29. ISA Design Challenges

• Allowing for future expansion of the chip requires some undefined opcodes.

• From the beginning there should be a balance between the number of undefined opcodes and

  1. the number of initial instructions, and

  2. the size of your opcodes (including special assignments.)

• Hard decisions:

  • Reduce the number of instructions in the initial instruction set?

  • Increase the size of the opcode?

  • Rely on an opcode prefix byte(s), which makes later added instructions longer?

• There are no easy answers to these challenges for CPU designers!