usr/src/libc/sys/syscall.s

# @(#)syscall.s 4.1 (Berkeley) 12/21/80
        .globl  _syscall
        .globl  cerror
_syscall:
        .word   0x0000
        movl    4(ap),r0                # syscall number
        subl3   $1,(ap)+,(ap)   # one fewer arguments
        chmk    r0                              # do it
        bcs             L1
        ret
L1:
        jmp             cerror