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The System system call (''syscall'') mechanism is the mechanism used by an applications to access kernel services. The userland (application /non-kernel) program loads certain registers designating the system service (syscall) desired and the arguments to that syscall, then invokes a software [[Computer_Architecture#Interrupts_and_Exceptions|interrupt]] or exception request which transfers control to the operating system kernel. Note that switching to kernel mode requires a change of [[Computer_Architecture#Execution_State,_Priviledge_State,_Rings,_or_Privilege_Level|processor mode]] (sometimes call the privilege level or exception level), because the kernel has access to instructions, memory structures, memory areas, and devices which userland programs can't access . The syscall numbers may vary between architectures, and the registers as well as the method used to invoke syscalls are architecture-specific. The processor modes and activity involved in a userland-to-kernel servicesswitch also vary by architecture.  High-level languages will wrap the syscall interface in basic wrappers or more advanced mechanisms. For example, in C, the ''write'' syscall can be accessed through the generic <code>syscall</code> wrapper, the <code>write()</code> wrapper, or through more complex functions such as <code>printf()</code>. [[Assembly Language|Assembly language]] programs will often access syscalls directly.

Every platform has a different syscall mechanism. Each mechanism is documented in the ABI standards for that platform, and a summary of various platforms is contained in the manpage for syscall(2) (run the command <code>man 2 syscall</code> to see this). To invoke a syscallin your own code, load the syscall number and arguments into the appropriate registers, and then invoke the platformarchitecture-specific syscall mechanism., using the details below:

* Arguments Integer and pointer arguments are placed in edi, esi, edx, ecx with spill over to the stack(refer to ABI documentation).

* Arguments Integer and pointer arguments are placed in rdi, rsi, rdx, rcx, r8d, r9d with spill over to the stack(refer to ABI documentation).

* Arguments Integer and pointer arguments are placed in r0-r7with spill over to the stack (refer to Procedure Call and ABI documentation).

== Syscall informationnames, numbers, and arguments == === Syscall names === A list of syscalls can be found in the manpage for <code>syscalls(2)</code> (note: this is different from the manpage for <code>syscall(2)</code> which is a generic syscall wrapper). View this with the command <code>man 2 syscalls</code> Documentation for the C wrappers for most syscalls can be found in the manpage for that wrapper, usually in section 2 of the manual (e.g., <code>write(2)</code>, which is accessed using the command <code>man 2 write</code>).

The details of each syscall's arguments and return value are listed in <nowiki><unistd.h></nowiki> (<code>/usr/include/unistd.h</code>) in a combination of comments and C extern declarations.For example, for the '''write''' syscall:

So this This documents that the ''write'' syscall takes three arguments: the file descriptor, a pointer to the message buffer, and the message size, and returns the number of bytes written or -1 if there was an error. === Documentation === A list of syscalls can be found in the manpage for <code>syscalls(2)</code> (note: this is different from the manpage for <code>syscall(2)</code> which is a generic syscall wrapper). Documentation for the C wrappers for most syscalls can be found in the manpage for that wrapper, usually in section 2 of the manual (e.g., <code>write(2)</code>, which is accessed using the command <code>man 2 write</code>).

== Using the syscall number macro definitions in asm code ==

The syscall macros can be used in a C an assembly program if that program is processed by the C preprocessor (cpp). The extension <code>,.S</code> (uppercase instead of lowercase) is used to designate an assembler source file that must be processed by cpp.

#include <asm/unistd.h>