At 02:29 PM 2/23/00 +1100, you wrote:
>John Orhan wrote:
>
>> I am trying to wrap my head around the paging system that the '505
>> uses. If I understand correctly, the memory is banked into four arrays
>> of around 255 bytes each.
>
>
>  On 12-bit instruction chips, bit 7 is missing from CALL instructions
>but not GOTOs, so you simply cannot call a subroutine in the second half
>of a "page" (256 bytes - exactly).

Lets clear this up. Bit 7 is not missing from a call instruction, bit-8 is.
A page is 512 * 12-bit words. You can call any location in the first half
of a 512 word page but a goto can go to any location in the page selected
by the PA0 and PA1 bits in the status register.


>
>  Otherwise, whatever bits are not contained in the GOTO or CALL
>instruction are picked for the high Program Counter byte from the
>register called PCLATH (or two of the status bits on the 12-bit cores).
>If you want to GOTO or CALL a routine on a chip where there are less
>bits stored in the GOTO or CALL instruction than the PC, you must set
>set PCLATH to correspond to the high address bits of that page.

(There is no PCLATH on the '505 but if you later use a 14-bit core PIC
you will need to learn a new page select scheme as Paul is describing
above.)

>
>  Note that on many chips, the GOTO or CALL instruction contains all the
>necessary bits.

Up to 2K only. You need to use the PCLATH register for all calls and goto's
above 2K. (PIC16C family, n/a for 17C and 18C Pics.)

>
>  Note also that if you change the low Program Counter by using it as
>the destination of an operation, the high PC will be loaded from PCLATH
>in the process, so you must also set PCLATH correctly prior to doing
>this.  It initialises to zero, so you may *not* need to set it if you
>want to branch to page zero and have never set it otherwise.

In the case of the '505 the upper bits come from PA0 and PA1 in the STATUS
register.

-Jim

>--
>  Cheers,
>        Paul B.
>
Regards,

Jim Robertson
NEWFOUND ELECTRONICS
________________________________________
Email: newfound@pipeline.com.au
http://www.new-elect.com
MPLAB compatible PIC programmers.
________________________________________