Modern CPUs always have a pile of registers to enable/disable features or configure things. These are often used to tweak low-level platform parameters or to hold "chicken bits", bits to "chicken out" and turn off a feature if it turns out to be buggy in silicon. It is extremely expensive to spin off a new chip with the feature fixed or disabled, so chip designers will always put in overrides that can be used in software to make stuff run without needing a new silicon revision. These registers are specific to a given CPU design, so they are not standardized, and OSes are not expected to use them except in platform-specific code.
Apple has a bunch of these in their custom CPU cores, and you can see some of them
here. Notice the names? ARM64_REG_HID0 and up. HID0 stands for
Hardware Implementation-Dependent Register 0, a naming convention that comes straight out of... PowerPC.
People who were around in the GameCube/Wii homebrew era might
recognize good old HID0, HID4, and friends. These are fully documented in the datasheet for the Wii's CPU, the
PPC 750CL (page 60 onwards). Apple marketed (almost identical) variants of this CPU as the G3. Fun fact, you can enable Nintendo's Paired Singles SIMD extension (developed for the GameCube's "Gekko") on some G3 Macs if you flip the secret HID bits that weren't documented for those models!
Other PowerPC cores also have HID registers, such as the
POWER9 (page 89), or the
Cell Broadband Engine (page 317, used in the PS3), or the
PPC 970FX a.k.a G5 used in other Macs (page 46). Of course, the bit definitions have nothing to do with the Apple Silicon registers, but it's nice to see Apple's silicon design team giving a shout-out to their PowerPC past!
