組合語言的問題?

[kinco]

這是朋友傳給我的作業答案~
但是我還是不懂~為什麼要用到EAX EBX ECX EDX 這麼多的指令

;*****This is Assembly Exam 01*****
INCLUDE Irvine32.inc
.data 
x1 QWORD 0A02B000190008400h 
x2 QWORD  800187008000A904h 
sum BYTE 9 DUP(?) 
.code
main PROC
  mov esi,offset sum
  mov EAX,0
  mov EBX,0
  mov EDX,0
  mov ECX,9
  mov EAX,DWORD PTR [x1]         
  add EAX,DWORD PTR [x2] 
  adc DX,0
  CLC
  mov EBX,DWORD PTR [x1+4]       
  add EBX,DWORD PTR [x2+4]
  add EBX,EDX
  adc DH,0 
  mov [esi],EAX
  mov [esi+4],EBX
  mov [esi+8],DL
  mov EDX,0
  mov edi,0
L1:
  movzx EDX,[sum+edi]
  push EDX
  inc edi
  loop L1
mov ECX,9
mov edi,0
L2:
  pop EDX
  mov [sum+edi],DL
  inc edi
  loop L2
mov esi,offset sum
mov ECX,9
  call DumpMem
  call DumpRegs
EXIT
main ENDP
END  main