Microprogrammed control

1
Microprogrammed control
2
Other variations of datapath

• Single bus design
• control is very simple
• but need more states in control
• downside?
• Dual bus design
• more things done in same clock
• always desirable?

3
Microprogramming

• More instructions gt more complex control
• e.g. x86 could require 1000s of states in FSM
• find a different representation for the FSM
  instead of circles and arcs!
• Control signal specification as an instruction
• microinstruction to disambiguate it from
  instruction set

4
Microinstruction

• Every instruction
• set of control signals to be activated
• specify next microinstruction (equivalent of next
  state of FSM)
• sequencing of microinstructions
• is the next microinstruction statically
  determinable?
• Analogous to control flow in high level languages

5
Microinstruction Format

• How to design?
• Share fields for mutually exclusive fields of
  control
• separate fields for control simultaneously active
• ALU, SRC1, SRC2, GPR control, memory actions,
  writes of other CPU registers, sequencing
• sequencing
• increment
• unconditional branch (e.g. back to F1 in FSM)
• dispatch (e.g. multi-way based on IR decode)

6

• Example microinstruction format
• label ALU, SRC1, SRC2, GPR control, memory
  control, PCwrite, sequence
• ALU add, sub, func
• SRC1 PC, A
• SRC2 B, 1, IR3-0SE, IR7-0
• GPR write ALU, read, write MDR
• memory read instr, read operand, write operand
• PCwrite ALU, ALUout-cond, none
• sequence mpc1, br, dispatch

7

• example microinstruction sequence
• F1
• add, PC, 1, -, read instr, ALU, seq
• F2
• add, PC, 1, -, read instr, ALU, seq
• IDRF
• -, -, -, read, none, dispatch
• implement other states of FSM similarly