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Structure and role of the processor and its components
Official AQA 7517 section 4.7.3.
0
Objectives
10
Flashcards
10
Questions
90 min
Study time
AqaA LevelComputer ScienceFundamentals of computer organisation and
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The processor and its components1 objectives
- The processor and its components: Explain the role and operation of a processor and its major components: • arithmetic logic unit • control unit • clock • general-purpose registers • dedicated registers, including: • program counter • current instruction register • memory address register • memory buffer register • status register.
The Fetch-Execute cycle and the role of registers within it1 objectives
- The Fetch-Execute cycle and the role of registers within it: Explain how the Fetch-Execute cycle is used to execute machine code programs including the stages in the cycle (fetch, decode, execute) and details of registers used.
The processor instruction set1 objectives
- The processor instruction set: Understand the term ‘processor instruction set’ and know that an instruction set is processor specific. Know that instructions consist of an opcode and one or more operands (value, memory address or register). A simple model will be used in which the addressing mode will be incorporated into the bits allocated to the opcode so the latter defines both the basic machine operation and the addressing mode. Students will not be expected to define opcode, only interpret opcodes in the given context of a question. For example, 4 bits have been allocated to the opcode (3 bits for basic machine operation, eg ADD, and 1 bit for the addressing mode). 4 bits have been allocated to the operand, making the instruction, opcode + operand, 8 bits in length. In this example, 16 different opcodes are possible (24 = 16).
Addressing modes1 objectives
- Addressing modes: Understand and apply immediate and direct addressing modes. Immediate addressing: the operand is the datum. Direct addressing: the operand is the address of the datum. Address to be interpreted as meaning either main memory or register. 78
Machine-code/assembly language operations1 objectives
- Machine-code/assembly language operations: Understand and apply the basic machine-code operations of: • load • add • subtract • store • branching (conditional and unconditional) • compare • logical bitwise operators (AND, OR, NOT, XOR) • logical • shift right • shift left • halt. Use the basic machine-code operations above when machine-code instructions are expressed in mnemonic form- assembly language, using immediate and direct addressing.
Interrupts1 objectives
- Interrupts: Describe the role of interrupts and interrupt service routines (ISRs); their effect on the Fetch-Execute cycle; and the need to save the volatile environment while the interrupt is being serviced.
Factors affecting processor performance1 objectives
- Factors affecting processor performance: Explain the effect on processor performance of: • multiple cores • cache memory • clock speed • word length • address bus width • data bus width.
Key terms
General-purpose registerDedicated registerFetch-Execute cycleCurrent instruction registerProcessor instruction setOperandAddressing modeMnemonicDirect addressingInterruptInterrupt service routine (ISR)Word length
Exam tips
- Use precise register descriptions: State both what a register stores and when it is used.
- Describe register transfers precisely: When explaining the fetch stage, state what is stored in each register and describe the order in which the address and instruction move.
Common mistakes
- Confusing the MAR and MBR: The memory address register stores the address of the memory location being accessed. The memory buffer register temporarily stores the data or instruction being transferred.
- Confusing the PC and MAR: The PC stores the address of the next instruction to be fetched. The MAR stores the address of the memory location currently being accessed.
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