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% ==================================================
% x64
% ==================================================
\subsection{x64 Calling Conventions}


\paragraph{Overview}

The x64 (64bit) architecture designed by AMD is based on Intel's x86 (32bit)
architecture, supporting it natively. It is sometimes referred to as x86-64,
AMD64, or, cloned by Intel, EM64T or Intel64.\\
On this processor, a word is defined to be 16 bits in size, a dword 32 bits
and a qword 64 bits. Note that this is due to historical reasons (terminology
didn't change with the introduction of 32 and 64 bit processors).\\
The x64 calling convention for MS Windows \cite{x64Win} differs from the
SystemV x64 calling convention \cite{x64SysV} used by Linux/*BSD/...
Note that this is not the only difference between these operating systems. The
64 bit programming model in use by 64 bit windows is LLP64, meaning that the C
types int and long remain 32 bits in size, whereas long long becomes 64 bits.
Under Linux/*BSD/... it's LP64.\\
\\
Compared to the x86 architecture, the 64 bit versions of the registers are
called rax, rbx, etc.. Furthermore, there are eight new general purpose
registers r8-r15.\\



\paragraph{\product{dyncall} support}

Currently, the MS Windows and System V calling conventions are supported.\\
\product{Dyncall} can also be used to issue syscalls on System V platforms by
using the syscall number as target parameter and selecting the correct mode.

\subsubsection{MS Windows}

\paragraph{Registers and register usage}

\begin{table}[h]
\begin{tabular*}{0.95\textwidth}{3 B}
Name                & Brief description\\
\hline
{\bf rax}           & scratch, return value\\
{\bf rbx}           & permanent\\
{\bf rcx}           & scratch, parameter 0 if integer or pointer\\
{\bf rdx}           & scratch, parameter 1 if integer or pointer\\
{\bf rdi}           & permanent\\
{\bf rsi}           & permanent\\
{\bf rbp}           & permanent, may be used as frame pointer\\
{\bf rsp}           & stack pointer\\
{\bf r8-r9}         & scratch, parameter 2 and 3 if integer or pointer\\
{\bf r10-r11}       & scratch, permanent if required by caller (used for syscall/sysret)\\
{\bf r12-r15}       & permanent\\
{\bf xmm0}          & scratch, floating point parameter 0, floating point return value\\
{\bf xmm1-xmm3}     & scratch, floating point parameters 1-3\\
{\bf xmm4-xmm5}     & scratch, permanent if required by caller\\
{\bf xmm6-xmm15}    & permanent\\
\end{tabular*}
\caption{Register usage on x64 MS Windows platform}
\end{table}

\paragraph{Parameter passing}

\begin{itemize}
\item stack parameter order: right-to-left
\item caller cleans up the stack
\item first 4 integer/pointer parameters are passed via rcx, rdx, r8, r9 (from left to right), others are pushed on stack (there is a
spill area for the first 4)
\item {\it non-trivial} C++ aggregates (as defined by the language) of any size, are passed indirectly via a pointer to a copy of the aggregate
\item aggregates (structs and unions) \textless\ 64 bits are passed like equal-sized integers
\item float and double parameters are passed via xmm0l-xmm3l
\item first 4 parameters are passed via the correct register depending on the parameter type - with mixed float and int parameters,
some registers are left out (e.g. first parameter ends up in rcx or xmm0, second in rdx or xmm1, etc.)
\item parameters in registers are right justified
\item parameters \textless\ 64bits are not zero extended - zero the upper bits contiaining garbage if needed (but they are always
passed as a qword)
\item parameters \textgreater\ 64 bits are passed by via a pointer to a copy (for aggregate types, that caller-allocated memory must be 16-byte aligned)
\item if callee takes address of a parameter, first 4 parameters must be dumped (to the reserved space on the stack) - for
floating point parameters, value must be stored in integer AND floating point register
\item caller cleans up the stack, not the callee (like cdecl)
\item stack is always 16byte aligned - since return address is 64 bits in size, stacks with an odd number of parameters are
already aligned
\item ellipsis calls take floating point values in int and float registers (single precision floats are promoted to double precision as
required by ellipsis calls)
\item if size of parameters \textgreater\ 1 page of memory (usually between 4k and 64k), chkstk must be called
\end{itemize}


\paragraph{Return values}

\begin{itemize}
\item return values of pointer, integral or aggregate (structs and unions) type (\textless=\ 64 bits) are returned via the rax register
\item floating point types are returned via the xmm0 register
\item for any other type \textgreater\ 64 bits (or for {\it non-trivial} C++ aggregates of any size), a hidden first parameter, with an address to the
return value is passed (for C++ thiscalls it is passed as {\bf second} parameter, after the this pointer)
\end{itemize}


\paragraph{Stack layout}

Stack frame is always 16-byte aligned.
% verified/amended: TP nov 2019 (@@@ no doc/disas_examples/x64.win.disas, yet...@@@)
Stack directly after function prolog:\\

\begin{figure}[h]
\begin{tabular}{5|3|1 1}
                                  & \vdots         &                                &                               \\
\hhline{~=~~}
register save area                & \hspace{4cm}   &                                & \mrrbrace{10}{caller's frame} \\
\hhline{~-~~}
local data                        &                &                                &                               \\
\hhline{~-~~}                            
\mrlbrace{7}{parameter area}      & arg n-1        & \mrrbrace{3}{stack parameters} &                               \\
                                  & \ldots         &                                &                               \\
                                  & arg 4          &                                &                               \\
                                  & r9 or xmm3     & \mrrbrace{4}{spill area}       &                               \\
                                  & r8 or xmm2     &                                &                               \\
                                  & rdx or xmm1    &                                &                               \\
                                  & rcx or xmm0    &                                &                               \\
\hhline{~-~~}
                                  & return address &                                &                               \\
\hhline{~=~~}
register save area                &                &                                & \mrrbrace{4}{current frame}   \\
\hhline{~-~~}
local data                        &                &                                &                               \\
\hhline{~-~~}
parameter area                    &                &                                &                               \\
\hhline{~-~~}
                                  & \vdots         &                                &                               \\
\end{tabular}
\caption{Stack layout on x64 Microsoft platform}
\end{figure}



\clearpage

\subsubsection{System V (Linux / *BSD / MacOS X)}

\paragraph{Registers and register usage}

\begin{table}[h]
\begin{tabular*}{0.95\textwidth}{3 B}
Name                & Brief description\\
\hline
{\bf rax}           & scratch, return value, special use for varargs (in al, see below)\\
{\bf rbx}           & permanent\\
{\bf rcx}           & scratch, parameter 3 if integer or pointer\\
{\bf rdx}           & scratch, parameter 2 if integer or pointer, return value\\
{\bf rdi}           & scratch, parameter 0 if integer or pointer\\
{\bf rsi}           & scratch, parameter 1 if integer or pointer\\
{\bf rbp}           & permanent, may be used as frame pointer\\
{\bf rsp}           & stack pointer\\
{\bf r8-r9}         & scratch, parameter 4 and 5 if integer or pointer\\
{\bf r10-r11}       & scratch\\
{\bf r12-r15}       & permanent\\
{\bf xmm0-xmm1}     & scratch, floating point parameters 0-1, floating point return value\\
{\bf xmm2-xmm7}     & scratch, floating point parameters 2-7\\
{\bf xmm8-xmm15}    & scratch\\
{\bf st0-st1}       & scratch, 16 byte floating point return value\\
{\bf st2-st7}       & scratch\\
\end{tabular*}
\caption{Register usage on x64 System V (Linux/*BSD)}
\end{table}

\paragraph{Parameter passing}

\begin{itemize}
\item stack parameter order: right-to-left
\item caller cleans up the stack
\item first 6 integer/pointer parameters are passed via rdi, rsi, rdx, rcx, r8, r9
\item first 8 floating point parameters \textless=\ 64 bits are passed via xmm0l-xmm7l
\item parameters in registers are right justified
\item parameters that are not passed via registers are pushed onto the stack (with their sizes rounded up to qwords)
\item parameters \textless\ 64bits are not zero extended - zero the upper bits contiaining garbage if needed (but they are always
passed as a qword)
\item integer/pointer parameters \textgreater\ 64 bit are passed via 2 registers
\item if callee takes address of a parameter, number of used xmm registers is passed silently in al (passed number doesn't need to be
exact but an upper bound on the number of used xmm registers)
\item aggregates (structs, unions (and arrays within those)) follow a more complicated logic (the following {\bf only considers field types supported by dyncall}):
\begin{itemize}
  \item {\it non-trivial} C++ aggregates (as defined by the language) of any size, are passed indirectly via a pointer to a copy of the aggregate
  \item aggregates \textgreater\ 16 bytes are always passed entirely via the stack
  \item all other aggregates are classified per qword, by looking at all fields occupying all or part of that qword, recursively
  \begin{itemize}
    \item if any field would be passed via the stack, the entire qword will
    \item otherwise, if any field would be passed like an integer/pointer value, the entire qword will
    \item otherwise the qword is passed like a floating point value
  \end{itemize}
  \item after qword classification, the logic is:
  \begin{itemize}
    \item if any qword is classified to be passed via the stack, the entire aggregate will
    \item if the size of the aggregate is \textgreater\ 2 qwords, it is passed via the stack (except for single floating point values \textgreater\ 128bits)
    \item all others are passed qword by qword according to their classification, like individual arguments
	\item however, an aggregate is never split between registers and the stack, if it doesn't fit into available registers it is entirely passed via the stack (freeing such registers for subsequent arguments)
  \end{itemize}
\end{itemize}
\item stack is always 16byte aligned - since return address is 64 bits in size, stacks with an odd number of parameters are
already aligned
\item no spill area is used on stack, iterating over varargs requires a specific va\_list implementation
\end{itemize}


\paragraph{Return values}

\begin{itemize}
\item return values of pointer or integral type are returned via the rax register (and rdx if needed)
\item floating point types are returned via the xmm0 register (and xmm1 if needed)
\item aggregates are first classified in the same way as when passing them by value, then:
\begin{itemize}
  \item for aggregates that would be passed via the stack (or for {\it non-trivial} C++ aggregates of any size), a hidden pointer to a non-shared,
  caller provided space is {\bf passed} as hidden, first argument; this pointer will be returned via rax
  \item otherwise, qword by qword is passed, using rax and rdx for integer/pointer qwords, and xmm0 and xmm1 for floating point ones
\end{itemize}
\item floating point values \textgreater\ 64 bits are returned via st0 and st1
\end{itemize}


\paragraph{Stack layout}

Stack frame is always 16-byte aligned. A 128 byte large zone beyond the
location pointed to by the stack pointer is referred to as "red zone",
considered to be reserved and not be modified by signal or interrupt handlers
(useful for temporary data not needed to be preserved across calls, and for
optimizations for leaf functions).
% verified/amended: TP nov 2019 (see also doc/disas_examples/x64.sysv.disas)
Stack directly after function prolog:\\

\begin{figure}[h]
\begin{tabular}{5|3|1 1}
                             & \vdots         &                                &                              \\
\hhline{~=~~}
register save area           & \hspace{4cm}   &                                & \mrrbrace{6}{caller's frame} \\
\hhline{~-~~}
local data (with padding)    &                &                                &                              \\
\hhline{~-~~}
\mrlbrace{3}{parameter area} & arg n-1        & \mrrbrace{3}{stack parameters} &                              \\
                             & \ldots         &                                &                              \\
                             & arg 6          &                                &                              \\
\hhline{~-~~}
                             & return address &                                &                              \\
\hhline{~=~~}
register save area           &                &                                & \mrrbrace{4}{current frame}  \\
\hhline{~-~~}
local data                   &                &                                &                              \\
\hhline{~-~~}
parameter area               &                &                                &                              \\
\hhline{~-~~}
                             & \vdots         &                                &                              \\
\end{tabular}
\caption{Stack layout on x64 System V (Linux/*BSD)}
\end{figure}


\clearpage

\subsubsection{System V syscalls}

\paragraph{Parameter passing}

\begin{itemize}
\item syscall is issued via the {\em syscall} instruction
\item kernel destroys registers rcx and r11
\item syscall number is set in rax
\item params are passed in the following registers in this order: rdi, rsi, rdx, rcx, r8, r9
\item no stack in use, meaning syscalls are in theory limited to six arguments
\item register rax holds the return value (values in between -4095 and -1 indicate errors)
\end{itemize}