Multiplication and Division

Shift instructions allow the bits of a register, or memory byte, to be moved one bit place to the left, or to the right. There are two types of shift instructions, logical and arithmetic. Logical shifts consider the contents of the register, or memory byte, as a pattern of binary digits when the shift is made. Arithmetic shifts treat the contents of the register, or memory byte, as a signed number, so that the effect of a left shift is equivalent to multiplication by two and a right shift is equivalent to division by two. The Z80 microprocessor in the ZX81 has one logical and two arithmetic shift instructions.