UNIX System Call Numbers

For AArch64/ARM64 Assembly tutorials see here

Usage

.global _start
.align 4

_start:     your_code_here
            ...
            mov X16, syscall_number     ; Load syscall number
            svc #0x80                   ; Example svc on macOS
            ...                         ; Other ops
            mov X16, 0                  ; Exit
            svc #0x80                   ; Call kernel

syscall_number: .quad your_call_num_here

Register	Explanation
X0	First argument register and also used for the return value of a function. When calling a function, the first parameter is passed in X0.
X1	Second argument register. Used to pass the second parameter to a function.
X2	Third argument register. Used to pass the third parameter to a function.
X3	Fourth argument register. Used to pass the fourth parameter to a function.
X4	Fifth argument register. Used to pass the fifth parameter to a function.
X5	Sixth argument register. Used to pass the sixth parameter to a function.
X6	Seventh argument register. Used to pass the seventh parameter to a function.
X7	Eighth argument register. Used to pass the eighth parameter to a function.
X8	Often used as a special-purpose register for system calls. In many ABIs, it holds the system call number when making a syscall.
X9	Temporary register. Caller-saved, so its value is not preserved across function calls. Used for intermediate calculations or temporary storage.
X10	Temporary register. Similar to X9, it is used for temporary values and is caller-saved.
X11	Temporary register. Another general-purpose register for temporary data or calculations.
X12	Temporary register. Caller-saved and used for intermediate calculations.
X13	Temporary register. Caller-saved and used for temporary or intermediate data.
X14	Temporary register. Another caller-saved register for general-purpose use.
X15	Temporary register. Caller-saved, and typically used for temporary calculations or data storage.
X16	Often reserved as an intra-procedure call (IPC) scratch register. It can also be used for special purposes like veneers or branching by the linker.
X17	Another IPC scratch register. Like X16, it is used for linker-related tasks or branch-to-register operations.
X18	Platform register. Its usage depends on the operating system. On macOS, it may be reserved for system purposes and should not be modified directly.
X19	Callee-saved register. Preserved across function calls, making it suitable for storing variables that must survive after calling other functions.
X20	Callee-saved register. Used for local variables or data that must be preserved across function calls.
X21	Callee-saved register. Preserved across function calls, often used for storing persistent values.
X22	Callee-saved register. Used for local or long-lived variables within a function.
X23	Callee-saved register. Preserved across function calls and used for persistent data.
X24	Callee-saved register. Used for storing variables that should not be overwritten during function calls.
X25	Callee-saved register. Typically used for storing persistent data within a function.
X26	Callee-saved register. Used for long-term data storage within a function, preserved across calls.
X27	Callee-saved register. Similar to other callee-saved registers, used for persistent or local variables.
X28	Callee-saved register. Another general-purpose register that retains its value across function calls.
X29	Frame Pointer (FP). Points to the current stack frame, helping with stack navigation and function call debugging.
X30	Link Register (LR). Stores the return address after a bl (branch with link) instruction. It tells the CPU where to return after a subroutine is complete.