At 01:29 PM 29/11/1998 +1300, Seth Fischer wrote:
>What does it mean to rotate a file _through the carry bit_ (RLF and
>RRF)?
>
>When executing RLF on portB (8 LEDs) I get this:
>
>portB = 00000001
>        RLF     portb
>portB = 00000011        ; I want only one bit high and 'walking' left
>        RLF     portB
>portB = 00000110
>        RLF     portB
>portB = 00001100
>
>What is happening?
>
>--
>Seth Fischer
>Auckland, New Zealand
>
>
You rotate through carry: E.g.;: 1101 0101      (carry e.g.; =0 - you probably
dont know)
rlf = 1010 1010  - lat zero comes from carry!   (carry now = former bit7 = 1)
rlf = 0101 0101  - last 1 comes from carry!     (carry again =1 )

When you start operation, carry may or may not be set.
Correct (secure) way is therefore:

clrf    PORTB
bcf     STATUS,C
bsf     PORTB,0

then: rlf  etc


best regards,

Hans




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