Print Page - Swapping bits in a byte

Title: Swapping bits in a byte
Post by: redbox on 22:43, 17 February 13

I have a byte loaded into A as follows:


LD A,(variable)

And I want to swap the following bits X and Y over in the byte - for example:

Code Select


128 64 32 16 8 4 2 1
0   0  X  X  X Y Y Y

becomes

Code Select


128 64 32 16 8 4 2 1
0   0  Y  Y  Y X X X

Any ideas?
All registers are available and ideally the result will end up in A.
No tables please, so I realise it'll probably be a combination of bit shifting (RRA etc) and OR and AND but can't think of the fastest way...?!

Title: Re: Swapping bits in a byte
Post by: Grim on 00:00, 18 February 13

Maybe something along the lines of:

Code Select


	; A = %00XXXYYY
	add a,a
	add a,a
	; A = %XXXYYY00
	add a,a
	ld c,a
	; C = A = %XXYYY000, Carry X
	adc a,a
	adc a,a
	adc a,a
	; A = %YY000XXX, Carry Y
	add a,c
	and %00111111
	; A = %00YYYXXX
	; 10µs

Title: Re: Swapping bits in a byte
Post by: TFM on 00:12, 18 February 13

Nice idea!! A RRCA approach wouldn't be more quick here :)

Title: Re: Swapping bits in a byte
Post by: redbox on 10:55, 18 February 13

Quote from: Grim on 00:00, 18 February 13
Maybe something along the lines of:

Thanks Grim, that's great.

Nice use of the carry as a buffer.

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General Category => Programming => Topic started by: redbox on 22:43, 17 February 13