In the 12C509 you _cannot_ call code in the second half of a 'page'. This is
because there are not enough bits available in the the instruction to define
the full target address.

"goto" instructions provide 9 bits to store the target address. This allows
use to specify any address in a 'page' of 512 locations.

You select the desired page of 512 locations by setting the PA0 bit in the
status register.

"call" instructions only provide 8 bits to store the target address. The
ninth bit is assumed to be a zero. Thus, the complete address is:

    PA0   0   K K K K K K K K

where the K bits are from the instruction.

Thus, you can only call to the first 256 locations in each 512 location
page.


Bob Ammerman
RAm Systems
(high performance, high function, low-level software)

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