Confusing brackets in MASM32

Question

I am trying to get to grips with MASM32 and am confused by the following:

I thought that brackets were used for indirection so if I have the a pre-defined variable

 .data
   item dd 42

then

 mov ebx, item

would put the contents of 'item', i.e. the number 42, into ebx and

 mov ebx, [item]

would put the address of 'item', i.e. where the 42 is stored, into ebx.

But the following code in a console app:

 mov ebx, item
 invoke dwtoa, ebx, ADDR valuestr 
 invoke StdOut, ADDR valuestr
 mov ebx, [item]
 invoke dwtoa, ebx, ADDR valuestr 
 invoke StdOut, ADDR valuestr

prints 42 twice. To get the address of 'item' I seem to need

 mov ebx, [OFFSET item]
 invoke dwtoa, ebx, ADDR valuestr 
 invoke StdOut, ADDR valuestr

Can anybody explain what square brackets are for in MASM, or point me at a good reference.

Answer 1

MASM is unusual for an assembly language in that is has types. MASM knows because of how you defined the symbol item that is a memory location of type DWORD. When you use it as an operand knows that you (probably) mean that you want the value stored at the address, not the value of the address. So it doesn't matter if you use item or [item] MASM assumes you mean the later. If you really want the address of item instead you need to use OFFSET item.

On the other hand if you had defined item as constant using item = 42 then mov ebx, item would load the immediate value. Because of this ambiguity, you need to know how item was defined to determine if it's an immediate operand or a memory operand, it's good idea to avoid using a bare symbol as an operand.

I should add that the square brackets [] mean pretty much nothing to MASM when you're just using symbols or numbers. They only mean something when you use them with registers. Here's some examples:

item    DD  42
const   =   43

    mov eax, item             ; memory operand (the value stored at item)
    mov eax, [item]           ; memory operand
    mov eax, OFFSET item      ; immediate operand (the address of item)
    mov eax, [OFFSET item]    ; immediate operand

    mov eax, const            ; immediate operand (43)
    mov eax, [const]          ; immediate operand
    mov eax, ds:[const]       ; memory operand (the value at address 43)
    mov eax, fs:30h           ; memory operand (the value at address 30h + fs base)
    mov eax, OFFSET const     ; immediate operand
    mov eax, [OFFSET const]   ; immediate operand

    mov eax, 42               ; immediate operand
    mov eax, ebx              ; register operand (the value of EBX)
    mov eax, [ebx]            ; indirect operand (the value pointed to by EBX)

So without registers square brackets only show your intent. You should put square brackets around symbols if you intend to use them as memory operands, and use OFFSET with symbols you intend to use as immediate values.