>> BTW, it is possible to properly use the RETLW as a lookup table in
>> PIC24/dsPIC assembly code.
>>            mov    <index value>,W0
>>           bra     W0
>>           retlw   valuefor0
>>           retlw   valuefor1
>>           retlw   valuefor2
>>          .etc.


>> This is significantly slower than using PSV, but is very simple to use.
>>
>> -- Bob Ammerman
>> RAm Systems

>It is 3 times slower than PSV (1 CALL + 1 BRA + 1 RETLW =3D 6 cycles
>total, against 1 MOVE + 1 Stall =3D 2 cycles total) and uses twice as many
>FLASH words.

Actually, it isn't quite that bad. Before using data stored in PSV you woul=
d
have to add the base address of the array:

    mov   <index value>, W0
    mov    #array_base_address_in_psv,W1
    mov.b  [W1+W0],W0

And this only works for a byte array. For a word array you'd need to scale
the index value:

    mov    <index value>,W0
    sl        W0,#1,W0
    mov    #array_base_address,W1
    mov    [W1+W0],W0

Of course RETLW can only handle 10 bit constants, not full word values.

>Besides you are forced to use two different access methods, one for
>constant data in FLASH and another for data in RAM.

True

>Isaac

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