3.4. Clearing¶

Clearing bits is the opposite of masking. Instead of wanting to keep certain bits, we want to 0 out specific bits while keeping other ones. For instance, we might want to clear out bits 25-29 from 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	1	1	0	1	1	1	0	1	0	1	1	0	1	1	1	0	1	1	1	1	0	1	1	1	0	1	1	0	1
1				B				A				D				D				E				E				D

To do so, we could mask by ANDing with 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
1	1	0	0	0	0	0	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1
C				1				F				F				F				F				F				F

Bits 25-29 would become 0 and everything else would remain as is. However, that pattern is too complex to represent as an immediate. Any time we are trying to clear a small number of bits, we will have a similar problem of needing a large mask to AND with.

Instead, we can use the bitwise clear instruction - it uses a pattern to specify which bits to clear (ANDing with a pattern specifies which bits to keep).

BICrd, rn, rm / #: BIt Clear. Take the pattern from rn, clear out the bits that are 1 in either register rm or immediate #, place the results in rd.

To continue the earlier sample, instead of AND with 0xC1FFFFFF (which is illegal) to say “keep bits 31-30 and 24-0”, we can BIC with 0x3E000000 to say “clear bits 29-25”:

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	1	1	1	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
3				E				0				0				0				0				0				0

LDR   r5, =0x1BADDEED

@Clear bits 25-29
BIC   r6, r5,  #0x3E000000

@Clear bits 16-31 (first two bytes)
BIC   r7, r5,  #0xFF000000   @clear first byte
BIC   r7, r7,  #0x00FF0000   @now clear second

Try sample