An implantable medical device (IMD) configured to provide stimulation therapy using an instruction set architecture (ISA) includes a main processor operating at a first frequency and a secondary processor operating at a second frequency lower than the first frequency. Example ISA may comprise assembly-language-like instructions that may be executed by the secondary processor for configuring one or more stimulation engines (SEs) to cause stimulation of select electrode sets of a lead system based on one or more pulse definitions and one or more timing definitions corresponding to a therapy program selection effectuated by a user at an external device.