16. Reducing x86 ISA to a Simplified Version

• Advances in computer architecture technology since 1978 made encoding of x86 instructions quite complex and somewhat illogical.

• Despite the lack of simplicity, x86 ISA well deserves studying its design and encoding.

• To cope with x86 complexity, let's pretend that we deal with a simplified version of the CPU:

  • there are only four 16-bit registers: AX, BX, CX, and DX

    • (therefore, register operands can be encoded with just two bits.)

  • the address bus is 16-bit with a maximum of 65,536 bytes of addressable memory.

  • there are only 20 instructions:

    • MOV (with two forms), ADD, SUB, CMP, AND, OR, NOT,
      JE, JNE, JB, JBE, JA, JAE, JMP,
      BRK, IRET, HALT, GET, and PUT.