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nasmdoc: corrections on 64-bit immediates/displacements

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Corrections the section on 64-bit immediates and displacements.  In
particular, immediates are sign-extended the same way displacements
are (and the same way 8-bit immediates are), so there is some use for
the 7-byte mov eax,dword imm form :(
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H. Peter Anvin 2007-09-25 20:36:45 -07:00
parent 64b3a9c56b
commit 4cba95cf81
1 changed files with 17 additions and 9 deletions
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doc/nasmdoc.src
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@ -6293,19 +6293,24 @@ 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:
The only instruction which takes a full \i{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:
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, or specify the
immediate as \c{DWORD}:
\c mov rax,foo ; 64-bit immediate
\c mov rax,qword foo ; (identical)
\c mov eax,foo ; 32-bit immediate, zero-extended
\c mov rax,dword foo ; 32-bit immediate, sign-extended
The only instructions which take a full 64-bit \e{displacement} is
The length of these instructions are 10, 5 and 7 bytes, respectively.
The only instructions which take a full \i{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
@ -6314,17 +6319,20 @@ displacement size as \c{QWORD}:
\c default abs
\c
\c mov eax,[foo] ; 32-bit absolute disp (-2..2 GB)
\c mov eax,[a32 foo] ; 32-bit absolute disp (0..4 GB)
\c mov eax,[foo] ; 32-bit absolute disp, sign-extended
\c mov eax,[a32 foo] ; 32-bit absolute disp, zero-extended
\c mov eax,[qword foo] ; 64-bit absolute disp
\c
\c default rel
\c
\c mov eax,[foo] ; 32-bit relative disp
\c mov eax,[a32 foo] ; d:o, address truncated to 32 bits(!)
\c mov eax,[qword foo] ; 32-bit relative disp(!)
\c mov eax,[qword foo] ; error
\c mov eax,[abs qword foo] ; 64-bit absolute disp
A sign-extended absolute displacement can access from -2 GB to +2 GB;
a zero-extended absolute displacement can access from 0 to 4 GB.
FIXME: THIS IS NOT YET CORRECTLY IMPLEMENTED
\H{unix64} Interfacing to 64-bit C Programs (Unix)