Kazinsal posted:

the VAX ISA is fuckin wild. it takes the idea of a complex instruction set computer and adds a dash of purestrain late 70s "dude this blow is amazing we need to fly to colombia for the weekend more often". it's a 16-register machine with a similar register layout to ARM, a base ISA that's more or less the PDP-11 ISA extended to 32 bits, a four-ring protection level system, four one-gigabyte segments, and a whole bunch of insane extra instructions for making assembly programming "easier". want instructions to implement doubly-linked ring buffer operations with optional multiprocessor-safe locking in a single machine cycle? VAX has those. want two- and extended three-operand versions of the C standard library string functions as microcoded instructions? VAX has that. need an instruction to do CRC16 and/or CRC32 on an arbitrary length string of bytes? VAX has one. have you ever wanted an instruction that's a whole implementation of a stream editor like sed? look no further than VAX! and if that's not enough and you want to emulate your own instruction set extensions in software, VAX has a mechanism for that.

later versions of the VAX ISA describe an alternate larger-exponent-smaller-mantissa 64-bit floating point format and a super large 128-bit floating point format, and extend all the instructions to allow use of those anywhere the normal 32-bit and 64-bit floats are usable. there's also a full set of instructions to convert between every type of float to each other, and to convert different sized integers to different sized floats and back, with different rounding/truncation options, and each option is its own opcode.

oh and of course since it's a full 32-bit minicomputer it's got all the nice fancy memory management features you'd want out of something that will have dozens of simultaneous users and hundreds of processes running like paging and per-page privilege level checking and segment limits so the OS can do things like auto-grow process stacks. and separate machine-level registers for setting the base and limit of the system segment on a context switch so you could implement syscall trampolines if you wanted to