fibonacci question

[slc234]

Hi I have a program that determines the seven numbers of the fibonacci sequence and puts them in the eax register with a loop. My question is: why do I have to move 12 into ecx, couldn't I move 7 instead since it is the first seven values? Is there another way to do this without decrementing it and by using code in data too?

Code Select Expand



INCLUDE Irvine32.inc

.code
main PROC

mov ebp, 0
mov edx, 1

mov ebx, edx
mov ecx, 12
L1:
mov eax, edx
mov ebp, eax
mov edx, ebx
add ebx, ebp
; call DumpRegs
; dec ecx
loop L1
exit
main ENDP
END main



[RuiLoureiro]

Hi I have a program that determines the seven numbers of the fibonacci sequence and puts them in the eax register with a loop. My question is: why do I have to move 12 into ecx, couldn't I move 7 instead since it is the first seven values? Is there another way to do this without decrementing it and by using code in data too?

Code Select Expand



INCLUDE Irvine32.inc

.code
main PROC

mov ebp, 0
mov edx, 1

mov ebx, edx
mov ecx, 12
L1:
mov eax, edx
mov ebp, eax
mov edx, ebx
add ebx, ebp
; call DumpRegs
; dec ecx
loop L1
exit
main ENDP
END main



You should move 7; you dont need dec ecx, but you need
this:
             mov   ecx, 7
             L1:
                   push ecx
                   ...
                   etc.
                   ...
                   pop   ecx
                   loop L1

[slc234]

Is there a way to do it without the push and pop statement?

[RuiLoureiro]

Is there a way to do it without the push and pop statement?

No because some procs inside L1 till loop L1 destroys ECX
                Yes if you dont use any proc inside that loop

[slc234]

It is a silly question, but how would I do it with no Proc inside the loop? I'm trying to do it the most basic way to understand it.

[raymond]

Not only can they destroy ECX, but also EDX and EAX. However, most external procedures are expected to preserve EBX, ESI and EDI. If you intend to call the DumpRegs procedure from within the loop without any push/pop, you could do it as follows. It is based on the standard Fibonacci sequence which starts with 1 and 1 as the F₁ and F₂ numbers.

ESI is initialized to the F₀ Fibonacci number
EDI is initialized to the F₁ Fibonacci number
EBX is used as the counter and initialized to the nth Fibonacci number you want to compute. 47 would be the maximum to use in EBX; the F₄₈ Fibonacci number exceeds  32 bits.

Code Select Expand

   mov esi,0
   mov edi,1
   mov ebx,7

@@:
   xchg esi,edi
   call DumpRegs
   add esi,edi
   dec ebx
   jnz @B

   exit

You could replace the DumpRegs procedure by another one which would display only the content of the ESI register.

[jj2007]

Not only can they destroy ECX, but also EDX and EAX. However, most external procedures are expected to preserve EBX, ESI and EDI. If you intend to call the DumpRegs procedure from within the loop without any push/pop,...

DumpRegs does not trash any registers, so you can safely use it inside a loop:

include \masm32\MasmBasic\MasmBasic.inc        ; download
include \masm32\MasmBasic\IrvineMb\Irvine32Mb.inc     ; attached

   Init
   mov eax, 11111111h
   mov ebx, 22222222h
   mov ecx, 33333333h
   mov edx, 44444444h
   call DumpRegs      ; displays the regs
   call DumpRegs      ; if DumpRegs changes them, they should now look different
   Inkey "bye"
   Exit
end start

Output:
  EAX=11111111  EBX=22222222  ECX=33333333  EDX=44444444
  ESI=00000000  EDI=00000000  EBP=0012FF94  ESP=0012FF8C
  EIP=0040103D  EFL=00000246  CF=0  SF=0  ZF=1  OF=0  AF=0  PF=1

  EAX=11111111  EBX=22222222  ECX=33333333  EDX=44444444
  ESI=00000000  EDI=00000000  EBP=0012FF94  ESP=0012FF8C
  EIP=00401042  EFL=00000246  CF=0  SF=0  ZF=1  OF=0  AF=0  PF=1

[raymond]

Thanks for the clarification about the DumpRegs procedure. However, using the suggested registers would not need to be modified if any other display procedure not preserving ALL registers was used.