What does `dup (?)` mean in TASM?

Question

I have this code here, but I'm unfamiliar with the syntax.

STACK16_SIZE    =       100h
stack16         db      STACK16_SIZE dup (?)

I think dup means we declare a variable of type array, as this is a stack, but I'm not sure. So what does dup mean in TASM, exactly?

Answer 1

STACK16_SIZE dup (?) means to duplicate the data in parenthesis by STACK16_SIZE times. It is equivalent to writing ?, ?, ?, ?, ... (100h times)

The data in parens is "uninitialized data". That is, memory is allocated, but not set to any particular value on load.

Assembly does not provide an array "type". If it does, it is only for debuggers for use when inspecting the data. However, in this code snippet, stack16 is a symbol with an address beginning a memory block of bytes—which is counter-intuitive and potentially a source of a subtle bug. For a CPU stack, it really ought to be defined as 16 bit words (dw) or 32 bit words (dd).

Answer 2

? means no particular value, uninitialized. DUP means duplicate.

So you get 100h bytes that are uninitialized.

Answer 3

Let's start with a different example. You can read 20 DUP (0) as "twenty duplicates of zero". The whole expression INPUTSTR DB 20 DUP (0) is equivalent to INPUTSTR DB 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0.

Now, because ? means "uninitialized value", 20 DUP (?) would give you 20 uninitialized bytes. And, in this specific case, where you have STACK16_SIZE DUP (?), you would get STACK16_SIZE uninitialized bytes.

This syntax is not TASM-specific. MASM supports it as well; take a look into the official MASM reference by Microsoft.