Convert ASM to C (not reverse engineer)

Getting the exact same source code back from a compiled program is basically impossible. But decompilers have been an area of research in computer science (e.g. the dcc decompiler, which was a PhD project).

There are various algorithms that can be used to do pattern matching on assembly code and generate equivalent C code, but it is very hard to do this in a general way that works well for all inputs.

You might want to check out Boomerang for a semi-recent open source effort at a generalized decompiler.

You can absolutely make a c program from assembler. The problem is it may not look like what you are thinking, or maybe it will. My PIC is rusty but using another assembler, say you had

add r1,r2

In C lets say that becomes

r1 = r1 + r2;

Possibly more readable. You lose any sense of variable names perhaps as values are jumping from memory to registers and back and the registers are being reused. If you are talking about the older pics that had what two registers an accumulator and another, well it actually might be easier because variables were in memory for the most part, you look at the address, something like

q = mem[0x12];
e = q;
q = mem[0x13];
e = e + q;
mem[0x12] = e;

Long and drawn out but it is clear that mem[0x12] = mem[0x12] + mem[0x13];

These memory locations are likely variables that will not jump around like compiled C code for a processor with a bunch of registers. The pic might make it easier to figure out the variables and then do a search and replace to name them across the file.

What you are looking for is called a static binary translation, not necessarily a translation from one binary to another (one processor to another) but in this case a translation from pic binary to C. Ideally you would want to take the assembler given in the app note and assemble it to a binary using the microchip tools, then do the translation. You can do dynamic binary translation as well but you are even less likely to find one of those and it doesnt normally result in C but one binary to another. Ever wonder how those $15 joysticks at wal-mart with pac-man and galaga work? The rom from the arcade was converted using static binary translation, optimized and cleaned up and the C or whatever intermediate language compiled for the new target processor in the handheld box. I imagine not all of them were done this way but am pretty sure some were.

The million dollar question, can you find a static binary translator for a pic? Who knows, you probably have to write one yourself. And guess what that means, you write a disassembler, and instead of disassembling to an instruction in the native assembler syntax like add r0,r1 you have your disassembler print out r0=r0+r1; By the time you finish this disassembler though you will know the pic assembly language so well that you wont need the asm to C translator. You have a chicken and egg problem.