27 lines
1.2 KiB
Markdown
27 lines
1.2 KiB
Markdown
# Parsing ACPI information
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> - [ACPI Specification](https://uefi.org/sites/default/files/resources/ACPI_6_3_final_Jan30.pdf)
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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).
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The bootloader will find the Root System Descriptor Pointer and provide it in a boot tag. Then the data just needs to be parsed.
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The RSDP contains a pointer to the Root System Descriptor Table which contains pointers to more System Descriptor Tables.
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The important fields for parsing a System Descript Tables are as follows:
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```c
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struct sdt {
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char signature[4];
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uint32_t length;
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uint8_t _[28];
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uint32_t data[];
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}__attribute__((packed));
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```
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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.
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The MADT in turn contains a number of fields with information - among other things - about Processor Local APICs, I/O Apics and IRQ redirections.
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Those are things I may be interested in, and therefore store in some global data structures.
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## Relevant files
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- `src/kernel/cpu/acpi.c` |