(NOTE: not the same scale as the CPU ! The motherborad is a lot bigger ! The CPU will fit in the white square on the lower right hand corner)
The mother board contains wiring to connect the CPU to other components of the computer:
Each pin signals a specific data, function or operation
Every signal pin has a signal name, e.g., MREQ (memory request), READ, etc
To assert a request, the manufacturer of the CPU can choose either the value 0 or the value 1 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
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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:
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You can see that some pins are asserted on high (= 1) (such as INTR), while other pins are asserted on low (= 0) (such as INTA) !!!
(Just remember that "Asserted" mean: "the function/request was activated")
- Address pins are used to
convey the
value of the
memory address
(and also the address of an
I/O device as will will
see later)
- Data pins are used to
convey the
data on the
data bus
- The 3rd group are
called control pins --- because
they carry
control signals between
the CPU and
memory/IO device.
We will discuss these pins next...
Control pins: (categorized)
- Bus control:
output pins 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
- IOREQ: if asserted, the CPU is making a request to an 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).
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- Here is a copy of the
M68000 data sheet:
click here
- The signals from the M68000
are given by the following diagram:
