Documentation

documentation/5 - acpi.md

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+# Parsing ACPI information
+
+> - [ACPI Specification](https://uefi.org/sites/default/files/resources/ACPI_6_3_final_Jan30.pdf)
+
+ACPI is a standard for controlling things like power and temperatures in computers. It's also useful for getting some information about the machine, such as number of processors - which is what Mittos will be using it for (at least at first).
+
+The bootloader will find the Root System Descriptor Pointer and provide it in a boot tag. Then the data just needs to be parsed.
+
+The RSDP contains a pointer to the Root System Descriptor Table which contains pointers to more System Descriptor Tables.
+
+The important fields for parsing a System Descript Tables are as follows:
+```c
+struct sdt {
+    char signature[4];
+    uint32_t length;
+    uint8_t _[28];
+    uint32_t data[];
+}__attribute__((packed));
+```
+
+So I step through the pointers in the RSDT data field until I find an SDT with signature "APIC". That should contain the Multiple APIC Description Table.
+
+The MADT in turn contains a number of fields with information - among other things - about Processor Local APICs, I/O Apics and IRQ redirections.
+Those are things I may be interested in, and therefore store in some global data structures.
+
+## Relevant files
+- `src/kernel/cpu/acpi.c`