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introduction.md

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+# Introduction
+
+There are things which are hard to do from regular user programs such as directly talking to hardware.
+
+Some operations can be done via system calls, but if you want flexibility and speed, using the kernel ring is fundamental
+
+However:
+
+- it would be very complicated to recompile the kernel and reboot every time you make some modification
+- the kernel would be huge if it were to support all possible hardware
+
+Modules overcome those two problems exactly because they can be loaded into the kernel *while it is running* and use symbols that the kernel chooses to export TODO which
+
+It then runs in the same address space as the kernel and with the same permissions as the kernel (basically do anything)
+
+Compiled modules are special object files that have a `.ko` extension instead of `.o` they also contain module specific metadata
+
+Device drivers (programs that enables the computer to talk to hardware) are one specific type of kernel modules
+
+Two devices can map to the same hardware!
+
+## Stable kernel interface
+
+Kernel modules can use any internal interface of the kernel, although some are more visible than others.
+
+But there is no stable kernel API for modules: if you don't add your driver to the kernel tree, it can break any time: <https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/stable_api_nonsense.txt?id=v4.0>
+
+## Configuration files
+
+If file it gets read, if dir, all files in dir get read:
+
+    sudo ls /etc/modprobe.d
+    sudo ls /etc/modprobe.conf
+
+Modules loaded at boot:
+
+    sudo cat /etc/modules
+
+## Hardware communication
+
+Talking to hardware always comes down to writing bytes in specific registers at a given memory addresses.
+
+Some processors implement a single address space for memory and other hardware devices, and others do not.
+
+However, since x86 is the most popular and it separates address spaces, every architecture must at least mimic this separation.
+
+On x86, the following specialized instructions exist for port IO:
+
+- `IN`: Read from a port
+- `OUT`: Write to a port
+- `INS/INSB`: Input string from port/Input byte string from port
+- `INS/INSW`: Input string from port/Input word string from port
+- `INS/INSD`: Input string from port/Input `doubleword` string from port
+- `OUTS/OUTSB`: Output string to port/Output byte string to port
+- `OUTS/OUTSW`: Output string to port/Output word string to port
+- `OUTS/OUTSD`: Output string to port/Output `doubleword` string to port
+
+However, you should avoid using those instructions directly in your device driver code since Linux functions abstract over multiple architectures (when possible) making your code more portable.
+
+Those instructions cannot be used from an user space program since the kernel prevents those from accessing hardware directly.
+
+The memory space for non-memory locations is called I/O ports or I/O space.
+
+To use a port, you must first reserve it. To see who reserved what:
+
+	sudo cat /proc/ioports