Alright, Charon aka Ma-Config showed me this patent, it has a detailed description of MDIL.
MDIL Instruction Set: Explaining Instruction Formats and Conventions in MDIL Code
This section discusses the binary encoding and the semantics of MDIL instructions.
What is MDIL? MDIL is a lower level of intermediate language than MSIL. It gives up on the goal of machine independence—in fact the acronym MDIL stands for Machine Dependent Intermediate Language. MDIL doesn't compete with MSIL—rather, going through MDIL is another step in the compilation pipeline going from source code in languages like C#, Visual Basic®, Managed C++ via MSIL to native code (mark of Microsoft Corporation). MDIL doesn't try to abstract properties of the target processor, but it still provides a layer of insulation against versioning of both managed code and the execution engine MDIL code is compiled for.
MDIL is not directly executable. Instead, a binder reads the MDIL code and produces executable machine code from it. MDIL code is thus reminiscent of object files resulting from the compilation of unmanaged code which are run through a linker to produce final executable machine code. However, pseudo instructions in MDIL may go further than fixups in traditional object files—rather than just fixing up parts of instructions (like an address field), MDIL pseudo instructions may generate new native instructions which possibly differ in length from the pseudo instruction. Thus the MDIL binder is more complicated than a linker because the binder has to adjust the jumps in the resulting native code so they still jump to their intended targets. With this added complexity also comes added power—MDIL code can be robust against base types adding new fields or virtual methods, so it can be considered a solution to the "brittle base class problem".