Microprocessor Design/Control and Datapath

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Most processors and other complicated hardware circuits are typically divided into two components: a datapath and a control unit. The datapath contains all the hardware necessary to perform all the necessary operations. In many cases, these hardware modules are parallel to one another, and the final result is determined by multiplexing all the partial results.

The control unit determines the operation of the datapath, by activating switches and passing control signals to the various multiplexers. In this way, the control unit can specify how the data flows through the datapath.

The width of the data path ...

There is only one mistake that can be made in a computer design that is difficult to recover from: not providing enough address bits for memory addressing and memory management.
Gordon Bell and Bill Strecker, 1975[1]
Almost any shortcoming in a computer architecture can be overcome except too small an address space; this is the reason that DEC was finally forced to design the VAX ("virtual address extension") as a replacement for the PDP-11.
Hollingsworth, Sachs, and Smith. 1987.[2]

For good code density, you want the ALU datapath width to be at least as wide as the address bus width. Then every time you need to increment an address, you can do it in a single instruction, rather than requiring multiple instructions to manipulate an address one piece at a time.[3][4]

After a person has designed the data path, that person finds all the control signal inputs to that datapath -- all the control signals that are needed to specify how data flows through that datapath.

  • Each general-purpose register needs at least one control signal to control whether it maintains the current value or loads a new value from elsewhere.
  • The ALU needs some control signals to tell it whether to add, subtract, etc.
  • The program counter section needs control signals to tell it whether the program counter gets reloaded with an incremented version of the previous value, or with some completely different branch value.
  • etc.

Once we know what control signals we need to generate, we need to design an Microprocessor Design/Instruction Decoder to generate those signals.


  1. Engineering Education "Today in History" PDP-11 minicomputer introduced by Gordon Bell 2009; referring to "What we learned from the PDP-11" by Gordon Bell and Bill Strecker, 1975. Early version of the PDP-11 had a 16-bit address space. ... also quoted in the book "Electronics"
  2. Hollingsworth, Sachs, and Smith. "The Fairchild Clipper: Instruction set architecture and processor implementation". Section 9.4: "Address Space Size". 1987.
  3. "It seems that the 16-bit ISA hits somehow the "sweet spot" for the best code density, perhaps because the addresses are also 16-bit wide and are handled in a single instruction. In contrast, 8-bitters need multiple instructions to handle 16-bit addresses." -- "Insects of the computer world" by Miro Samek 2009.
  4. "it just really sucks if the largest datum you can manipulate is smaller than your address size. This means that the accumulator needs to be the same size as the PC -- 16-bits." -- Allen "Opcode considerations"