Add August 2021 changes for acpi (#70)

Author: IsaacWoods

content/this-month/2021-08/index.md

@@ -6,6 +6,7 @@ date = 0000-01-01
 month = "August 2021"
 authors = [
     "phil-opp",
+    "IsaacWoods",
     # add yourself here
 ]
 +++
@@ -36,6 +37,38 @@ The `x86_64` crate provides various abstractions for `x86_64` systems, including
 
 In August, …
 
+### [`acpi`](https://github.com/rust-osdev/acpi)
+
+The `acpi` repository contains crates for parsing the ACPI tables – data structures that the firmware of modern computers use to relay information about the hardware to the OS.
+
+There was a fair amount of progress this month, including adding support for a major AML feature: buffer fields.
+These are objects that represent bit-level slices of buffer objects, allowing a section of a buffer to be read and
+written. This requires us to support multiple namespace objects operating on the same underlying data -
+currently this is handled simply with spinlocks, but we're looking to improve this situation in the future.
+With this, we grew support for the `DefCreateBitField`, `DefCreateByteField`, `DefCreateWordField`,
+`DefCreateDWordField`, `DefCreateQWordField`, and `DefCreateField` opcodes, as well as logic for reading and
+writing `Integer`s and `Buffer`s out of them. <span class="gray">(published as `aml v0.16.0`)</span>
+
+The `acpi` crate also saw improvement, and unfortunately some more breakage. We now support X2APIC entries
+appearing in the MADT, and so have changed the representation of Processor UIDs and Local APIC IDs in
+`acpi::platform::Processor` to `u32`, to support the wider IDs that X2APIC uses to support more processors. On
+older platforms that only support APIC, the upper bits will simply always be zeroed, and so these values can be
+cast down to `u8` when needed. Note that for complex legacy reasons, you cannot rely upon ACPI for detecting
+whether a platform has X2APIC support or not, and so whether the MADT actually contained X2APIC entries is not exposed
+by this interface - this is by design. <span class="gray">(published as `acpi v4.0.0`)</span>
+
+Other changes were:
+- [`aml`: the contents of the `namespace` module were made public](https://github.com/rust-osdev/acpi/pull/107).
+  Thanks to [@Andy-Python-Programmer](https://github.com/Andy-Python-Programmer) for this contribution!
+- [`aml`: store locals and method arguments in arrays](https://github.com/rust-osdev/acpi/commit/c1597aba3d39344834292637fb81e2f2971d6c04). If you're manually calling AML methods
+  that take arguments, you will need to update how you create them.
+- [`aml`: support the `DefFatal` opcode](https://github.com/rust-osdev/acpi/commit/514e55df07acbca93dfd4eb2db3cdd6fdea5aaf5). `DefFatal` will cause a panic by default, but this behaviour can be overridden by implementing `Handler::handle_fatal_error`.
+- [`aml`: implement the `DefWhile` opcode](https://github.com/rust-osdev/acpi/commit/06409b360ef30b3b08b56865f3ee380315751f14)
+- [`aml`: implement the `DefBreak` opcode](https://github.com/rust-osdev/acpi/commit/da5f5cec8096d2ebd5697212e282abbeaed6edb7)
+- [`aml`: implement the `DefContinue` opcode](https://github.com/rust-osdev/acpi/commit/ed0400092e18598c73ca6048fb96b2522237808d)
+- [`aml`: implement the `DefIncrement` and `DefDecrement` opcodes](https://github.com/rust-osdev/acpi/commit/b854d5491e48e5a4f332ff259ce185cb357261d0)
+- [`aml`: implement the `ToInteger` opcode](https://github.com/rust-osdev/acpi/commit/00a61d8b7471dae725283296f4ee9c0c20013156)
+
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