Oops...

Replying to my own post to fix a couple errors, save one instruction, and
adhere to the original specification more closely.

~ Bob Ammerman
RAm Systems

> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of Bob Ammerman
> Sent: Friday, August 11, 2017 2:19 PM
> To: 'Microcontroller discussion list - Public.' <piclist@mit.edu>
> Subject: RE: [PIC] pic16f628a memory bank switching in a large circular
buffer.
>=20
> Here is an implementation of the OPs actual requirements, as I understand
> them. One of the neat things about this implementation is that the
routines
> are isochronous, which means that they always take exactly the same
> amount of time.
>=20
> Again, untested code!
>=20
> -- Bob Ammerman
> RAm Systems
>=20
>=20
> Again, we have the two snippets of code already defined, converted into
> macros
>=20
> FIXINDEX:
> Convert W from a number in the range 00h..9Fh into a number in one of the
> ranges 20h..6F or A0h..0EFh
>=20
> BUMPINDEX
> Increment W circularly, wrapping from 09Fh back to zero.
>=20
>   ; INSP is the insertion pointer into
>   ; the buffer. OFFSET is the number of
>   ; samples 'back' in the buffer for output
>=20
>   ;=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D
>   ; INSERTION ROUTINE:
>=20
>   MOVF	INSP,W
>   BUMPINDEX
>   MOVWF INSP
>   FIXINDEX
>   MOVWF FSR
>   MOVF  INBYTE,W
>   MOVWF INDF
>   RETURN
>=20
>   ;=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D
>   ; OUTPUT ROUTINE
>=20
>   ; Convert the offset from
>   ; a distance backward to
>   ; a distance forward
>=20
>   MOVWF	OFFSET,W
>   SUBLW 0
>=20
>   ; Now compute the desired location
>   ; in the buffer
>=20
>   ADDWF INSP,W
>=20
>   ; at this point we have to
>   ; wrap any value greater
>   ; than 09Fh. The next instruction
>   ; will set carry if we have a sum
>   ; greater then 9Fh.
>=20
>   ADDLW 60H
>=20
>   ; now we subtract 0A0h if we
>   ; are over 9Fh. Note that we
>   ; we use ADDLW instead of
>   ; SUBLW because the latter
>   ; computes 'literal minus W'
>   ; rather than 'W minus literal',
>   ; which is what we want.
>=20
>   BTFSC STATUS,C
>   ADDLW 100h-0A0h
>=20
>   ; Finally subtract back out
>   ; the 60H we added above. The
>   ; same comment about ADDLW vs.
>   ; SUBLW applies
>=20
>   ADDLW 100h-060h
>=20
>   ; Convert the value to a pointer
>   ; and get the byte
>=20
>   FIXINDEX
>   MOVWF	FSR
>   MOVF  INDF,W
>=20
>   ; Done!
>=20
>   RETURN


--=20
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