NOTE: all numbers are in HEX below. The trick below
   works because we are working MODULO 100 hex.

Input:   W is a pointer ranging from 00h thru 9Fh
Output:  W points to the correct register in bank 0 or 1

; At entry, the value of W is as follows:
;  00..4F - we want to map to page 0 addresses 20..6F
;  50..9F - we want to map to page 1 addresses A0..EF

; We start by adding B0. This will set STATUS,C to 1,
; if we have a page 1 address, and to 0 if we have a=20
; page 0 address. After this add, we will have:
;  B0..FF - which we want to map to page 0 addresses 20..6F
;  00..4F - which we want to map to page 1 addresses A0..EF

   ADDLW 0B0h=09

; Now, if carry is set, we add 30. After this conditional
; add, we will have:
;  B0..FF - which we want to map to page 0 addresses 20..6F
;  30..7F - which we want to map to page 1 addresses A0..EF

   BTFSC STATUS,C
   ADDLW 030h

; One more add gives us our answer. By adding 70 we get:
;  20..6F for page 0 addresses
;  A0..EF for page 1 addresses

   ADDLW 070h

-- Problem solved in 4 instructions!

~ Bob Ammerman
RAm Systems

--=20
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