Paging and Banking Issues

Paging and Banking Issues

In many cases, RAM allocation will span multiple banks, and executable code will span multiple pages. In these cases, it is necessary to perform proper bank and page set-up to properly access the labels. However, since the absolute addresses of these variable and address labels are not known at assembly time, it is not always possible to place the proper code in the source file. For these situations, two new directives, BANKSEL and PAGESEL have been added. These directives instruct the linker to generate the correct bank or page selecting code for a specified label. An example of how code should be converted is shown below.

Absolute Code

LIST P=12C509

#include "P12C509.INC"

Varl EQU H'10'

Var2 EQU H'30'

...

MOVLW InitialValue

BCF FSR, 5

MOVWF Varl

BSF FSR, 5

MOVWF Var2

BSF STATUS, PA0

CALL Subroutine

...

Subroutine CLRW ;In Page 1

...

RETLW 0

Relocatable Code

LIST P=12C509

#include "P12C509.INC"

UDATA

Varl RES 1

Var2 RES 1

...

CODE

MOVLW InitialValue

BANKSEL Varl

MOVWF Varl

BANKSEL Var2

MOVWF Var2

PAGESEL Subroutine

CALL Subroutine

...

Subroutine CLRW

...

RETLW 0