4.9 Instruction Words

• A program consists of a sequence of binary-coded instructions that the CPU must fetch from memory, decode and execute
• For most computers. instruction words convey two types of info:
• the operation to be performed (op code)
• the address of the operand (data) that is to be operated on (operand address)
• Three basic 68HC11 instruction formats:
• single byte (8 bit op code, no operand address)
• two byte (8 bit op code, 8 bit operand address)
• three byte (8 bit op code, low-order byte operand address, high-order byte operand address)
• Exception: Instructions that use the Y register will have a prebyte precedes the op code byte (indicate internal Y register usage)
• Some instructions will have 2-byte op code: a prebyte and the op code