Pervasive formal verification of operating systems and hypervisors is, due to their safety-critical aspects, a highly relevant area of research. Many implementations consist of both assembler and C functions. Formal verification of their correctness must consider the correct interaction of code written in these languages, which is, in practice, ensured by using matching application binary interfaces (ABIs). Also, these programs must be able to interact with hardware. We present an integrated operational small-step semantics model of intermediatelanguage C and Macro-Assembler code execution for pervasive operating systems and hypervisor verification. Our semantics is based on a compiler calling convention that defines calleeand caller-save registers. We sketch a theory connecting this semantic layer with an ISA-model executing the compiled code for use in a pervasive verification context. This forms a basis for soundness proofs of tools used in the VerisoftXT project and is a crucial step towards arguing formal correctness of execution of the verified code on a gate-level hardware model.