(NOTE: not the same scale as the CPU ! The motherborad is a lot bigger !)
Some pins signals a specific function/operation, and every signal pin has a signal name, e.g., MREQ (memory request), READ, etc
To signal a request, the manufacturer of the CPU can choose 0 or 1 value for that signal.
For example, the signal MREQ means "Memory Request", i.e., the CPU requests a transfer operation with the memory. The manufacturer of the CPU can choose
- MREQ = 1 to indicate that the CPU wants to request a memory transfer operations, OR
- MREQ = 0 to indicate that the CPU wants to request a memory transfer operations
To denote the fact that MREQ = 1 to indicate that the CPU asserts the memory request, the MREQ signal is written on the spec (specification) sheet as:
On the other hand, if the manufacturer of the CPU decides that MREQ = 0 indicates that the CPU asserts the memory request, the MREQ signal is written on the spec sheet as:
- Bus control: output oins of the CPU that are used to tell the memory
and IO devices which operation they should perform.
The most important signals are:
- MREQ: if asserted, the CPU is making a request to the memory, and if not asserted, the CPU is not making a request to the memory.
- IOREQ: if asserted, the CPU is making a request to an IO device and if not asserted, the CPU is not making a request to any IO device.
- READ: if asserted, the CPU is making a read request and if not asserted, the CPU is making a write request.
- Note: the CPU may not any read/write requests at all. In this case, both MREQ and IOREQ are not asserted.
- Status: output pins indicating the current status of the CPU
(allows the memory and IO devices to check on the state of the CPU)
- Interrupt: pins for IO devices to synchronize with the CPU.
The most important signals are:
- IREQ = interrupt request. Signal sent by an IO device to request the attention of the CPU
- IACK = interrupt acknowledge. Signal sent by the CPU to signal the IO device that it is ready to serve the IO device
- Bus arbitration:
pins used by CPU to request/acquire the right to use the system bus
Every device connected to the system bus (including the CPU) must first make a request to use the system bus before it can actually use it. This must be done because it is not possible to allow multiple devices to transmit their electrical signals simultaneously on the system bus.
- Co-processor signaling: signal used by the CPU to communicate with its "co-processor" (e.g., math co-processor that perform complex calculations).