[PDF] Shift and Rotate Instructions The Shift and Rotate instructions

How does the Rol instruction work?

The ROL (rotate left) instruction shifts each bit to the left. The highest bit is copied into the Carry flag and the lowest bit position. No bits are lost.

Why does the 8086 use Reg bits?

Unfortunately, there aren’t enough bits in the 8086’s opcode byte to support all instructions, so the 8086 uses the reg bits in the mod-reg-r/m byte as an opcode extension. For example, inc, dec, and negdo not require two operands, so the 80x86 CPUs use the reg bits as an extension to the eight bit opcode.

How many logical instructions does the 80x86 family provide?

The 80x86 family provides ?ve logical instructions, four rotate instructions, and three shift instructions. The logical instructions are and, or, xor, test, and not; the rotates are ror, rol, rcr,and rcl; the shift instructions are shl/sal, shr, and sar. The 80386 and later processors provide an even richer set of operations.

How does the 80x86 instruction set page 275 work?

The 80x86 Instruction Set Page 275 tion does not modify the value of operand2, it uses a temporary copy of operand2during the shift. The immediate operand speci?es the number of bits to shift. If the count is n, then shldshifts bit n-1 into the carry ?ag. It also shifts the H.O. nbits of operand2into the L.O. nbits of operand1.