CALL calls a subroutine, by means of pushing the current
instruction pointer (IP) and optionally CS as well on the
stack, and then jumping to a given address.
CS is pushed as well as IP if and only if the call is a far
call, i.e. a destination segment address is specified in the
instruction. The forms involving two colon-separated arguments are
far calls; so are the CALL FAR mem forms.
The immediate near call takes one of two forms (call imm16/imm32,
determined by the current segment size limit. For 16-bit operands,
you would use CALL 0x1234, and for 32-bit operands you would use
CALL 0x12345678. The value passed as an operand is a relative offset.
You can choose between the two immediate far call forms
(CALL imm:imm) by the use of the WORD and DWORD keywords:
CALL WORD 0x1234:0x5678) or CALL DWORD 0x1234:0x56789abc.
The CALL FAR mem forms execute a far call by loading the
destination address out of memory. The address loaded consists of 16
or 32 bits of offset (depending on the operand size), and 16 bits of
segment. The operand size may be overridden using \c{CALL WORD FAR
mem} or CALL DWORD FAR mem.
The CALL r/m forms execute a near call (within the same
segment), loading the destination address out of memory or out of a
register. The keyword NEAR may be specified, for clarity, in
these forms, but is not necessary. Again, operand size can be
overridden using CALL WORD mem or CALL DWORD mem.
As a convenience, NASM does not require you to call a far procedure
symbol by coding the cumbersome CALL SEG routine:routine, but
instead allows the easier synonym CALL FAR routine.
The CALL r/m forms given above are near calls; NASM will accept
the NEAR keyword (e.g. CALL NEAR [address]), even though it
is not strictly necessary.