Geotronix wrote:
>
> I am working on a project using a 16F877 and am having difficulty in
> indirectly addressing RAM locations.  I would like to send a byte of data
> to RAM location 0x120, by using the following:
>
> movfw Offset                            ;contains the value 20h
> movwf FSR
> movfw Data1
> bcf STATUS,RP0
> bsf STATUS,RP1
> movwf INDF
>
> YET...when simulated, Data1 is moved to 0x020.
> It seems that no matter which page I switch to prior to executing the movwf INDF statement, Data1 is placed into 0x020.
> Can anyone help?

Some PICs have more than 256 bytes of RAM available, (16F877 for
example), but the FSR can only hold values from 0 - 255. Therefore, to
indirectly access RAM addresses above 255, there needs to be another
address bit available.

This bit is in the STATUS register and is called IRP.

For example

if IRP = 0, you can indirectly access RAM 0x00 - 0xFF
(Banks 0 and 1)
if IRP = 1, you can indirectly access RAM 0x100 - 0x1FF
(Banks 2 and 3)

This bit only works in conjunction with the FSR register and indirect
addressing and is effectively bit 9 of the indirect address.

STATUS RP0 and RP1 are not used with indirect addressing, only direct
addressing.

Your code should be modified like this...

movfw Offset    ;contains the value 20h
movwf FSR
bsf STATUS,IRP  ; indirectly access banks 2 and 3
movfw Data1
movwf INDF      ; written to 0x120

--
Best regards

Tony

mICros
http://www.bubblesoftonline.com
mailto:sales@bubblesoftonline.com

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