Question

2 forms in constant declaration:

len equ 2
len: equ 2

Are they the same?

If not then what is the advantage or disadvantage of each declaration form?

Answer 1

The functional difference between these two:

len  equ 2
len: equ 2

is no difference at all. As stated on the NASM doc page:

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.)