Скачать или смотреть x64 Assembly Tutorial 17: Rotate Instructions

In this, the 17th, tutorial we'll be going through four rotate instructions, ROR, ROL, RCR and RCL. These instructions perform a related operation to the shifts but there is a subtle difference. The rotate instructions don't fill the vacant bits with 0's or 1's like the shift instructions. They fill the vacant bits with the bits that have been rotated out.

The rotate instructions are useful becuase sometimes you might want to perform a shift but maintain the bits in the operand. If you do a shift, the data shifted out is discarded. If you use rotate, you can rotate in the opposite direction and get the original bit pattern back again.

The rotate through the carry flag are interesting because they treat the operand as if it had one extra bit, the Carry Flag. If you rotate through the carry flag with an 8 bit register you're really using 9 bits. Likewise if you rotate through the carry flag a 64 bit register, you're actually rotating 65 bits!

All diagrams I've ever seen of this instruction draw the register as per usual as a flat row of bits. They describe the operation with arrows leaving one end of the operand and coming back in the other end. I think this is easier to view as a wheel.