This is for a phase accumulator in a DDS -- the lookup table is 1024
entries, so the 24-bit accumulator needs to be truncated to 10-bits, or
truncating 16 bits of the high and middle byte to 10 bits. The PIC in
question is an 18F1320, and the table is aligned at 0x1C00. This means that
TBLPTRH and TBLPTRL need to be updated for each read, with 0x1C added to the
upper two bits of the 10-bit value for TBLPTRH, and the lower 8-bits of the
10-bit value copied to TBLPTRL. My first effort is below, but I think there
must be a faster way to do it --

ACCLOOP
 movlw FRQL    //frequency increment low byte
 addwf ACCM    //increment accumulator low byte
 movlw FRQH    // freq incr high byte
 addwfc ACCH    //incr acc high byte
 movff ACCH, SHDH //shadow acc high byte
 rlncf SHDH
 rlncf SHDH, W    //rotated twice, two highest bits of 10-bit value now two
LSBs in W
 andlw b'00000011' //leaves just those two bits
 addlw 0x1C    //add table offset
 movwf TBLPTRH    //TBLPTRH ready
 rlncf ACCL, W
 rlncf W
 andlw b'00000011'
 movwf SHDL //upper two bits of ACCL, which will be lower two bits of 10-bit
value readied in SHDL
 rlncf SHDH, W
 andlw b'11111100' //upper six bits (originally bits 5...0 of ACCH)
 addwf SHDL, W
 movwf TBLPTRL

    Would you guys know of standard/faster ways to do truncations to such
odd bit values?

Thanks,
Ishaan

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