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Document NASM behaviour for 64-bit immediates and displacements

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Document (intended) NASM behaviour for 64-bit immediates and
displacements.
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H. Peter Anvin 2007-09-25 16:01:07 -07:00
parent b0aff17908
commit f664bf1ed6
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@ -6282,6 +6282,51 @@ just from 32-bit platforms but from each other. If a specific size
data type is desired, it is probably best to use the types defined in
the Standard C header \c{<inttypes.h>}.
In 64-bit mode, the default instruction size is still 32 bits. When
loading a value into a 32-bit register (but not an 8- or 16-bit
register), the upper 32 bits of the corresponding 64-bit register are
set to zero.
\H{id64} Immediates and displacements in 64-bit mode
In 64-bit mode, immediates and displacements are generally only 32
bits wide. NASM will therefore truncate most displacements and
immediates to 32 bits.
The only instruction which takes a full 64 bit immediate is:
\c MOV reg64,imm64
NASM will produce this instruction whenever the programmer uses \c{MOV}
with an immediate into a 64-bit register. If this is not desirable,
simply specify the equivalent 32-bit register, which will be
automatically zero-extended by the processor:
\c mov rax,foo ; 64-bit immediate
\c mov eax,foo ; 32-bit immediate, zero-extended
The only instructions which take a full 64-bit \e{displacement} is
loading or storing, using \c{MOV}, \c{AL}, \c{AX}, \c{EAX} or \c{RAX}
(but no other registers) to an absolute 64-bit address. Since this is
a relatively rarely used instruction (64-bit code generally uses
relative addressing), the programmer has to explicitly declare the
displacement size as \c{QWORD}:
\c default abs
\c
\c mov eax,[foo] ; 32-bit absolute displacement (-2..2 GB)
\c mov eax,[a32 foo] ; 32-bit absolute displacement (0..4 GB)
\c mov eax,[qword foo] ; 64-bit absolute displacement
\c
\c default rel
\c
\c mov eax,[foo] ; 32-bit relative displacement
\c mov eax,[a32 foo] ; d:o, address truncated to 32 bits(!)
\c mov eax,[qword foo] ; 32-bit relative displacement(!)
\c mov eax,[abs qword foo] ; 64-bit absolute displacement
FIXME: THIS IS NOT YET CORRECTLY IMPLEMENTED
\H{unix64} Interfacing to 64-bit C Programs (Unix)
On Unix, the 64-bit ABI is defined by the document: