1

im trying to do a simple program calculating a recursive function where:

  1. X1 = -1
  2. X2 = -2
  3. Xn = X(n-1) * X(n-2)

Using x64 assembly. I have two problems i cant come up with solution for:

  • a) how can i make it like a "real" function - argument passed in %rax, function result located in %rax. As you can see for now my argument is passed through %rdi and result is in %rax.

  • b) how can i pass the argument and results through stack? i tried it but failed (stack-version lines are commented out). it has something to do with calculating address i suppose, could someone modify it so i get the grasp how it should have been done?

    .data
SYSREAD = 0
SYSWRITE = 1
SYSEXIT = 60
NUMBER = 4

STDOUT = 1 
STDIN = 0

EXIT_SUCCESS = 0

.text
.global main

 main:
# --- --- --- --- --- --- --- --- --- --- --- ---
mov $0, %rax
mov $0, %r15
mov $0, %rdi
mov $NUMBER, %rdi

# push %rdi
call recursion
# pop %rax
jmp end

# --- --- --- --- --- --- --- --- --- --- --- ---
                                # function
                                # X1 = -1
                                # X2 = -2
                                # Xn = X(n-1) razy X(n-2)
                                # input   => %rax = n
                                # output  => %rax = X(n)
recursion:
# pop %rdi

cmp $1, %rdi                    # is n = 1? 
je recursion_end_1

cmp $2, %rdi                    # is n = 2?
je recursion_end_2
                                # n in %rdi
dec %rdi                        # (n-1)
push %rdi                       # (n-1) into stack
call recursion

pop %rdi                        # (n-1) in %rdi
dec %rdi                        # (n-2)
push %rax                       # (n-2) into stack
call recursion

pop %rcx                        # value of (n-2)
imul %rcx                       # multiplication
push %rax
jmp recursion_end               # result in %rax

recursion_end_1:
mov $-1, %rax                   # n = 1
ret

recursion_end_2:
mov $-2, %rax                   # n = 2
ret

recursion_end:
ret

# --- --- --- --- --- --- --- --- --- --- --- ---

end:
movq $SYSEXIT, %rax             # wychodzenie
movq $EXIT_SUCCESS, %rdi
syscall
JPK
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    "real" functions **do** get the first argument in `rdi` as per the normal sysv abi convention. – Jester May 09 '17 at 22:37
  • About stack: http://stackoverflow.com/q/1395591/4271923 By the very first `pop %rdi` inside `recursion` you are taking return address out of stack, not argument (arguments are above that, so `mov rdi,[rsp+8]` is probably close to what you want). Don't hesitate to step into this code in debugger, and watch values in `rsp` + open memory window to that area, to see how those values are written into memory + how `push/pop/call/ret` operates. Every minute of poking around in debugger usually equals to 10 minutes of reading/explaining. – Ped7g May 10 '17 at 13:27

0 Answers0