ACPI Machine Language (AML)

This is implemented in aml.

AML is a language used to describe the hardware of the system, and it is used by the ACPI to configure the system, check spec here (this is what's used to implement this parser).

This is an AML parser that is able to parse the code inside the DSDT and SSDT tables.

Currently, we do not use the parsed code, as we need to build an interpreter for it, so that we can emulate executing it and get the data we need.

Why this is needed?

There are some details of the hardware that is hidden in AML, such as:

• Finding out the sleep commands for the system, i.e. which registers to write to, and what values to write.
• Finding out interrupts configuration, for devices that share the same interrupt line.

Generally, most OSes will use acpica which is a tool that can parse and execute AML code.

Some annoying parts

Just to share here, since this is a documentation and all.

Generally, parsing AML is not that hard, its a binary format, we can read the opcode and know what term type we need.

The issue is one thing, method calls.

Method calls are encoded as such, NameString followed by N arguments as TermArg (which is a term type). The issue is:

• The expression itself doesn't provide the number of arguments.
• The method call can happen before the method is defined (method definition provide number of arguments).
• The method can be external (I think).

So, we have one choice (acpica also does it) and that is to "guess" the number of arguments. And this is very error prone, as a Term can be also considered an expression, like NameString can be considered as a variable if it is not a method call. very messy :'(

The current implementation I have I think is quite good, and I got to fix all the bugs I found, but of course there could be more.

This seems kinda "complaining" XD, but I just wanted to share the experience.