1 files changed, 47 insertions, 0 deletions
diff --git a/PGU/CHAP9/count-chars.s b/PGU/CHAP9/count-chars.s
new file mode 100644
index 0000000..7e12791
--- /dev/null
+++ b/PGU/CHAP9/count-chars.s
@@ -0,0 +1,47 @@
+# Count the characters in a string (until a null byte is reached)
+# It's supposed to behave similarly with strlen()
+#
+# Returns the count in %rax
+#
+# - %rcx: Char count
+# - %rdx: current Char address
+# - %al: current char
+#
+
+.type count_chars, @function
+.globl count_chars
+
+# We receive the string addr in the stack.
+# Remember %rsp + 8 contains the return value
+.equ ST_STRING_ADDRESS, 16
+
+count_chars:
+	pushq %rbp
+	movq %rsp, %rbp
+
+	#initialize counter
+	movq $0, %rcx
+
+	# Start address of string:
+	movq ST_STRING_ADDRESS(%rbp), %rdx
+
+count_loop_begin:
+
+	# Grab char
+	movb (%rdx), %al
+
+	cmpb $0, %al
+	je count_loop_end
+
+	# We are not done yet...
+	incq %rcx
+	incq %rdx
+	jmp count_loop_begin
+
+count_loop_end:
+
+	movq %rcx, %rax
+
+	movq %rbp, %rsp
+	popq %rbp
+	ret