Add x86-assembly

x86-assembly/src/stack/stack.s

@@ -0,0 +1,48 @@
+.text
+	.global _start
+
+_start:
+	# Push some 64-bit integers onto the stack.
+	push $3
+	push $2
+	push $1
+
+	# Let's print "Hey\n", using the stack.
+	# man ascii
+	# |---------+----+-----+-----+----|
+	# | ASCII   |  H |   e |   y | \n |
+	# |---------+----+-----+-----+----|
+	# | Decimal | 72 | 101 | 121 | 10 |
+	# |---------+----+-----+-----+----|
+	# | Hex     | 48 |  65 |  79 | 0A |
+	# |---------+----+-----+-----+----|
+
+	# x86 is little-endian: it stores the least significant byte
+	# of a multi-byte value at the lowest memory address.
+	# Since the write system call reads memory from low to high address,
+	# we put the first character ('H' = 0x48) in the least significant byte.
+	#    <-- high           low -->
+	#               \n y e H
+	push $0x000000000A796548
+
+	# %rsp points to the top of the stack (the lowest address of our push,
+	# where 'H' is stored).
+	# We can pass this address to the kernel.
+	# write will start at 'H', and work up to '\n'.
+	mov $1, %rax     # write
+	mov $1, %rdi     # stdout
+	lea (%rsp), %rsi # address of buffer
+	# Try changing the number of bytes to 9, 17, 25.
+	# Then call the program with strace.
+	# You'll be able to see the previous numbers we pushed onto the stack.
+	mov $4, %rdx     # number of bytes
+	syscall
+
+	# Remember, the stack is LIFO.
+	pop %rdi # pop the "Hey\n" string
+	pop %rdi # pop 1
+	pop %rdi # pop 2
+
+	# We'll use the last popped value in %rdi as our exit code.
+	mov $231, %rax
+        syscall