Add new code

Signed-off-by: Carlos Maiolino <[email protected]>

1 files changed, 73 insertions, 0 deletions

diff --git a/PGU/OLD/chapter4/factorial.s b/PGU/OLD/chapter4/factorial.s
new file mode 100644
index 0000000..749f57e
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+++ b/PGU/OLD/chapter4/factorial.s
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+##################################################################
+# Calculate the factorial of a number, using recursive functions #
+##################################################################
+
+
+#
+# %rbx holds the number to factor
+#
+.section .data
+
+.section .text
+
+.globl _start
+_start:
+	pushq $5
+	call factorial2
+	addq $8, %rsp	# Turn back the stack pointer
+			# Not really needed here, but good practice
+	movq %rax, %rbx
+	movq $1, %rax
+	int $0x80
+
+.type factorial, @function
+
+# This function is garbage, can be much more clean than this shit.
+
+factorial:
+	pushq %rbp		# Regular initial function stuff
+	movq %rsp, %rbp		# Save %rbp and reset it to %rsp
+
+	movq 16(%rbp), %rbx	# Get the argument passed into %rbx
+	movq %rbx, %rcx		# Copy it to %rcx and decrement it
+	decq %rcx		# here. %rcx will be the next element in the
+				# factorial.
+				#
+	cmpq $1, %rcx		# Compare it to 1 and exit if true, we are at
+	je exit_factorial	# the end of the factorial
+
+	pushq %rbx		# If not one, save the original argument in the
+	pushq %rcx		# stack and push the next value as argument to
+	call factorial		# the recursive call
+	popq %rcx		# pop value to restore stack pointer
+	popq %rbx		# restore %rbx and multiply it
+	imul %rax, %rbx		# by the result
+
+exit_factorial:
+	movq %rbx, %rax		# Move the result to %rax
+	movq %rbp, %rsp		# And do standard clean up before
+	popq %rbp		# returning.
+	ret
+
+# This is a clean version after extra thought on this
+
+.type factorial2, @function
+factorial2:
+	pushq %rbp		# Basic function setup, save %rbp and
+	movq %rsp, %rbp		# setup it to current %rsp
+
+	movq 16(%rbp), %rax	# Get the argument passed into
+	cmpq $1, %rax		#
+	je factorial2_end	# We are at the last element in the factorial
+				# Just return it
+	decq %rax
+	pushq %rax
+	call factorial2
+	movq 16(%rbp), %rbx
+	imulq %rbx, %rax
+
+factorial2_end:
+	movq %rbp, %rsp
+	popq %rbp
+	ret
+