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diff --git a/PGU/CHAP4/factorial.s b/PGU/CHAP4/factorial.s
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+# Recursion example - calculates the factorial of a number
+
+# Everything goes in the stack, no data section needed
+.section .data
+
+.section .text
+
+.globl _start
+
+# This is pointless unless we are exporting this function to other programs or
+# files. It's just here for completeness.
+.global factorial
+
+_start:
+	pushq $4	# Push argument
+	call factorial	# call command pushes the next instruction's address on
+			# the stack, when we return to it, it will pop the
+			# address off of the stack and point the Instruction
+			# pointer to it
+
+	addq $8, %rsp	# Cleanup stack - Remove the argument from it
+			# Remember by cleanup, we just move the stackpointer
+			# back a word (8 bytes in x86_64) times the number of
+			# arguments pushed in the stack.
+
+	movq %rax, %rdi # Move result into exit() argument register %rdi
+			# Return values are always stored in %rax...
+
+	movq $60, %rax	# Setup %rax to exit() syscall number
+	syscall		# Invoke kernel's syscall
+
+
+# Tell linker this is a function
+# Export factorial as a function
+.type factorial, @function
+
+factorial:
+	# Create the stack frame for this function...
+	pushq %rbp		# Save %rbp in the stack
+	movq %rsp, %rbp		# And current stack pointer in %rbp
+
+	movq 16(%rbp), %rax	# Retrieve argument from stack, remember, we're
+				# dealing with quad words, %rbp is at the
+				# beginning of the function's stack frame,
+				# 8(%rbp) is the function's return address setup
+				# by call command and
+				# 16(%rbp) is our argument pushed in the stack
+				# by the function's caller
+
+	cmpq $1, %rax		# If we are at 1, nothing to be done...
+	je end_factorial
+
+	decq %rax		# Otherwise we decrease our current operand
+	pushq %rax		# and send it again to factorial function
+	call factorial		# here...
+
+	movq 16(%rbp), %r8	# Retrieve our current argument again
+	imulq %r8, %rax		# and multiply it by the result of the factorial
+				# below (num factorial = num * factorial of
+				# previous num, ex. 4! == 4 * 3!)
+
+end_factorial:
+
+	## Clean up function's stack frame ##
+	movq %rbp, %rsp		# We are done with this factorial, restore
+	popq %rbp		# %rsp and %rbp and
+	ret			# return to the caller