5. Variable-length Opcodes

• We can make some opcodes longer than n bits...

  • ...and that is the secret to reducing the size of a typical program on the CPU!

  (This strategy is acceptable only for CISC processors; RISC^(*) processors prefer uniform 32-bit or 64-bit instructions.)

• Assuming that CPU is capable of reading byte-sized quantities from memory, each opcode must be some even multiple of 8-bits long.

• Another point to consider is the space for instruction operands:

  • RISC designers include all operands in their opcode.

  • CISC designers, including x86, place constants and address displacements (offsets) apart from the opcode.

• ______________

• ^(*) CISC stands for complex instruction set computer design,
  while RISC is a reduced instruction set computer.