We report on the design and implementation of an extensible programming language and its intrinsic support for formal verification. Our language is targeted at low-level programming of infrastructure like operating systems and runtime systems. It is based on a cross-platform core combining characteristics of assembly languages and compiler intermediate languages. From this foundation, we take literally the saying that C is a "macro assembly language": we introduce an expressive notion of certified low-level macros, sufficient to build up the usual features of C and beyond as macros with no special support in the core. Furthermore, our macros have integrated support for strongest postcondition calculation and verification condition generation, so that we can provide a high-productivity formal verification environment within Coq for programs composed from any combination of macros. Our macro interface is expressive enough to support features that low-level programs usually only access through external tools with no formal guarantees, such as declarative parsing or SQL-inspired querying. The abstraction level of these macros only imposes a compile-time cost, via the execution of functional Coq programs that compute programs in our intermediate language; but the run-time cost is not substantially greater than for more conventional C code. We describe our experiences constructing a full C-like language stack using macros, with some experiments on the verifiability and performance of individual programs running on that stack.