I'm trying to jump to a specific address in HI-TECH compiler and the address is given dynamically.
I tried using asm() function but it looks like the argument must be a constant char[]. Is there another solution to jump to an address using goto or another function like goto_address(address) in CCS Compiler?
I tried this:
long address = 0xA20;
char addr[15];
sprintf(addr, "goto %s", address)
asm(addr); // Here argument error.
Thanks.
This is the kind of thing most easily done in assembler.
If you need to do this regularly, you can create a assembler subroutine that follows the C subroutine linkage conventions. This would take a 24 bit argument that is the address to jump to. Remember to have the subroutine pop its own return address from the stack before doing the jump. The subroutine would write bits 23:16 of the address to PCLATU, bits 15:8 to PCLATH, and then bits 7:0 to PCL. That last instruction will actually do the jump, so everything else needs to be set up properly before the write to PCL.
The asm() directive you're trying to use is supposed to be used to 'embed' a single, properly formatted, assembler instruction - written as a C-style string of characters.
In other words, something like asm(" BCF 0,3" );
The compiler looks at that string during compile time and inserts that exact assembler instruction into your code - that's why its asking you for a constant char[] - it has to be a correctly formatted constant string when your code is compiled. So what you're trying to do by building the string with sprintf during execution can't ever work.
In addition, if you look at the description of the GOTO assembler instruction for your PIC, you'll see that it takes a constant fixed value as the address to jump to. The address forms a part of the instruction at compile time and you can't change it on-the-fly during execution.
What you're trying to do might be better served using function pointers. See C coding design - function pointers? . But you'll need to be careful about managing & not mangling the stack.
You are very confused between what happens at compile time vs. what happens at run time. The argument to asm() must be a string that is fully specified at compile time. If the address value is only known at run time, then you need to come up with a sequence of instructions that can take a run-time value and copy it to the PC.
One of the most direct ways to do this on most architectures — and I believe this will work on PIC18 — is to PUSH the address value onto the return stack and then execute a RETURN instruction.
GOTO instruction should be enough. But your solution (as well as mine) also implies the problem of passing the address value to inline assembly.
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GOTO requires that the address be known at compile time. Passing a run-time value from regular C code to asm() code (and back again) is covered in the documentation.
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Commented
Oct 31, 2014 at 16:05
In order to do a computed goto you can change the program counter.
On the PIC18F4550 the program counter is composed of 3 registers, PCU (bits 20:16), PCH (bits 15:8) and PCL (bits 7:0).
Only the PCL can be written directly. In order to change the value off all the registers simultaneously you have to write to the PCLATU, PCLATH and PCL registers as explained on the section 5.1.1 of the datasheet.
First you should write the most significant bits to PCLATU and PCLATH, then you write the least significant bits to the PCL, this also copies the PCLATU and PCLATH to PCU and PCH respectively.
