From d98f46ce647846b0aa30b2e16a30fd4e152a1bf5 Mon Sep 17 00:00:00 2001
From: Carlos Maiolino <cem@kernel.org>
Date: Thu, 10 Jul 2025 22:55:07 +0200
Subject: Add new code

Signed-off-by: Carlos Maiolino <cem@toxiclabs.cc>
---
 PGU/OLD/chapter3/exit.s | 70 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)
 create mode 100644 PGU/OLD/chapter3/exit.s

(limited to 'PGU/OLD/chapter3/exit.s')

diff --git a/PGU/OLD/chapter3/exit.s b/PGU/OLD/chapter3/exit.s
new file mode 100644
index 0000000..632ebed
--- /dev/null
+++ b/PGU/OLD/chapter3/exit.s
@@ -0,0 +1,70 @@
+# A useless program, which the only thing this shit does is call exit syscall
+# The exit status code can be read using
+
+# `echo $?` on shell, which will show the exit status of the last program run.
+
+
+# %eax holds the system call number
+# %ebx holds the return status
+
+
+# Assembler directives (or pseudo-operations).
+#
+# Everything starting with a period, is an instruction to the assembler itself.
+# They are handled by the assembler program, not actually executed by the
+# computer.
+
+# .section command breaks the program down into sections.
+
+# .data section lists any memory storage which will be needed for data
+.section .data
+
+# .text section lists the program instructions
+.section .text
+
+# Instructs the assembler that _start is "important to remember".
+# _start is a symbol, which is going to be replaced by something else either
+# during assembly or linking
+
+# Symbols are generally used to mark locations of programs or data, so you can
+# refer to them by name instead of by location number
+
+# .globl means that the assembler should not discard this symbol after assembly,
+# because the linker will need it.
+
+# _start is a special symbol that always needs to be marked with .globl, because
+# it marks the location of the start of the program.
+
+# If the program is not marked this way, when the computer loads the program, it
+# won't know where to begin running the program.
+
+.globl _start
+
+# Defines the value of _start label. Label == a symbol followed by a colon.
+# Labels defines a symbol's value.
+
+# Labels tell the assembler to make the symbol's value be wherever the next
+# instruction or data element will be.
+
+_start:
+
+# different from the book, I'm using x86_64 instructions and its registers, so,
+# instead of using %eax and %ebx, I'm using %rax and %rbx directly. The movl
+# instruction needed to be changed to movq (move quad) to adapt to the 64-bit
+# registers
+# mov: (move) 16-bit
+# movl: (move long) 32-bit
+# movq: (move quad) 64-bit
+
+#ins $(val), (dest) == immediate mode
+movq $1, %rax		# Move value 1 into %eax register
+			# %eax will hold the syscall number
+			# by the linux's calling convention
+
+movq $100, %rbx		# Move value 0 into %ebx register
+			# %ebx will hold the exit() syscall
+			# argument.
+
+int $0x80		# Interrupt 0x80 will wake up the kernel
+			# execute the syscall loaded into %eax
+
-- 
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