Processor instruction set architectures (ISAs) are typically specified using a mixture of prose and pseudocode. We present ongoing work on expressing such specifications rigorously and automatically translating them to interactive theorem prover definitions, making them amenable to mechanised proof. Our ISA descriptions are written in Sail—a custom ISA specification language designed to support idioms from various processor vendor’s pseudocode, with lightweight dependent typing for bitvectors, targeting a variety of use cases including sequential and concurrent ISA semantics. From Sail we aim to portably generate usable theorem prover definitions for multiple provers, including Isabelle, HOL4, and Coq. We are focusing on the full ARMv8. 3-A specification, CHERI-MIPS, and RISC-V, together with fragments of IBM POWER and x86.