1.4. Rotation

The rotate instruction shifts all the bits in a register to the right by a given amount. If we take this pattern:

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	1	1	0	1	1	1	1	0	0	0	0	0	0	0	0
0				0				0				0				6				F				0				0

And shift it right 1 bit, we get:

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	1	1	0	1	1	1	1	0	0	0	0	0	0	0
0				0				0				0				3				7				8				0

Notice that shifting one bit completely changes the hex version of the pattern. If we want to shift by one hex digit, we should shift the original pattern 4 bits.

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	1	1	0	1	1	1	1	0	0	0	0
0				0				0				0				0				6				F				0

Tip

Each 4-bits of rotation shifts the hex digits by 1

Bits that shift past position 0 wrap around to bit 31. Here is the pattern shifted 6 more bits:

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
1	1	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	1	1	0	1	1
C				0				0				0				0				0				1				B

---

RORrd, rn, rm / #

ROtate Right. Rotate rn to the right the number of bits in rm or #. Result in rd.

MOV   r1, #0x12      @ r1 = 0000 0000 ... 0000 0001 0010

@Rotate r1 right by 1 bit
ROR   r2, r1, #1     @ r2 = 0010 0000 ... 0000 0000 1001

@Rotate r1 right by 4 bits - one hex char
ROR   r3, r1, #4     @ r3 = 0010 0000 ... 0000 0000 0001

Try sample

MOV r1, #0x12 @ r1 = 0000 0000 ... 0000 0001 0010 @Rotate r1 right by 1 bit ROR r2, r1, #1 @ r2 = 0010 0000 ... 0000 0000 1001 @Rotate r1 right by 4 bits - one hex char ROR r3, r1, #4 @ r3 = 0010 0000 ... 0000 0000 0001

There is no rotate left instruction, because we do not need one. A 31-bit rotate is the same as rotating left 1 bit, a 30-bit rotate the same as rotating left 2 bits, …. If we want to rotate left 12 bits, we could instead rotate 20 to the right:

MOV   r1, #0x12      @ r1 = 0x00000012

@Rotate r1 left 12 bits by ROR 20 bits (32 - 20 = 12); answer in r6
ROR   r6, r1, #20    @ r6 = 0x00012000

Try sample

MOV r1, #0x12 @ r1 = 0x00000012 @Rotate r1 left 12 bits by ROR 20 bits (32 - 20 = 12); answer in r6 ROR r6, r1, #20 @ r6 = 0x00012000

Tip

To rotate left X bits, we rotate 32 - X to the right

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