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/*

 * The SYSENTER syscall mechanism can be used for syscalls with

 * four or fewer arguments. To pass these four arguments, we

 * use four registers: EDX, ECX, EBX, ESI. The syscall number

 * is passed in EAX. We use EDI to remember the return address

 * and EBP to remember the stack. The INT-based syscall mechanism

 * can actually handle six arguments plus the syscall number

 * entirely in registers.

 */

.global sysenter_handler

sysenter_handler:

	pushl %ebp	# remember user stack

	pushl %edi	# remember return user address

	pushl %gs	# remember TLS

	pushl %eax	# syscall number

	subl $8, %esp	# unused sixth and fifth argument

	pushl %esi	# fourth argument

	pushl %ebx	# third argument

	pushl %ecx	# second argument

	pushl %edx	# first argument

	movw $16, %ax

	movw %ax, %ds

	movw %ax, %es

	cld

	call syscall_handler

	addl $28, %esp	# remove arguments from stack

	pop %gs		# restore TLS

	pop %edx	# prepare return EIP for SYSEXIT

	pop %ecx	# prepare userspace ESP for SYSEXIT

	sysexit		# return to userspace