Hi BAJ.

Thanks for the help.  I thought there must be something like that, given there
is no indexing register or indirect opcode.

I assume that once the FSR and (IRP-status bit 7 - looked it up) have been
set, by referencing location INDF with clrf or movlf etc will perform the
indirection for you?

I like the 'no bankswitching' technique.

Cheers,
Mark

> -----Original Message-----
> From: piclist-bounces@mit.edu 
> [mailto:piclist-bounces@mit.edu]On Behalf
> Of Byron A Jeff
> Sent: 18 October 2004 13:47
> To: Microcontroller discussion list - Public.
> Subject: Re: [PIC]Stepping through buffer in assembler?
> 
> 
> On Mon, Oct 18, 2004 at 01:04:46PM +0100, Mcgee, Mark wrote:
> > Hi
> > 
> > I'm using PIC16F628A.
> > 
> > I've been wondering how would you move along a 
> buffer/array, reading or
> > writing to each location in a loop?  I can't see any 
> obvious indirect memory
> > access instructions in the reference manual.
> 
> The SFR combo you're looking for is FSR/INDF
> 
> > 
> > In my old 6502/'6510 assembler days I'd do something like (IIRC);
> > LDA #0
> > LDX #5
> > loop:
> > STA buffer_start,X
> > DEX
> > BNE loop
> 
> In PIC the eqivalent would be something like (unchecked code 
> warning...):
> 
> movlw 5
> movwf count 
> movlw buffer_start
> movwf fsr
> loop:
> clrf indf
> incf fsr,F
> decfsz count,f
> goto loop
> 
> > ...
> > 
> > Obviously, PIC only has the W register, and doesn't have 
> index registers...Is
> > there some 16-bit address to set, and some indirect 
> instruction or something?
> 
> It does have index register(s). But they are special function 
> registers as
> opposed to out of memory registers likw W.
> FSR holds the address, while INDF references the target of 
> that address.
> 
> And the 18F family actually has 3 pairs of them, along with 
> auto inc/dec
> instructions for the FSR registers. Yet another advantage of 
> the 18F parts.
> 
> Finally be aware that FSR is only 8 bits. If the memory you 
> need to access
> is beyond that, then you have to manipulate the indirect upper bits in
> some register (STATUS or OPTION can't remember off the top of 
> my head).
> 
> One advntage of FSR is that it is 8 bits so you can 
> indirectly address up
> to 256 bytes (in)directly, while you can only access 128 
> bytes directly without
> switching banks. That's why sometimes I'll do code like:
> 
> movlw TRISB ; In bank 1
> movwf FSR
> clrf INDF   ; make port B all outputs
> 
> Without bothering to fiddle with the banking bits.
> 
> Hope this helps,
> 
> BAJ
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