Buses

FETCH

The memory address of the next instruction from the MAR is placed on the address bus. The read line of the control bus is activated The data is placed onto the data bus and transferred to the MDR (which stores it until executed)

EXECUTE

The processor carries out the instruction

FETCH-EXECUTE CYCLE

This describes how the processor gets instructions from memory (RAM) and carries them out.

DATA BUS

Used to transfer data between parts of the CPU and other parts of the computer. The width of the data bus is measured by the number of lines on the bus. Each line can carry one bit. A 32 bit bus can transfer 32 bits at a time.

CLOCK LINE

a regular pulse to time operations. A clock of 600MHz (megahertz) generates a pulse 600,000,000 times a second.

RESET LINE

clear the CPU of data and fetch instructions from known location.

READ LINE

to start a memory read once the address is on the address bus.

CONTROL BUS

The control bus is different from the other 2 in that it is a series of separate lines each with a different purpose which may operate at different times.

DECODE

The control unit decodes the instruction

NMI

(Non Maskable Interrupts) - requests from external devices that cannot be ignored.

WRITE LINE

- to start a memory write once the address is on the address bus and the data is on the data bus.

ADDRESS BUS

Holds the address of a memory location being accessed The more lines on the address bus the more locations the system can address. Maximum number of addresses = 2 width of address bus

MEMORY READ OPERATION

set up the address bus with address of the memory location to be accessed by storing in MAR activate the read line on the control bus Place the data onto the data bus and transfer along to the CPU.

MEMORY WRITE OPERATION

set up the address bus with the address of the memory location by placing it in MAR set up the data bus with the data by placing it onto the MDR activate the write line on the control bus the data will now be transferred along the data bus into the correct memory location.

INTERRUPT

signals from external devices e.g. printers, disk drives that require servicing. Some can be ignored.