Can somebody help me out with this expression written in assembly using arithmetic operations?

Question

The expression: EAX = 7*EAX - 2*EBX - EBX/8
My code:

start:
    mov EAX,9
    mov EDX ,7
    mul EDX ; EAX<-EAX*EDX
    mov EDX,EAX ; EDX<-EAX*EDX<=>(7*9)
    mov EBX,8
    mov EAX,2
    mul EBX; EAX<- EAX*EBX (2*8)
    mov ECX,EAX
    sub EDX,ECX; <=>7*EAX-2*EBX (7*9-2*8)
    mov EAX,EBX
    div EBX ; EAX <=> 8/8
    sub EDX,EAX; <=> 7*9-2*8-8/8
    push 0
    call exit
end start

Answer 1

EAX = 7*EAX - 2*EBX - EBX/8

It is not because an expression contains multiplication (*) or division (/), that your solution should be using these operations.

• For 7*EAX you can use the simpler imul eax, 7.
• Instead of doubling EBX, you could simply subtract EBX twice.
• And for calculating an eighth of EBX, you should simply shift its value 3 times to the right.

imul eax, 7
sub  eax, ebx
sub  eax, ebx
shr  ebx, 3
sub  eax, ebx