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[[Category:Assembly Language]]
The system call (''syscall'') mechanism is used by 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 switch also vary by architecture.
High-level languages 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.
== Syscall Mechanism ==
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 syscall in your own code, load the syscall number and arguments into the appropriate registers, and then invoke the architecture-specific syscall mechanism, using the details below:
* The syscall is invoked with <code>svc 0</code>
== 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>).
=== Syscall numbers ===
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.
The syscall macros can be used in 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.
