You need to add 0x2000000 to the call number using a syscalls.master file. I'm using the XNU bds/kern/syscalls.master file. Here's a function in the syscalls.master file that I'm going to call:
4 AUE_NULL ALL { user_ssize_t write(int fd, user_addr_t cbuf, user_size_t nbyte); }
In terms of which registers to pass arguments to, it's the same as 64-bit Linux. Arguments are passed through the rdi, rsi, rdx, r10, r8 and r9 registers, respectively. The write function takes three arguments, which are described in the following assembly:
mov rax, 0x2000004 ; sys_write call identifier
mov rdi, 1 ; STDOUT file descriptor
mov rsi, myMessage ; buffer to print
mov rdx, myMessageLen ; length of buffer
syscall ; make the system call
