What is a micro-instruction?

A micro-instruction is a simple command that makes the hardware operate properly. The format is unique to each computer, but our example has a 24 bit micro-instruction. Our instruction is broken down into nine parts:

• Bits 0-6: ADDR This part of the micro-instruction is used to specify a point in the code to jump to next. Whether or not the jump is made depends upon the result of the ALU operation (see bits 21-23) and the COND field (see bits 7-8). It is just the number of another micro-instruction in the control store.
• Bits 7&8: COND This part of the micro-instruction determines whether a jump in the code should be made, or if simply the next instruction is to be executed. It can have the following values:
  
  • 00: Never jump.
  • 01: Jump if the ALU's N bit is 1.
  • 10: Jump if the ALU's Z bit is 1.
  • 11: Always jump.
• Bits 9-11: A This part of the micro-instruction is used to determine what register will be the first input to the ALU. It is simply a number from zero to seven.
• Bits 12-14: B This part of the micro-instruction is used to determine what register will be the second input to the ALU. If the second input is not used, this field's value does not matter.
• Bits 15-17: C This part of the micro-instruction is used to determine where the output from the ALU is stored, and again is just the number of a register.
• Bit 18: ENC This part of the micro-instruction is used to determine whether or not the ALU's output is stored in the register specified by the C field (see bits 15-17). If the instruction were just testing to see if a number was zero, for example, the result would not need to be stored anywhere, only the Z flag from the ALU would be important.
• Bit 19: WR This part of the micro-instruction is used to determine whether or not the contents of the MBR register are written to RAM. It also uses the MAR register to determine which address to write to. Every write takes two cycles (in our example), so a write started in one instruction must be continued in the next.
• Bit 20: RD This part of the micro-instruction is used to determine if the MBR register is filled with a word from RAM. It uese the MAR register to determine which address to read from. Like a write, a read also requires two cycles.
• Bits 21-23: ALU This part of the micro-instruction is used to determine what operation the ALU performs on its inputs.
  
  • 000: Do nothing, just output the first input
  • 001: NOT
  • 010: Left Shift
  • 011: Right Shift
  • 100: Add
  • 101: Subtract
  • 110: AND
  • 111: OR