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]

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.[2][3]


  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. "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.
  3. "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"