Can I multiply a register's value by an immediate number to add the result to another register? - linux

Question

Learning Assembly with NASM, Ubuntu, 32 bits.
My array in .data:
ary db 1,2,2,4,5 ; Five elements of one byte each
And some number:
tmp db 2 ; Holds the number 2
Let's say I want to print the element at index 4 in the array (so it would be 5).
I know I could do this:
mov EAX,4
mov EBX,0
mov ECX,ary ; Put the array's address in ECX
add ECX,4 ; Move address four bytes to the right
add byte [ECX],'0' ; The value at this address to ASCII
mov EDX,1
int 0x80
However, for whatever reasons, I decided that instead of writing the constant number 4, I want to do it by multiplying my variable (which is 2) by 2.
This is the updated code:
mov EAX,[tmp] ; Put the number 2 in EAX
mov ECX,ary ; Put the array's address in ECX
add ECX,EAX * 2 ; Move (2 * 2) = 4 bytes to the right
add byte [ECX],'0' ; Decimal to ASCII
mov EAX,4
mov EBX,0
mov EDX,1
int 0x80
This doesn't work at add ECX,EAX * 2:
invalid operand type
But why? Doesn't ECX evaluate to 2? Being equivalent to
add ECX,2 * 2
Curiously, these do work:
add ECX,EAX * 1 ; Moves by 2
add ECX,EAX * 0 ; Moves by 0
The above suggests me that the answer is no. And the reason that multiplying by 1 or 0 works is because the assembler doesn't actually need to do any multiplication to know the answer in the first place.
Does this mean that to achieve what I want, I do have to use the mul instruction?

Answer 1

You CAN do multiplication and adding in one instruction if you use lea:
lea ECX,[ECX+EAX*2]

Answer 2

In x86, although lea supports multiplication by a constant, the add instruction doesn't support an operand that multiples a register by a constant. It supports additive offsets, but not multiplication. I assume, as you noted, that the assembler is being somewhat forgiving in this case in the accepted syntax of add ECX,EAX*0 and add ECX,EAX*1 as being equivalent to add ECX,0 and add ECX,EAX, respectively.
You would instead need do something like this:
mov ECX,ary ; Put the array's address in ECX
mov EAX,[tmp] ; Put the number 2 in EAX
shl EAX,1 ; (instead of mul EAX,2)
add ECX,EAX ; Move (2 * 2) = 4 bytes to the right
add byte [ECX],'0' ; Decimal to ASCII
mov EAX,4
mov EBX,0
mov EDX,1
int 0x80

Answer 3

The instruction LEA can be used to provide two additions and one limited multiplication at once. The common syntax is:
lea reg, [offset+reg+const*reg]
Here, reg is any register, offset is some constant number and const is one of 1, 2, 4 or 8 constant.
This way, this instruction is very powerful is order to compute some pretty complex equations:
The equation from the question:
add ECX,EAX * 2
can be computed this way:
lea ecx, [ecx+2*eax]
There are many other uses:
lea eax, [ebx+2*ebx] ; eax = 3*ebx
lea eax, [eax+4*eax] ; eax = 5*eax
lea eax, [ecx+8*ecx] ; eax = 9*ecx
lea eax, [1234+ebx+8*ecx]
Note, that FASM allows shorter syntax for the above examples:
lea eax, [3*ebx]
lea eax, [5*eax]
lea eax, [9*ecx]
Additional advantage of lea instruction is that it does not affects the flags. The execution speed of this instruction is very fast on all x86 CPU.