Parts of the CPU
• Control Unit
• Arithmetic & Logic Unit
• Send out the signals to fetch instructions
• Interpret those instructions
• Send out signals to enable instructions to
be carried out.
• This is known as the fetch execute cycle
Arithmetic & Logic Unit
• Carries out arithmetic calculations
• Carries out logic operations (AND/OR)
• Temporary storage locations in processor
• Store data during calculations
• Store the address of the next instruction to
• Hold the instruction as it is being decoded
• Program Counter (PC) - Stores the memory address of
the next program instruction
• Instruction Register (IR) - Holds the code for the current
• Accumulator (ACC) - Holds values before and after
processing by the ALU
• Memory Data Register (MDR) - Holds data prior to a
memory write or after a memory read.
• Memory Address Register (MAR) - Holds the address of
the memory location to be accessed.
• Address Bus
• Data Bus
• Control Bus
• Other internal buses which transfer data
inside the CPU
• used by the processor to pinpoint the
memory location, needed
• this is a one-way bus.
• used to transfer the data
• its size will usually match the word size i.e.
the size of the memory locations
• this is a two-way bus.
• initiates and controls whatever process is taking
• Read Line – initiates memory read operation.
• Write Line - initiates memory write operation.
• Clock – Generates pulse to synchronise components.
• Interrupt – Signal to processor of an interrupt like a key press
or mouse click. Processor saves stack and deals with the
• NMI – Non-Maskable Interrupt. Interrupt which cannot be
• Reset – Clears all registers, aborts program and gives control
back to the operating system.
The Fetch Execute Cycle In
Click on the next slide to show the
1 101 1 101
PC IR 2 256
Data Bus 3 241
ACC MDR 5
WRITE LINE 10
Arithmetic Logic Unit Copy contents memory IR
Add onethe the of MDR tois transferred to the
The contents ofprogram counter
Decode the instruction
Execute to instruction
read control line is copied into the MAR
The program counter is activated