More details of the structure of a CPU

A CPU (processor) consists of a chip on a casing with many connection pins:

The CPU (processor) is inserted into a CPU socket on a mother board (and connects to the motherboard by it pins):

Buses on the motherboard

The motherboard contains wiring (= buses) that connects the CPU to other components (= memory and IO devices) of the computer:

Pins and signal names

  • The pins on a CPU chip are used as inputs and outputs to the CPU.

  • Each pin signals a specific:

      1. data bit
      2. address bit or
      3. a command/status

  • Each pin of the CPU is connected to one wire on the motherboard

  • Every pin on the CPU has a signal name

    E.g.:

      • MREQ (memory request)
      • READ (read operation)
      • etc

Terminology: asserted
 

  • Asserted:

      • We say that a signal is "asserted" if the CPU is requesting the operation that is conveyed by the signal.

      • Important fact:

          • Signal is asserted   ≠   signal value = 1 !!!

          • To assert a request, the manufacturer of the CPU can choose either the value 0 or the value 1 for the signal !!

How to denote that a signal is asserted using the value 1
 

A signal that is asserted when the signal value = 1 is denoted by: SignalName

Example:

How to denote that a signal is asserted using the value 0
 

A signal that is asserted when the signal value = 0 is denoted by: SignalName

Example:

Example of signal names (the 8085A CPU)

Notice that some signals are asserted by 1 (e.g.: INTR) and other signals are asserted by 0 (e.g.: INTA)

The pins on a CPU

There are 3 groups of pins on a CPU:

 

The pins on a CPU

There are 3 groups of pins on a CPU:

The Address pins convey the address of a memory location

The pins on a CPU

There are 3 groups of pins on a CPU:

The Address pins can also convey the address of an IO device

The pins on a CPU

There are 3 groups of pins on a CPU:

The Data pins convey the data transfered between the CPU and memory or an IO device

The pins on a CPU

There are 3 groups of pins on a CPU:

The Control pins convey command/status info between the CPU and memory/IO device

Categories of signals in the control pin group

  • Bus control:

      • output pins of the CPU that are used to send commands to the memory and IO devices

  • Processor status:

      • output pins given status information of the CPU (allows memory/IO-debice to check if the CPU is ready or not)

  • Interrupt control:

      • Signals used by IO devices to synchronize with the CPU (discussed later)

  • Bus arbitration:

      • Signals used by CPU and IO devices to request/acquire the use of the data bus for transfer

  • Co-processor signaling: used in older processer to communicate with a (Math) co-processor

Sample (M68000) Processor and its signals

This is the data sheet of the M68000 CPU:

Notice the control signals on the data sheet