Nicy stuff ... :)

main.asm

;; main.asm

segment     .data

promptfirstnumber db    "Enter the first number: ", 0
promptsecondnumber db   "Enter the second number: ", 0
scanfnumber db          "%d", 0
printsum    db          "%d + %d = %d", 10, 0
a           dd          0
b           dd          0

segment     .text
            global      main
            extern      printf
            extern      scanf
            extern      sum

main:

            ;; Make room for two local variables ...
            enter       4, 0

            ;; Push ebx on the stack, following C calling conventions ...
            push        ebx

            ;; Initialize the two local variables ...
            mov         dword [ebp-4], a
            mov         dword [ebp-8], b

            ;; Ask user to type in the first number ...
            push        promptfirstnumber
            call        printf
            add         esp, 4

            ;; Scan the first number ...
            mov         ebx, [ebp-4]
            push        ebx
            mov         ebx, scanfnumber
            push        ebx
            call        scanf
            add         esp, 8

            ;; Ask user to type in the second number ...
            push        promptsecondnumber
            call        printf
            add         esp, 4

            ;; Scan the second number ...
            mov         ebx, [ebp-8]
            push        ebx
            mov         ebx, scanfnumber
            push        ebx
            call        scanf
            add         esp, 8

            ;; Push the first number and the second number on the stack ...
            mov         ebx, [ebp-8]
            mov         ebx, [ebx]
            push        ebx
            mov         ebx, [ebp-4]
            mov         ebx, [ebx]
            push        ebx

            ;; Call sum ...
            call        sum

            ;; Restore the stack ...
            add         esp, 8

            ;; Print out the result ...
            mov         ebx, eax
            push        eax
            mov         ebx, [ebp-8]
            mov         ebx, [ebx]
            push        ebx
            mov         ebx, [ebp-4]
            mov         ebx, [ebx]
            push        ebx
            mov         ebx, printsum
            push        ebx
            call        printf
            add         esp, 16

            ;; Restore the value of ebx ...
            pop         ebx

            ;; Free room made for the locals ...
            leave

            ;; Return back ...
            ret

sum.asm

;; sum.asm

segment     .text
            global      sum

sum:

            ;; Make room for no local variables ...
            enter       0, 0

            ;; Push ebx on the stack, following C calling conventions ...
            push        ebx

            ;; Initialize the value of eax ...
            mov         eax, 0

            ;; Add the value of a [ebp+8] to eax ...
            add         eax, [ebp+8]

            ;; Add the value of b [ebp+12] to eax ...
            add         eax, [ebp+12]

            ;; Restore the value of ebx ...
            pop         ebx

            ;; Free no room made for the locals ...
            leave

            ;; Return back ...
            ret