28. ISA Design Considerations

• Instruction set architecture design that can stand the test of time is a true intellectual challenge.

• It takes several compromises between space and efficiency to assign opcodes and encode instruction formats.

• Today people are using Intel x86 instruction set for purposes never intended by original designers.

• Extending the CPU is a very difficult task.

• The instruction set can become extremely complex.

• If x86 CPU was designed from scratch today, it would have a totally different ISA!

• Software developers usually don't have a problem adapting to a new architecture when writing new software...

  • ...but they are very resistant to moving existing software from one platform to another.

• This is the primary reason the Intel x86 platform remains so popular to this day.