24. Simplified Special Opcode Instructions (not x86!)

• There are four possible one-operand instruction classes, specified by 2-bit ii[4:3] field:

  1. The first encoding ii[4:3]=00 further expands the instruction set with a set of zero-operand instructions

  2. The second opcode ii[4:3]=01 is also an expansion opcode codifies all of the simplified jump instructions.

  3. The third opcode ii[4:3]=10 is the NOT instruction, a bitwise logical not operation that inverts all the bits in the destination register or memory operand.

  4. The fourth opcode ii[4:3]=11 is currently unassigned:

     • Any attempt to execute unassigned opcode will halt the processor with an illegal instruction error.

     • CPU designers often reserve unassigned opcodes to extend the instruction set at a future date. Intel did so when moving from the 80286 processor to the 80386.