Why are you keeping an offset from pointer_in, instead of just keeping
pointer_out as a virtual pointer in its own right?

~ Bob Ammerman
RAm Systems

> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of noname
> Sent: Tuesday, August 08, 2017 3:55 PM
> To: Microcontroller discussion list - Public. <piclist@mit.edu>
> Subject: Re: [PIC] pic16f628a memory bank switching in a large circular
buffer.
>=20
> Follow up question:
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>=20
> Following the same method as outlined by Bob, is it possible to map the
> (absolute) pointer_out directly from (abs) pointer_in? I have tried
several
> ways on paper, but I do not get to a simple solution.
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>=20
> So starting with a valid (abs) pointer_in (mapped in the range 0x20-0x6F
or
> 0xA0-0xEF)
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> I need to add the offset that can be anything between 0 and 0xA0, and the=
n
> account for =B4bankgaps=B4 and finally land in the same range 0x20-0x6F a=
nd
> 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.
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>=20
> 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.
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> Nonetheless my code is some 30% smaller now:
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> Current code:
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>=20
> getpointers_table160
>     decfsz  virt_pointer_in, w                                 ; if 0 the=
n
point to top of the
> buffer again
>     goto    _skipifset                                                ; i=
f
not 0 then skip next line
>     addlw   CIRCULAR_BUFFER_LENGTH    ; + 0xA0
> _skipifset
>     movwf    virt_pointer_in                                    ; wreg
holds the current
> virt_pointer_in minus 1
>=20
>     addlw   h'B0'-1                                                   ;
carry set if bank1
>     btfsc   STATUS,C
>     addlw   h'30'
>     addlw   h'70'                                                       ;
bank set 0x20-6F or 0xA0-EF
>     movwf   pointer_in                                             ; now
wreg holds the abs value of
> pointer_in
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>
; calc the pointer out
>     movf    pointer_offset, w                                    ; offset
from 0 to 0xA0
>     addwf   virt_pointer_in, w                                 ;
virt_pointer_in is used as ref
> here
>     btfsc   STATUS, C
>     addlw   h'60'
>     addlw   h'60' - 1                                                  ;
Carry is set if >0xff
>     btfss   STATUS, C
>     addlw   h'A0'
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>
; 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
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>=20
>=20
>=20
>=20
> ________________________________
> 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
> buffer.
>=20
> NOTE: all numbers are in HEX below. The trick below
>    works because we are working MODULO 100 hex.
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> Input:   W is a pointer ranging from 00h thru 9Fh
> Output:  W points to the correct register in bank 0 or 1
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> ; 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
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> ; 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
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>    ADDLW 0B0h
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> ; 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
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>    BTFSC STATUS,C
>    ADDLW 030h
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> ; One more add gives us our answer. By adding 70 we get:
> ;  20..6F for page 0 addresses
> ;  A0..EF for page 1 addresses
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>    ADDLW 070h
>=20
> -- Problem solved in 4 instructions!
>=20
> ~ Bob Ammerman
> RAm Systems
>=20
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>=20
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