问题描述
从ARM报价 MRC 文件，
If MRC is meant to copy contents of a single coprocessor register into a single ARM register, what is the reason for CRm? Also, I'm not quite sure what the reason for an opcode is either, but that's of lesser relevance.
user786653 may have hit the nail on the head with that link.
I know the old fpu, the fpa, used the coprocessor interface and if you decode the fpa floating point instructions I think you will find they line up with the opcode pattern for a generic coprocessor (mrc/mcr).
My point is I wouldnt get worked up over every field in that instruction, some field are likely dont cares as far as the coprocessor is concerned. The ARM processor and assembler dont necessarily know what you have hooked up and this is a generic way to put bit patterns into a machine instruction. You need to focus on the specific coprocessor you are talking to, its TRM specifically the TRM with matching version numbers for that logic (if possible, if from ARM) assuming that coprocessor is from arm, if it is someone elses coprocessor then use their documentation. the fields being used by each operation in that coprocessor are hopefully defined well enough. If they dont talk about what one of the fields means I would assume it a dont care. Cut and paste their mrc/mcr line in the manual if there, if not google around maybe find some already well used code and cut and paste that pattern and/or try different things for the undefined fields and see if that changes the operation.
Generically though the mrc/mcr field definitions are not set in stone, a vendor can create a coprocessor and do whatever they want with those fields. Like the FPA perhaps going so far as to change the assembler/disassembler to create a new extension to the instruction set. the arm core may care about the ARM register itself, If there is a TRM for the coprocessor interface itself I wouldnt be surprised if a coprocessor read has a control line that tells the arm to or not to modify the specified arm register based on the coprocessor driving that signal.
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