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(nasm.info)Section B.4.18

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B.4.18. `CALL': Call Subroutine
-------------------------------

     CALL imm                      ; E8 rw/rd             [8086]
     CALL imm:imm16                ; o16 9A iw iw         [8086]
     CALL imm:imm32                ; o32 9A id iw         [386]
     CALL FAR mem16                ; o16 FF /3            [8086]
     CALL FAR mem32                ; o32 FF /3            [386]
     CALL r/m16                    ; o16 FF /2            [8086]
     CALL r/m32                    ; o32 FF /2            [386]

   `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 `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.