There once was an assembler by Parallax that converted 8051 type instructions into (often multiple) PIC instructions. I've never played with it, and I have no idea how close to 8051 instructions they were.
But, yes, the architectures are so different that any expectation of taking a program *written* for an 8051 and converting it to PIC is likely to be false. The idea of the Parallax system was to simplify the learning curve for 8051 programmers who wanted to try PICs, not to convert existing programs.
If the program is already in 8051 assembler, it's almost surely best to stick with an 8051 variant.
Best regards, Spehro Pefhany
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