The Yhc runtime is a stack-based spineless G-Machine (See /Machine). This page details the instruction set that the machine uses.
A more up to date page is at http://www-users.cs.york.ac.uk/~ndm/yhc/bytecodes.html, which is automatially generated from the bytecode.xml file.
Instructions have 'variants', which are different specializations of the instruction: For example if the general instruction is PUSH n then
- PUSH_n is a specialized 1 byte instruction of PUSH n
- PUSH_P1 n is a 2 byte instruction of PUSH n
- PUSH_P2 n is a 3 byte instruction of PUSH n
Some instructions don't have three byte versions. Here 'PUSH n' refers to the two byte version, or the general 'PUSH n' instruction, depending on context.
The compiler will choose the smallest available bytecodes to encode any particular instruction.