14.2: Example, Sum and Average
- Page ID
- 19947
The following is a simple example of a main that calls an assembly language function, stats(), to compute the integer sum and integer average for a list of signed integers. The main and the function are in different source files and are presented as an example of how multiple source files are used. The example itself is really too small to actually require multiple source files.
Assembly Main
The main is as follows:
; Simple example to call an external function.
; ----------------------------------------------------------
; Data section
section .data
; -----
; Define standard constants
LF equ 10 ; line feed
NULL equ 0 ; end of string
TRUE equ 1
FALSE equ 0
EXIT_SUCCESS equ 0 ; success code
SYS_exit equ 60 ; terminate
; -----
; Declare the data
1st1 dd 1, -2, 3, -4, 5
dd 7, 9, 11
1en1 dd 8
1st2 dd 2, -3, 4, -5, 6
dd -7, 10, 12, 14, 16
len2 dd 10
section .bss
sum1 resd 1
ave1 resd 1
sum2 resd 1
ave2 resd 1
; ----------------------------------------------------------
extern stats
section .text
global _start
_start:
; ----
; Call the function
; HLL Call: stats (1st, len, &sum, &ave);
mov rdi, lst1 ; data set 1
mov esi, dword [len1]
mov rdx, sum1
mov rcx, ave1
call stats
mov rdi, lst2 ; data set 2
mov esi, dword [len2]
mov rdx, sum2
mov rcx, ave2
call stats
; -----
; Example program done
exampleDone:
mov rax, SYS_exit
mov rdi, EXIT_SUCCESS
syscall
The above main can be assembled with the same assemble command as described in Chapter 5, Tool chain. The extern statement will ensure that the assembler does not generate errors.
14.2.2 Function Source
The function, in a different source file is as follows:
; Simple example void function.
; **********************************************************
; Data declarations
; Note, none needed for this example.
; If needed, they would be declared here as always.
section .data
; **********************************************************
section .text
; --------------------------------------------------
; Function to find integer sum and integer average
; for a passed list of signed integers.
; Call:
; stats(lst, len, &sum, &ave);
; Arguments Passed:
; 1) rdi - address of array
; 2) rsi - length of passed array
; 3) rdx - address of variable for sum
; 4) rcx - address of variable for average
; Returns:
; sum of integers (via reference)
; average of integers (via reference)
global stats
stats:
push r12
; -----
; Find and return sum.
mov r11, 0 ; i=0
mov r12d, 0 ; sum=0
sumLoop:
mov eax, dword [rdi+r11*4] ; get lst[i]
add r12d, eax ; update sum
inc r11 ; i++
cmp r11, rsi
jb sumLoop
mov dword [rdx], r12d ; return sum
; -----
; Find and return average.
mov eax, r12d
cdq
idiv esi
mov dword [rcx], eax ; return average
; -----
; Done, return to calling function.
pop r12
ret
The above source file can be assembled with the same assemble command as described in Chapter 5, Tool chain. No extern statement is required since no external functions are called.
Assemble and Link
Assuming the main source file is named main.asm and the functions source file is named stats.asm, the following command will perform the assemble and link.
yasm -g dwarf2 -f elf64 main.asm -l main.lst
yasm -g dwarf2 -f elf64 stats.asm -l stats.lst
ld -g -o main main.o stats.o
The files names can be changed as desired.
As usual, the debugger is started with the ddd <executable> command. It would be appropriate to note that using the debugger step command will step into the function, including showing the function source code (even if the source is in a different file). The debugger next command will execute the entire function and not show the source. If the function is working, the next command would be most useful. In order to debug the function, the step command would be most useful.
If the “-g” option is omitted, the debugger will not be able to display the source code.