CPT4 - Notes from Past Papers

Past Papers used:

- Jan 07

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Binery Representation

HEXDECIMAL

Hex translate set of 4 binery at a time, e.g. 0000

Each set of 4 can be represented by: 1,2,3,4,5,6,7,8,9,A,B,C,D,E,F

E.g. 0001 1110 = 1E

TWO'S COMPLEMENT

0 = Positive 1 = Negative

If number is positive (Starts with a 0) the number is translated as pure binery.

If number is negative (Starts with a 1) the number is inverted (0 to 1's, 1 to 0's) and then translated as pure binery with 1 added to the result.

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Fetch/Execute Cycle

The stages of the fetch/execute cycle are:

- Processor copies the contents of the program counter into the MAR.

- Instruction is transferred via the data bus to the MBR.

- The contents of the MBR are copied into the CIR.

- Instruction in the CIR is decoded.

- Program Counter is updated to address the next instruction.

- The instruction is executed.

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Interrupt Handling

The Interrupt Handling Process is as follows:

- INstruction being executed finishes execution.

- Other interrupts are disabled.

- Contents of Program Counter are stored into memory.

- Contents of all other registers stored in memory.

- Source of interrupt identified.

- Program Counter is loaded with the address of the appropiate ISR.

- ISR is executed.

- Contents of registers are retrieved.

- Program counter is restored from memory.

- Interrupts are restored.

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Examples of Interrupts

An interrupt could be:

- A mouse button click.

- A disc tranfer has been completed.

- A timer needs to be updated.

- A program has performed an illegal operation.

- A piece of hardware has failed.

In each case an interrupt signal is generated and sent along a control wire to the processor.

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Batch and Interactive Processing

These are two modes of operation of a computer system.

BATCH Processing continues from beginning to end without user intervention, the process is delayed until all data has been entered.

A batch process may be controlled be Job Control Language (JCL), the JCL contains a batch header, the following items may be contained in the batch header: JobID, Priority, Job completion time, main memory required, name of file to be executed, devices/hardware required.

INTERACTIVE User and computer in two way communication, processing is carried out as the user enters the data.

Interactive process are given higher priority than Batch processing, this is because the user requires an immediate response.

Other features that may effect the priority of a process are: proximity of the end of a job, proximity of the deadline for a batch process, time waiting in inactive list, resources required,estimated running time.

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