Shifting: sll, sla, srl, sra

Yesterday you set, cleared and tested bits with and, or and xor. Today you move them. Four instructions shift all bits of a number by a number of places:

Shifting is arithmetic#

One place to the left doubles a number, one place to the right halves it and cuts off the remainder:

move r0 %0000_0101   # 5
sll r1 r0 3          # %0010_1000 = 40, i.e. 5 * 8
srl r2 r1 2          # %0000_1010 = 10, i.e. 40 / 4

With decimals this does not work: before every bit instruction the chip converts the value into an integer, the fractional part is dropped.

Logical or arithmetic?#

For positive numbers srl and sra are the same. For negative numbers they are not: negative numbers have ones all the way on the left. sra shifts those ones in – the sign stays, −64 becomes −16. srl shifts in zeros and turns a small negative number into a huge positive one.

move r3 -64
sra r4 r3 2          # -16

Pulling out fields#

Often several small values live in one number, say four bits for a state starting at bit 4. First shift right, then cut off with a mask:

srl r5 r0 4          # bit 4 slides down to bit 0
and r5 r5 %1111      # keep only the lower four bits

The day after tomorrow you learn ext, which does exactly these two steps in one. But you only understand how it works inside with srl and and.