A-level Computer Science₇₅₁₇

Have an understanding and knowledge of the basic internal components of a computer system.

Although exam questions about specific machines will not be asked, it might be useful to base this section on the machines used at the centre.

Understand the role of the following components and how they relate to each other:

• processor
• main memory
• address bus
• data bus
• control bus
• I/O controllers.

Understand the need for, and means of, communication between components. In particular, understand the concept of a bus and how address, data and control buses are used.

Be able to explain the difference between von Neumann and Harvard architectures and describe where each is typically used.

Embedded systems such as digital signal processing (DSP) systems use Harvard architecture processors extensively.

Von Neumann architecture is used extensively in general purpose computing systems.

Understand the concept of addressable memory.

Be able to describe the stored program concept: machine code instructions stored in main memory are fetched and executed serially by a processor that performs arithmetic and logical operations.

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.

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.

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).

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.

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.

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.

Explain the effect on processor performance of:

• multiple cores
• cache memory
• clock speed
• word length
• address bus width
• data bus width.

Know the main characteristics, purposes and suitability of the devices and understand their principles of operation.

Devices that need to be considered are:

• barcode reader
• digital camera
• laser printer
• RFID.

Explain the need for secondary storage within a computer system.

Know the main characteristics, purposes, suitability and understand the principles of operation of the following devices:

• hard disk
• optical disk
• solid-state disk (SSD).

SSD = NAND flash memory + a controller that manages pages, and blocks and complexities of writing. Based on floating gate transistors that trap and store charge. A block, made up of many pages, cannot overwrite pages, page has to be erased before it can be written to but technology requires the whole block to be erased. Lower latency and faster transfer speeds than a magnetic disk drive.

Compare the capacity and speed of access of various media and make a judgement about their suitability for different applications.