Our program will take several command line arguments and add them together printing out the result in the terminal.
Writing our program
Our program begins by using the POP instruction to get the number of passed arguments off the stack. This value is stored in ECX (originally known as the counter register). It will then loop through these arguments popping each one off the stack and performing our addition logic. As we know, arguments passed via the command line are received by our program as strings. Before we can add the arguments together we will need to convert them to integers otherwise our result will not be correct. We do this by calling our Ascii to Integer function (atoi). This function will convert the ascii value into an integer and place the result in EAX. We can then add this value to EDX (originally known as the data register) where we will store the result of our additions. If the value passed to atoi is not an ascii representation of an integer our function will return zero instead. When all arguments have been converted and added together we will print out the result and call our quit function.
How does the atoi function work
Converting an ascii string into an integer value is not a trivial task. We know how to convert an integer to an ascii string so the process should essentially work in reverse. Firstly we take the address of the string and move it into ESI (originally known as the source register). We will then move along the string byte by byte (think of each byte as being a single digit or decimal placeholder). For each digit we will check if it's value is between 48-57 (ascii values for the digits 0-9). We will also check if the value is a linefeed or null terminating byte meaning we have reached the end of the string.
Once we have performed all these checks and determined that the byte can be converted to an integer will perform the following logic. We will subtract 48 from the value – converting the ascii value to it's decimal equivalent. We will then add this value to EAX (the general purpose register that will store our result). We will then multiple EAX by 10 as each byte represents a decimal placeholder and continue the loop.
When all bytes have been converted we need to do one last thing before we return the result. The last digit of any number represents a single unit (not a multiple of 10) so we have multiplied our result one too many times. We simple divide it by 10 once to correct this and then return.
What is the BL register
You may have noticed that the atoi function references the BL register. So far in these tutorials we have been exclusively using 32bit registers. These 32bit general purpose registers contain segments of memory that can also be referenced. These segments are available in 16bits and 8bits. We wanted a single byte (8bits) because a byte is the size of memory that is required to store a single ascii character. If we used a larger memory size we would have copied 8bits of data into 32bits of space leaving us with 'rubbish' bits - because only the first 8bits would be meaningful for our calculation.
The EBX register is 32bits. EBX's 16 bit segment is referenced as BX. BX contains the 8bit segments BL and BH (Lower and Higher bits). We wanted the first 8bits (lower bits) of EBX and so we referenced that storage area using BL.
Almost every assembly language tutorial begins with a history of the registers, their names and their sizes. These tutorials however were written to provide a foundation in NASM by first writing code and then understanding the theory. The full story about the size of registers, their history and importance are beyond the scope of this tutorial but we will return to that story in later tutorials.
Only the new function in this file 'atoi' is shown below.
; int atoi(Integer number)
; Ascii to integer function (atoi)
push ebx ; preserve ebx on the stack to be restored after function runs
push ecx ; preserve ecx on the stack to be restored after function runs
push edx ; preserve edx on the stack to be restored after function runs
push esi ; preserve esi on the stack to be restored after function runs
mov esi, eax ; move pointer in eax into esi (our number to convert)
mov eax, 0 ; initialise eax with decimal value 0
mov ecx, 0 ; initialise ecx with decimal value 0
xor ebx, ebx ; resets both lower and uppper bytes of ebx to be 0
mov bl, [esi+ecx] ; move a single byte into ebx register's lower half
cmp bl, 48 ; compare ebx register's lower half value against ascii value 48 (char value 0)
jl .finished ; jump if less than to label finished
cmp bl, 57 ; compare ebx register's lower half value against ascii value 57 (char value 9)
jg .finished ; jump if greater than to label finished
cmp bl, 10 ; compare ebx register's lower half value against ascii value 10 (linefeed character)
je .finished ; jump if equal to label finished
cmp bl, 0 ; compare ebx register's lower half value against decimal value 0 (end of string)
jz .finished ; jump if zero to label finished
sub bl, 48 ; convert ebx register's lower half to decimal representation of ascii value
add eax, ebx ; add ebx to our interger value in eax
mov ebx, 10 ; move decimal value 10 into ebx
mul ebx ; multiply eax by ebx to get place value
inc ecx ; increment ecx (our counter register)
jmp .multiplyLoop ; continue multiply loop
mov ebx, 10 ; move decimal value 10 into ebx
div ebx ; divide eax by value in ebx (in this case 10)
pop esi ; restore esi from the value we pushed onto the stack at the start
pop edx ; restore edx from the value we pushed onto the stack at the start
pop ecx ; restore ecx from the value we pushed onto the stack at the start
pop ebx ; restore ebx from the value we pushed onto the stack at the start
; Calculator (ATOI)
; Compile with: nasm -f elf calculator-atoi.asm
; Link with (64 bit systems require elf_i386 option): ld -m elf_i386 calculator-atoi.o -o calculator-atoi
; Run with: ./calculator-atoi 20 1000 317
pop ecx ; first value on the stack is the number of arguments
mov edx, 0 ; initialise our data register to store additions
cmp ecx, 0h ; check to see if we have any arguments left
jz noMoreArgs ; if zero flag is set jump to noMoreArgs label (jumping over the end of the loop)
pop eax ; pop the next argument off the stack
call atoi ; convert our ascii string to decimal integer
add edx, eax ; perform our addition logic
dec ecx ; decrease ecx (number of arguments left) by 1
jmp nextArg ; jump to nextArg label
mov eax, edx ; move our data result into eax for printing
call iprintLF ; call our integer printing with linefeed function
call quit ; call our quit function
~$ nasm -f elf calculator-atoi.asm
~$ ld -m elf_i386 calculator-atoi.o -o calculator-atoi
~$ ./calculator-atoi 20 1000 317