What's the purpose of the rotate instructions (ROL, RCL on x86)?
I always wondered what's the purpose of the rotate instructions some CPUs have (ROL, RCL on x86, for example). What kind of software makes use of these instructions? I first thought they may be used for encryption/computing hash codes, but these libraries are written usually in C, which doesn't have operators that map to these instructions.
Has anybody found an use for them? Why where they added to the instructions set?
Answer
Rotates are required for bit shifts across multiple words. When you SHL the lower word, the high-order bit spills out into the carry. To complete the operation, you need to shift the higher word(s) while bringing in the carry to the low-order bit. RCL is the instruction that accomplishes this.
High word Low word CF
Initial 0110 1001 1011 1001 1100 0010 0000 1101 ?
SHL low word 0110 1001 1011 1001 1000 0100 0001 1010 1
RCL high word 1101 0011 0111 0011 1000 0100 0001 1010 1
ROL and ROR are useful for examining a value bit-by-bit in a way that is (ultimately) non-destructive. They can also be used to shunt a bitmask around without bringing in garbage bits.