Add new code

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

1 files changed, 90 insertions, 0 deletions

diff --git a/PGU/OLD/chapter4/power.s b/PGU/OLD/chapter4/power.s
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+# Simple example of functions in ASM, following System V ABI
+# and C calling convention
+
+# Everything is stored in registers, so, we have nothing in data section
+.section .data
+
+.section .text
+
+.globl _start
+
+_start:
+	pushq $3		# Second argument
+	pushq $2		# First argument
+	call power		# call function power()
+
+	addq $16, %rsp		# Remove arguments from the stack
+				# Could have popped'em, but we don't
+				# need them anymore, so, just move
+				# the stack pointer back
+
+	pushq %rax		# Save return value of power()
+
+# Call power () again
+	pushq $2		# Push second argument
+	pushq $5		# Push first argument
+	call power
+
+	addq $16, %rsp		# Move stack pointer back
+
+	popq %rbx		# Second answer is already in %eax
+				# (returned from power), first answer
+				# was pushed into the stack before, so,
+				# just pop it into %ebx.
+
+	addq %rax, %rbx		# Sum up both results (and %ebx is already the
+				# exit()'s return value
+
+# Call exit()
+
+	movq $1, %rax
+	int $0x80
+
+
+
+
+####################
+#		   #
+# Function power() #
+#		   #
+####################
+
+# Remember the return address (where the program should keep executing after
+# function return), is also pushed into the stack by the 'call' command.
+
+# Remember, in x86_64 architecture, the Stack grows 'downwards', so, every time
+# we want to look back in the stack, we need to add to the current stack
+# pointer, other than subtract.
+
+# Ex:
+# 56	<- stack 'bottom'
+# 48	pushq
+# 40	pushq
+# 32	<- %rsp (top of the stack)
+
+#Tread symbol 'power' as a function
+.type power,@function
+
+power:
+	pushq %rbp
+	movq %rsp, %rbp
+	movq 16(%rbp), %rcx		# Holds the Base number. 16 because:
+					# Current %rsp (pushq %rbp) plus the
+					# implicit return address pushed by
+					# the call command
+
+	movq 24(%rbp), %rbx		# Holds the power (pushed first before
+					# function call
+	movq %rcx, %rax
+
+loop:
+	cmpq $1, %rbx
+	je return
+	imul %rcx, %rax			# The result is already stored in %rax
+	decq %rbx
+	jmp loop
+
+return:
+	movq %rbp, %rsp
+	popq %rbp
+	ret