docs/develop: More reorganization.

* There is now a 'busses' folder, and the extant USB/SDHCI/Bluetooth/etc.
   docs now live in it, instead of various other places.
 * kernel/ports is now kernel/arch, like it is in src/system.
   SPARC documentation is now in there, too.
 * VM files (these are rather outdated) are now in kernel/vm.
 * SCSI ASC info removed, this is easily available online and
   it doesn't seem to be very relevant.

docs/develop/kernel/arch/long double.md

@@ -0,0 +1,48 @@
+Notes on long double support
+============================
+
+The "long double" type is different on each architecture. Depending on the
+available hardware and ABI conventions, performance compromises, etc, there
+may be many implementations of it. Here is a summary for our convenience.
+
+128-bit IEEE
+------------
+
+Platforms: Sparc, ARM64, RISC-V
+
+This is the standard long double type from IEEE754. It has 1 sign bit,
+15 exponent bit, and 112 fractional part bits. It is the natural extension
+of the 64bit double.
+
+Sparc specifies this type in their ABI but no implementation actually has
+the instructions, they instead trigger a trap which would software emulate
+them. However, gcc short circuits this by default and calls C library
+support functions directly.
+
+64-bit IEEE
+-----------
+
+Platforms: ARM
+
+This is the same representation as plain "double". ARM uses this for simplicity.
+
+80-bit
+------
+
+Platform: x86, x86\_64, m68k
+
+This intermediate format is used by x86 CPUs internally. It may end up being
+faster than plain double there. It consists of a 64bit fractional part, 15
+exponent bits, and 1 sign bit. This is convenient because the fractional part
+is a relatively easy to handle 64bit number.
+
+m68k uses a similar format, but padded to 96 bits (the extra 16 bits are unused).
+
+double double
+-------------
+
+Platforms: PowerPC?
+
+This is also a 128bit type, but the representation is just two 64bit doubles.
+The value is the sum of the two halves. This format allows faster emulation
+than a "true" 128bit long double, and the precision is almost as good.