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Before instance variables were used to store the actual data While this worked it was a double mental loop and (more importantly) did not allow the same memory access Ie, in the interpreter the types for args had to be set correctly But they don't need to be until we walk the list, when we can get the types from the method. In short, now the fake memory may be used as memory, by indexing into it We now have to use attr reader/writers that map to the fake memory And while this is possible, it means a lot more self. than i would like Also the compiler whill have to undo those attr to instance acess at some point
Risc Machine
The RiscMachine, is an abstract machine with registers. Think of it as an arm machine with normal instruction names. It is not however an abstraction of existing hardware, but only of that subset that we need.
Our primary objective is to compile typed code to this level, so the register machine has:
- object access instructions
- object load
- object oriented call semantics
- extended (and extensible) branching
- normal integer operators (but no sub word instructions)
All data is in objects.
The register machine is aware of Parfait objects, and specifically uses Message and Frame to express call semantics.
Calls and syscalls
The RiscMachine only uses 1 fixed register, the currently worked on Message. (and assumes a program counter and flags, neither of which are directly manipulated)
There is no stack, rather messages form a linked list, and preparing to call, the data is pre-filled into the next message. Calling then means moving the new message to the current one and jumping to the address of the method. Returning is the somewhat reverse process.
Syscalls are implemented by one Syscall instruction. The Risc machine does not specify/limit the meaning or number of syscalls. This is implemented by the level below, eg the arm/interpreter.
Interpreter
There is an interpreter that can interpret compiled register machine programs. This is very handy for debugging (and nothing else).
Even more handy is the graphical interface for the interpreter, which is in it's own repository: rubyx-debugger.
Arm / Elf
There is also a (very straightforward) transformation to arm instructions. Together with the also quite minimal elf module, arm binaries can be produced.
These binaries have no external dependencies and in fact can not even call c at the moment (only syscalls :-)).