What if you need to shift a 5 bit number in, through and out of a byte using bit0 to bit4 and not affect bit5 - bit7 because, for example, bits 5-7 are running the column select (via a 3 to 8) for a matix LED and the lower 5 bits are connected to the colum.
If you had two 8 bit wide registers next to each other like the following. The 5 bit value in byte 2 is shifted left throughout byte 1 and out.
byte1 byte2 XXX00000 xxx11111 start state XXX00001 xxx11110 XXX00011 xxx11100 XXX00111 xxx11000 XXX01111 xxx10000 XXX11111 xxx00000 XXX11110 xxx00000 XXX11100 xxx00000 XXX11000 xxx00000 XXX10000 xxx00000 XXX00000 xxx00000 end state
X = can not be changed.
x = not used
Dmitry Kiryashov [zews at AHA.RU] says:
1. You only need to shift single one register. rlf byte1,W ;get new bit from carry xorwf byte1,W ;get difference andlw B'00011111' ;0's will mask unchanged bits xorwf byte1,F ;update only required bits 2. You need to shift in such way through many registers. rlf byte1,W ;get new bit from carry andlw B'00111111' ;mask msb's but last addlw B'11100000' ;copy last to carry xorwf byte1,W andlw B'00011111' ;see explanation of xorwf byte1,F ;this in example above ;then ; rlf byte2,W andlw B'00111111' addlw B'11100000' ; ;and so on as many bytes as you need ;)
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<A HREF="http://www.piclist.com/techref/microchip/math/bit/rotpart.htm"> PIC Microcontoller Bit Math Method Rotate some bits without affecting others</A>
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