13.1: Calling System Services

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A system service call is logically similar to calling a function, where the function code is located within the operating system. The function may require privileges to operate which is why control must be transferred to the operating system.

When calling system services, arguments are placed in the standard argument registers. System services do not typically use stack-based arguments. This limits the arguments of a system services to six (6), which does not present a significant limitation.

To call a system service, the first step is to determine which system service is desired. There are many system services (see Appendix C). The general process is that the system service call code is placed in the rax register. The call code is a number that has been assigned for the specific system service being requested. These are assigned as part of the operating system and cannot be changed by application programs. To simplify the process, this text will define a very small subset of system service call codes to a set of constants. For this text, and the associated examples, the subset of system call code constants are defined and shown in the source file to help provide complete clarity for new assembly language programmers. For more experienced programmers, typically developing larger or more complex programs, a complete list of constants is in a file and included into the source file.

If any are needed, the arguments for system services are placed in the rdi, rsi, rdx, r10, r8, and r9 registers (in that order). The following table shows the argument locations which are consistent with the standard calling convention.

Register	Usage
rax	Call code (see table)
rdi	1st argument (if needed)
rsi	2nd argument (if needed)
rdx	3rd argument (if needed)
r10	4th argument (if needed)
r8	5th argument (if needed)
r9	6th argument (if needed)

This is very similar to the standard calling convention for function calls, however the 4th argument, if needed, uses the r10 register.

Each system call will use a different number of arguments (from none up to 6). However, the system service call code is always required.

After the call code and any arguments are set, the syscall instruction is executed. The syscall instruction will pause the current process and transfer control to the operating system which will attempt to perform the service specified in the rax register. When the system service returns, the process will be resumed.