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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
1 changed files with 45 additions and 0 deletions
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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 data type is desired, it is probably best to use the types defined in
the Standard C header \c{<inttypes.h>}. 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) \H{unix64} Interfacing to 64-bit C Programs (Unix)
On Unix, the 64-bit ABI is defined by the document: On Unix, the 64-bit ABI is defined by the document: