PIC Microcontoller Memory Method

File Select Register

• The File Select Register (FSR) (register 4) points to other File Registers and allows them to be accessed by reading or writing the IND register (register 0).
  
• Use the following to load a variable address into FSR:

  movlw   variable_name
  movwf   FSR
  

  Using < movlw > is the key.
  

• If you touch FSR in interrupt code be sure to save/restore it!
  
• The LFSR instruction on the 18C series is broken, don't use it.

  DS80058B      PIC18CXX2 Rev. B Silicon Errata Sheet
  
  "Using the LFSR instruction to load a value into the
  specified FSR register may also corrupt a RAM location.
  Do not use the LFSR instruction."
  

Bob Ammerman says:

Another use or two or three of FSR:

To pass an argument by reference to a subroutine so that one subroutine can be used with varying data. I use this for code that manages debounce state on buttons.

To allow the same code to be used to hit two different I/O ports.

And my favorite:

I sometimes use 'open-collector' type communication channels. To implement these one would typically set the port register bit to zero, and then use the TRIS register bit to control the output (zero or float).

In order to avoid bank switch inside tight loops, I'll do this:

    movlw    TRISB
    movwf    FSR

now I can:

    bcf        INDF,n
 or
    bsf        INDF,n

to output my bits without having to bank switch.

Andrew Warren says:

Remember that in the 16C5x processors, the unused upper bits of the FSR always read as "1".

On the 16C54, for instance, the upper three bits are set that way, which means that the FSR will always contain an address in the range 0xE0-0xFF.

Therefore, when you test the FSR for equality to some number, you must either AND the FSR with 00011111 or OR the number with 11100000 before the comparison.

That's a drag, but if you're just using the FSR to clear RAM, the "111" in the upper three bit-positions actually helps:

        MOVLW   5       ;Start clearing RAM at register 0x05.
        MOVWF   FSR

    LOOP:

        CLRF    INDF
        INCFSZ  FSR     ;Stop clearing RAM at register 0x1F.
        GOTO    LOOP

The "INCFSZ" will get us out of the loop after we've cleared register 0x1F, even though our code doesn't explicitly compare FSR to 0x1F anywhere. It works like this:

The FSR starts at 11100101 (we write 0x05 to it, but the high 3 bits are always set), then we iterate through the loop until the FSR holds 11111111 (0x1F with the high 3 bits set).

At that point, our INCFSZ reads the FSR (11111111) and increments it to 00000000. That triggers the "skip" part of the INCFSZ, even though FSR will hold 11100000, not 00000000, after we exit the loop.

One note about PICs with > 256 bytes of RAM is that you need to take note of the STATUS,IRP bit. This is the 9th FSR bit so you can access RAM above 255. STATUS,RP0 & RP1 are not used with indirect addressing.

File registers

Software Stacks

Moving blocks of data in memory

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