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(nasm.info)Section 3.1

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3.1. Layout of a NASM Source Line
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   Like most assemblers, each NASM source line contains (unless it is a
macro, a preprocessor directive or an assembler directive: see Note:
Chapter 4 and Note: Chapter 5) some combination of the four fields

     label:    instruction operands        ; comment

   As usual, most of these fields are optional; the presence or absence
of any combination of a label, an instruction and a comment is allowed.
Of course, the operand field is either required or forbidden by the
presence and nature of the instruction field.

   NASM uses backslash (\) as the line continuation character; if a
line ends with backslash, the next line is considered to be a part of
the backslash- ended line.

   NASM places no restrictions on white space within a line: labels may
have white space before them, or instructions may have no space before
them, or anything. The colon after a label is also optional. (Note that
this means that if you intend to code `lodsb' alone on a line, and type
`lodab' by accident, then that's still a valid source line which does
nothing but define a label. Running NASM with the command-line option
`-w+orphan-labels' will cause it to warn you if you define a label
alone on a line without a trailing colon.)

   Valid characters in labels are letters, numbers, `_', `$', `#', `@',
`~', `.', and `?'. The only characters which may be used as the _first_
character of an identifier are letters, `.' (with special meaning: see
*Note Section 3.9::), `_' and `?'. An identifier may also be prefixed
with a `$' to indicate that it is intended to be read as an identifier
and not a reserved word; thus, if some other module you are linking
with defines a symbol called `eax', you can refer to `$eax' in NASM
code to distinguish the symbol from the register.

   The instruction field may contain any machine instruction: Pentium
and P6 instructions, FPU instructions, MMX instructions and even
undocumented instructions are all supported. The instruction may be
prefixed by `LOCK', `REP', `REPE'/`REPZ' or `REPNE'/`REPNZ', in the
usual way. Explicit address-size and operand-size prefixes `A16',
`A32', `O16' and `O32' are provided - one example of their use is given
in Note: Chapter 9. You can also use the name of a segment register
as an instruction prefix: coding `es mov [bx],ax' is equivalent to
coding `mov [es:bx],ax'. We recommend the latter syntax, since it is
consistent with other syntactic features of the language, but for
instructions such as `LODSB', which has no operands and yet can require
a segment override, there is no clean syntactic way to proceed apart
from `es lodsb'.

   An instruction is not required to use a prefix: prefixes such as
`CS', `A32', `LOCK' or `REPE' can appear on a line by themselves, and
NASM will just generate the prefix bytes.

   In addition to actual machine instructions, NASM also supports a
number of pseudo-instructions, described in *Note Section 3.2::.

   Instruction operands may take a number of forms: they can be
registers, described simply by the register name (e.g. `ax', `bp',
`ebx', `cr0': NASM does not use the `gas'-style syntax in which
register names must be prefixed by a `%' sign), or they can be
effective addresses (see *Note Section 3.3::), constants (*Note Section
3.4::) or expressions (*Note Section 3.5::).

   For floating-point instructions, NASM accepts a wide range of
syntaxes: you can use two-operand forms like MASM supports, or you can
use NASM's native single-operand forms in most cases. Details of all
forms of each supported instruction are given in Note: Appendix B.
For example, you can code:

             fadd    st1             ; this sets st0 := st0 + st1
             fadd    st0,st1         ; so does this
     
             fadd    st1,st0         ; this sets st1 := st1 + st0
             fadd    to st1          ; so does this

   Almost any floating-point instruction that references memory must
use one of the prefixes `DWORD', `QWORD' or `TWORD' to indicate what
size of memory operand it refers to.