose:       To print a string to the console
    # input:         $a0 - The address of the string to print.
    # returns:       None
    # side effects:  The String is printed to the console.
    .text
    PrintString:
        addi $v0, $zero, 4
        syscall
        jr $ra
    
    
    # subprogram:    Exit
    # author:        Charles Kann
    # purpose:       to use syscall service 10 to exit a program
    # input:         None
    # output:        None
    # side effects:  The program is exited
    .text
    Exit:
        li $v0, 10
        syscall
    
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