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⬅️ Previous capture (2023-01-29)

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"Michael J. Mahon" wrote:
> If you want to simulate a ROL that does not include carry,
> it is only necessary to preset the carry bit to the high bit
> of the shifted byte.  For example, if the A register is being
> shifted, then preceding the ROL with a CMP #$80 will cause
> the ROL to shift the high bit to the low bit position.

You can also use ASL followed by ADC #0.  Swapping nibbles
in a byte can be done by:

LDA #value      abcd efgh
ASL             bcde fgh0
ADC #0          bcde fgha
ASL             cdef gha0
ADC #0          cdef ghab
ASL             defg hab0
ADC #0          defg habc
ASL             efgh abc0
ADC #0          efgh abcd

I got this from Apple Assembly Line.

--
Paul
Monroe, Michigan USA



Paul R. Santa-Maria wrote:

>"Michael J. Mahon" wrote:
>> If you want to simulate a ROL that does not include carry,
>> it is only necessary to preset the carry bit to the high bit
>> of the shifted byte.  For example, if the A register is being
>> shifted, then preceding the ROL with a CMP #$80 will cause
>> the ROL to shift the high bit to the low bit position.
>
>You can also use ASL followed by ADC #0.  Swapping nibbles
>in a byte can be done by:
>
>LDA #value      abcd efgh
>ASL             bcde fgh0
>ADC #0          bcde fgha
>ASL             cdef gha0
>ADC #0          cdef ghab
>ASL             defg hab0
>ADC #0          defg habc
>ASL             efgh abc0
>ADC #0          efgh abcd
>
>I got this from Apple Assembly Line.

It's interesting how the way the problem is framed influences
the solution.  ;-)

I thought of it as cancelling the effect of including the carry
in the ROL, so I copied the high bit to the carry after the shift.

Bob S-C thought of it as getting the carried-out bit back into
the accumulator, so he copied the carry to bit 0 after the shift.

Cool.  ;-)

-michael

Check out amazing quality sound for 8-bit Apples on my
Home page:  http://members.aol.com/MJMahon/



I wrote...

>It's interesting how the way the problem is framed influences
>the solution.  ;-)
>
>I thought of it as cancelling the effect of including the carry
>in the ROL, so I copied the high bit to the carry after the shift.

Of course, I meant to say, _before_ the shift.  ;-(

-michael

Check out amazing quality sound for 8-bit Apples on my
Home page:  http://members.aol.com/MJMahon/