3.2.1 Outline the structure of the central processing unit (CPU) including
the functions of the control unit (CU), the arithmetic and logic unit
(ALU), primary memory and address buses.
Teaching Note:
Students are expected to be able to reproduce a basic diagram
illustrating the CPU and to know that each location in primary
memory has a unique address.
Sample Question:
Draw and label a basic diagram of a CPU. [4 marks]
JSR Notes:
First, from the teaching notes, do note that - uncharacteristically for this syllabus - you are expected to reproduce an actual diagram. And remember that the diagram in the textbook is not quite correct in that the ALU and the Control Unit are the parts that are within the CPU itself, not all the other things under the "Primary Memory" section. Though note that often times there is some cache in the CPU itself - this is called Level 1 cache. Meantime most cache of it is not built into the CPU, yet is close by - this is called Level 2 cache.
You can find lots of other CUP Diagrams on the Internet, and though they vary hugely in composition and detail, they all do have in common the two main parts: control unit and arithmetic logic unit.
In terms of the teaching note about knowing that each memory address is unique, the idea is that this must be the case, or data would get confused. And in a 32 bit system, since the registers that hold the addresses are 32 bits, the number of unique addresses that can be generated is 2 ^ 32, or around 4 billion. This is why in 32 bit systems, more than 4 GB of RAM memory cannot be addressed. In 64 bit computers - common today - the limit is 64 ^ 2 unique addresses, i.e. 16 billion billion!
