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(nasm.info)Section 2.2.2

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2.2.2. NASM Requires Square Brackets For Memory References
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   NASM was designed with simplicity of syntax in mind. One of the
design goals of NASM is that it should be possible, as far as is
practical, for the user to look at a single line of NASM code and tell
what opcode is generated by it. You can't do this in MASM: if you
declare, for example,

     foo     equ     1
     bar     dw      2

   then the two lines of code

             mov     ax,foo
             mov     ax,bar

   generate completely different opcodes, despite having
identical-looking syntaxes.

   NASM avoids this undesirable situation by having a much simpler
syntax for memory references. The rule is simply that any access to the
_contents_ of a memory location requires square brackets around the
address, and any access to the _address_ of a variable doesn't. So an
instruction of the form `mov ax,foo' will _always_ refer to a
compile-time constant, whether it's an `EQU' or the address of a
variable; and to access the _contents_ of the variable `bar', you must
code `mov ax,[bar]'.

   This also means that NASM has no need for MASM's `OFFSET' keyword,
since the MASM code `mov ax,offset bar' means exactly the same thing as
NASM's `mov ax,bar'. If you're trying to get large amounts of MASM code
to assemble sensibly under NASM, you can always code `%idefine offset'
to make the preprocessor treat the `OFFSET' keyword as a no-op.

   This issue is even more confusing in `a86', where declaring a label
with a trailing colon defines it to be a `label' as opposed to a
`variable' and causes `a86' to adopt NASM-style semantics; so in `a86',
`mov ax,var' has different behaviour depending on whether `var' was
declared as `var: dw 0' (a label) or `var dw 0' (a word-size variable).
NASM is very simple by comparison: _everything_ is a label.

   NASM, in the interests of simplicity, also does not support the
hybrid syntaxes supported by MASM and its clones, such as `mov
ax,table[bx]', where a memory reference is denoted by one portion
outside square brackets and another portion inside. The correct syntax
for the above is `mov ax,[table+bx]'. Likewise, `mov ax,es:[di]' is
wrong and `mov ax,[es:di]' is right.