1.1.1 Architecture - Registers within the CPU Flashcards
What is a register and what do they do in the CPU?
A register is a small and fast type of memory in the CPU. They hold temporary data while a software program is running.
What are the three main buses in the CPU?
The Data Bus
The Address Bus
The Control Bus
What does the Data Bus carry and where to and from?
The data bus carries both instructions and data around the system, for example from the CPU to RAM or to the graphics card.
What does the Address Bus carry and where to and from?
The Address Bus carries storage addresses of data or instructions from the Memory Address Register to main memory.
What does the Control Bus carry and where to and from?
The Control Bus carries signals around the system to co-ordinate actions within the CPU and also control external components such as the hard disk, graphics card etc.
What is the purpose of the accumulator?
The accumulator temporarily stores various pieces of data within the CPU.
This includes any data that has been processed or results of calculations made by the ALU.
Why is the accumulator so important?
The accumulator is a register, which means it is the fastest type of memory. This means it can be accessed much quicker than the main memory - which is where the data would be stored otherwise.
When an address specified by the MAR is examined, where are its contents copied into?
The contents are copied into the MDR, which holds them temporarily.
What is the purpose of the MAR?
The MAR temporarily stores the address of the next data or instruction in main memory.
What is the purpose of the CIR?
The Current Instruction Register holds the current instruction ready for decoding and executing by the Control Unit.
What are the two parts of the instructions that are held in the CIR?
The operation code (opcode) - machine code that has to be carried out by the CPU
The operand - the second part of the instruction that the opcode acts upon. It contains data, a memory address or both.
What is opcode?
Operation code is machine code that has to be carried out by the CPU. It is one of the two parts that makes up instructions.
What is the operand?
The operand is the second part of instructions, which contains data, a memory address or both. Opcode acts upon the operand.
What is the purpose of the PC?
The program counter stores the location of the next instruction.
When it is time for the next instruction to be fetched, the content of the PC is loaded into the MAR. It is then incremented (in a liner program).
What is the alternative to incrementing the PC after a cycle?
If the program is not linear, the PC can be made to jump to another location entirely. This is called a branch instruction.
