Source: man syscall
Every architecture has its own way of invoking and passing arguments to the kernel. The details for various architectures are listed in the two tables below.
The first table lists the instruction used to transition to kernel
mode, (which might not be the fastest or best way to transition to
the kernel, so you might have to refer to vdso(7)), the register used
to indicate the system call number, and the register used to return
the system call result.
| arch/ABI | instruction | syscall # | retval | Notes |
|---|---|---|---|---|
| arm/OABI | swi NR | - | a1 | NR is syscall # |
| arm/EABI | swi 0x0 | r7 | r0 | |
| arm64 | svc #0 | x8 | x0 | |
| blackfin | excpt 0x0 | P0 | R0 | |
| i386 | int $0x80 | eax | eax | |
| ia64 | break 0x100000 | r15 | r8 | See below |
| mips | syscall | v0 | v0 | See below |
| parisc | ble 0x100(%sr2, %r0) | r20 | r28 | |
| s390 | svc 0 | r1 | r2 | See below |
| s390x | svc 0 | r1 | r2 | See below |
| sparc/32 | t 0x10 | g1 | o0 | |
| sparc/64 | t 0x6d | g1 | o0 | |
| x86_64 | syscall | rax | rax | See below |
| x32 | syscall | rax | rax | See below |
For s390 and s390x, NR (the system call number) may be passed directly with "svc NR" if it is less than 256.
The x32 ABI uses the same instruction as the x86_64 ABI and is used on the same processors. To differentiate between them, the bit mask __X32_SYSCALL_BIT is bitwise-ORed into the system call number for system calls under the x32 ABI.
On a few architectures, a register is used to indicate simple boolean failure of the system call: ia64 uses r10 for this purpose, and mips uses a3.
The second table shows the registers used to pass the system call arguments.
| arch/ABI | arg1 | arg2 | arg3 | arg4 | arg5 | arg6 | arg7 | Notes |
|---|---|---|---|---|---|---|---|---|
| arm/OABI | a1 | a2 | a3 | a4 | v1 | v2 | v3 | |
| arm/EABI | r0 | r1 | r2 | r3 | r4 | r5 | r6 | |
| arm64 | x0 | x1 | x2 | x3 | x4 | x5 | - | |
| blackfin | R0 | R1 | R2 | R3 | R4 | R5 | - | |
| i386 | ebx | ecx | edx | esi | edi | ebp | - | |
| ia64 | out0 | out1 | out2 | out3 | out4 | out5 | - | |
| mips/o32 | a0 | a1 | a2 | a3 | - | - | - | See below |
| mips/n32,64 | a0 | a1 | a2 | a3 | a4 | a5 | - | |
| parisc | r26 | r25 | r24 | r23 | r22 | r21 | - | |
| s390 | r2 | r3 | r4 | r5 | r6 | r7 | - | |
| s390x | r2 | r3 | r4 | r5 | r6 | r7 | - | |
| sparc/32 | o0 | o1 | o2 | o3 | o4 | o5 | - | |
| sparc/64 | o0 | o1 | o2 | o3 | o4 | o5 | - | |
| x86_64 | rdi | rsi | rdx | r10 | r8 | r9 | - | |
| x32 | rdi | rsi | rdx | r10 | r8 | r9 | - |
The mips/o32 system call convention passes arguments 5 through 8 on the user stack.
Note that these tables don't cover the entire calling convention — some architectures may indiscriminately clobber other registers not listed here.