This is xnu-8019. See this file in:
/*
 * Copyright (c) 1999-2010 Apple Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 * 
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 * 
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 * 
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 * 
 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
 */
/* Copyright (c) 1992 NeXT Computer, Inc.  All rights reserved.
 *
 *	File:	libc/ppc/sys/fork.s
 *
 * HISTORY
 * 18-Nov-92  Ben Fathi (benf@next.com)
 *	Created from M88K sources
 *
 * 11-Jan-92  Peter King (king@next.com)
 *	Created from M68K sources
 */
 
/*
 * All of the asm stubs in this file have been adjusted so the pre/post
 * fork handlers and dyld fixup are done in C inside Libc. As such, Libc
 * expects the __fork asm to fix up the return code to be -1, 0 or pid
 * and errno if needed.
 */

#include "SYS.h"

#if defined(__i386__)

LEAF(___fork, 0)
	subl  $28, %esp   // Align the stack, with 16 bytes of extra padding that we'll need

	movl 	$ SYS_fork,%eax; 	// code for fork -> eax
	UNIX_SYSCALL_TRAP		// do the system call
	jnc	L1			// jump if CF==0

	CALL_EXTERN(tramp_cerror)
	movl	$-1,%eax
	addl	$28, %esp   // restore the stack
	ret
	
L1:
	orl	%edx,%edx	// CF=OF=0,  ZF set if zero result	
	jz	L2		// parent, since r1 == 0 in parent, 1 in child
	
	//child here...
	xorl	%eax,%eax	// zero eax
	REG_TO_EXTERN(%eax, __current_pid);
L2:
	addl	$28, %esp   // restore the stack
	// parent ends up here skipping child portion
	ret

#elif defined(__x86_64__)

LEAF(___fork, 0)
	subq  $24, %rsp   // Align the stack, plus room for local storage

	movl 	$ SYSCALL_CONSTRUCT_UNIX(SYS_fork),%eax; // code for fork -> rax
	UNIX_SYSCALL_TRAP		// do the system call
	jnc	L1			// jump if CF==0

	movq	%rax, %rdi
	CALL_EXTERN(_cerror)
	movq	$-1, %rax
	addq	$24, %rsp   // restore the stack
	ret
	
L1:
	orl	%edx,%edx	// CF=OF=0,  ZF set if zero result	
	jz	L2		// parent, since r1 == 0 in parent, 1 in child
	
	//child here...
	xorq	%rax, %rax
	PICIFY(__current_pid)
	movl	%eax,(%r11)
L2:
	// parent ends up here skipping child portion
	addq	$24, %rsp   // restore the stack
	ret
	UNWIND_EPILOGUE

#elif defined(__arm__)
	
MI_ENTRY_POINT(___fork)
	stmfd	sp!, {r4, r7, lr}
	add	r7, sp, #4

	mov	r1, #1					// prime results
	mov	r12, #SYS_fork
	swi	#SWI_SYSCALL				// make the syscall
	bcs	Lbotch					// error?

	cmp	r1, #0					// parent (r1=0) or child(r1=1)
	beq	Lparent

	//child here...
	MI_GET_ADDRESS(r3, __current_pid)
	mov	r0, #0
	str	r0, [r3]		// clear cached pid in child
	ldmfd   sp!, {r4, r7, pc}

Lbotch:
	MI_CALL_EXTERNAL(_cerror)			// jump here on error
	mov	r0,#-1					// set the error
	// fall thru
Lparent:	
	ldmfd   sp!, {r4, r7, pc}			// pop and return

#elif defined(__arm64__)

#include <mach/arm64/asm.h>
	
MI_ENTRY_POINT(___fork)
	ARM64_STACK_PROLOG
	PUSH_FRAME
	// ARM moves a 1 in to r1 here, but I can't see why.
	mov		x16, #SYS_fork				// Syscall code
	svc		#SWI_SYSCALL				// Trap to kernel
	b.cs	Lbotch						// Carry bit indicates failure
	cbz		x1, Lparent					// x1 == 0 indicates that we are the parent

	// Child
	MI_GET_ADDRESS(x9, __current_pid)	// Get address of cached "current pid"
	mov		w0, #0	
	str		w0, [x9]					// Clear cached current pid				
	POP_FRAME							// And done
	ARM64_STACK_EPILOG

Lbotch:
	MI_CALL_EXTERNAL(_cerror)			// Handle error
	mov		w0, #-1						// Return value is -1
Lparent:
	POP_FRAME							// Return
	ARM64_STACK_EPILOG

#else
#error Unsupported architecture
#endif