----- Original Message ----- 
From: "Hopkins" <rdhopkins@ihug.co.nz>
To: "'PICLIST'" <piclist@mit.edu>
Sent: Thursday, December 23, 2004 9:42 PM
Subject: [PIC]Circular buffer


> I understand the idea of using pointers that chase each other in a
> circular fashion but I do not know how to declare a memory block (da
> command?) and then how to setup the pointers to store and retrieve
data.
>
> The plan is to store LCD messages in EEPROM that are only altered if
the
> operator wishes to alter a message.
>
> Most of the time the EEPROM table will be copied into ram and used
> there.
>
> Another buffer is to be used with a keypad that will send data via an
> I2C link to another processor. This buffer will stay in ram the whole
> time.
>
> I am programming in assembler using a 16F876.
>
> Scott Dattalo explained the procedure with a C example but I do not at
> this stage understand C enough to be able to use it.

Here is some code that make help you out.  Of course you will have to
modify some of the values.

STARTOFRAM    equ           0x20              ;First Usable RAM Location
for 16f628
ENDOFRAM      equ           0x6F              ;Last Usable RAM Location
for 16f628
FIXEDAREA     equ           0x70              ;Start of shared memory
between banks
BANK2RAM      equ           0xA0              ;Start of Bank 2 RAM
ENDOFBANK2    equ           0xEF              ;End of Bank 2 RAM

              cblock BANK2RAM
;
; This must be last thing in cblock (circular q uses all available ram)
;

QData
              endc

              cblock FIXEDAREA
; Make sure we can always get to these
QHead
QEnd
              endc


STARTOFQ      equ            QData            ; Start of Circular Q
Memory
ENDOFQ        equ            ENDOFBANK2 + 1
SIZEOFQ       equ            ENDOFQ - STARTOFQ ;Number of bytes that q
will hold

This code will stick values into the queue.

;The following code commented out will cause bytes received when queue
is full to be dropped at the expense of
;  extra cpu cycles.  The current setup is faster to execute but will
cause q to wrap around, which causes SIZEOFQ
;  bytes to be lost.
;              incf        QEnd, W             ;Testing for a q wrap
;              xorlw       ENDOFQ              ;Are we at the end of
memory
;              btfss       STATUS, Z
;               goto       $ + 3
;;Point back to beginning of Q
;              movlw       STARTOFQ            ;Address of start of Q
;              goto        $ + 2
;              incf        QEnd, W
;              xorwf       QHead, W
;              btfsc       STATUS, Z
;               goto dropit

;Stick received byte into circular queue
              incf        QEnd, F             ;Increment pointer
              movfw       QEnd                ; and load into W
              xorlw       ENDOFQ              ;Are we at the end of
memory
              btfss       STATUS, Z
               goto       AppendByte
;Point back to beginning of Q
              movlw       STARTOFQ            ;Address of start of Q
              movwf       QEnd
AppendByte
              movfw       QEnd                ;Get address to place byte
into
              movwf       FSR                 ; and set up indirect
pointer to it
              movfw       serialData          ;Get the byte received and
              movwf       INDF                ; stick it into the Q

To extract values from the queue use code that looks something like
this.

Main
              movfw       QHead               ;Get current start of q
pointer and
              xorwf       QEnd, W             ; compare to end of q
pointer
              btfsc       STATUS, Z           ;If equal then nothing in
the q to process
               goto Main
;Extract received byte from circular queue
              incf        QHead, F            ;Increment pointer
              movfw       QHead                ; and load into W
              xorlw       ENDOFQ              ;Are we at the end of the
queue
              btfss       STATUS, Z
               goto       Extract_Byte          ;no, just pull one out
;Point back to beginning of Q
              movlw       STARTOFQ            ;Address of start of Q
              movwf       QHead
Extract_Byte
              movfw       QHead               ;Get address to get byte
from
              movwf       FSR                 ; and set up indirect
pointer to it
              movfw       INDF                ; Load byte from queue
into W

Good Luck, YMMV



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