Follow up question:


Following the same method as outlined by Bob, is it possible to map the (ab=
solute) pointer_out directly from (abs) pointer_in? I have tried several wa=
ys on paper, but I do not get to a simple solution.


So starting with a valid (abs) pointer_in (mapped in the range 0x20-0x6F or=
 0xA0-0xEF)

I need to add the offset that can be anything between 0 and 0xA0, and then =
account for =B4bankgaps=B4 and finally land in the same range 0x20-0x6F and=
 0xA0-0xEF . I tried something by adding 0xB0 to the offset (is it smaller =
or larger than 50?) but I diverge in such bookkeeping I cannot oversee.


My current code utilises the virtual pointer_in as reference, but I think I=
 can go even faster if I took the absolute pointer_in as reference.

Nonetheless my code is some 30% smaller now:


Current code:


getpointers_table160
    decfsz  virt_pointer_in, w                                 ; if 0 then =
point to top of the buffer again
    goto    _skipifset                                                ; if =
not 0 then skip next line
    addlw   CIRCULAR_BUFFER_LENGTH    ; + 0xA0
_skipifset
    movwf    virt_pointer_in                                    ; wreg hold=
s the current virt_pointer_in minus 1

    addlw   h'B0'-1                                                   ; car=
ry set if bank1
    btfsc   STATUS,C
    addlw   h'30'
    addlw   h'70'                                                       ; b=
ank set 0x20-6F or 0xA0-EF
    movwf   pointer_in                                             ; now wr=
eg holds the abs value of pointer_in

                                                                           =
       ; calc the pointer out
    movf    pointer_offset, w                                    ; offset f=
rom 0 to 0xA0
    addwf   virt_pointer_in, w                                 ; virt_point=
er_in is used as ref here
    btfsc   STATUS, C
    addlw   h'60'
    addlw   h'60' - 1                                                  ; Ca=
rry is set if >0xff
    btfss   STATUS, C
    addlw   h'A0'

                                                                           =
       ; At this point wreg =3D virtual pointer_out
    addlw   h'B0'            ; carry set if bank1
    btfsc   STATUS,C
    addlw   h'30'
    addlw   h'70'            ; bank set 0x20-6F or 0x80-EF
    movwf   pointer_out
    return





________________________________
Van: piclist-bounces@mit.edu <piclist-bounces@mit.edu> namens Bob Ammerman =
<picram@roadrunner.com>
Verzonden: maandag 7 augustus 2017 19:52
Aan: 'Microcontroller discussion list - Public.'
Onderwerp: RE: [PIC] pic16f628a memory bank switching in a large circular b=
uffer.

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
; 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

; 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

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