Mill Computing, Inc. Forums The Mill Architecture Memory Reply To: Memory

Ivan Godard
Post count: 627

This is all filed, but was at most alluded to in the talks so far. so here’s a more complete explanation.

All transfers use one of two modes: indirect through a pointer on the belt (no offset), or (up to 32-bit manifest) signed offset from the entry address of the current function. There is also a LEA form that returns the offset-from-function-entry result as a pointer dropped to the belt.

All address modes (load, store, and LEA) comprise a base, an up-to-32-bit manifest signed offset, and optionally an index from the belt with a manifest scale factor that left-shifts the index by 0-4 bits. The base may be either a pointer from the belt or one of a small set of specRegs that contain addresses. Currently these are:
cpReg base of code region for load module
cppReg base of constant pool in load module
dpReg base of static data region
fpReg base of stack frame, or NaR if no frame
inpReg base of inbound memory-passed function arguments, or NaR if none
tpReg base of thread-local storage, or NaR if none

This list is expected to change as we discover issues when porting the OSs. In particular, cpReg and cppReg are likely to go away; there may be an outpReg added; and there may be a few levels of display added (for support of languages with closures). With these, all addresses in the code itself are static and no load-time relocation is necessary.

cpReg, cppReg, and dpReg are initialized by the loader when the process is created. fpReg and inpReg are managed directly by the hardware via the call, retn, and stackf operations. The values of all these registers are currently readable by the rd operation, but that too may go away. They are reachable by MMIO, as used by the init ROM in the power-up sequence to set the initial execution environment.