Almost every computer you have used follows a design described in 1945: one memory holds both the program and its data, and a processor fetches and runs the instructions one by one. Learn the parts of that processor and the job of every register inside it.
In the Von Neumann model, a computer has a single processor and one memory. The stored program concept means that program instructions and data are held together in the same main memory, and instructions are fetched from it and executed one at a time, in sequence. Inside the processor, the control unit (CU) decodes instructions and sends control signals, the arithmetic and logic unit (ALU) does calculations and comparisons, the system clock times every step, and registers hold the values in use. A special purpose register has one fixed job (PC, MAR, MDR, CIR, IX, status register); a general purpose register holds any data being processed (the accumulator, ACC).
Click each part in the lab to see what it does. The Von Neumann model is the reason a computer can switch from a game to a spreadsheet: the program is just more data loaded into memory.
"Data and programs are stored in memory" is not enough to describe the stored program concept. Say they are stored together in the same memory and that instructions are fetched and executed one at a time.
Three components inside the processor do the work, and they all depend on the memory beside it. Exam questions ask for their purpose, so learn one precise sentence for each.
| Component | Purpose |
|---|---|
| Arithmetic and logic unit (ALU) | Carries out arithmetic (such as addition and subtraction), logic operations (AND, OR, XOR), shifts and comparisons. |
| Control unit (CU) | Decodes the instruction in the CIR and sends control signals along the control bus to manage the transfer of data and the order of operations. |
| System clock | Produces regular timing pulses that synchronise all the components, so every step of the cycle happens in step. |
| Immediate access store (IAS) | Holds the instructions and data of the programs currently running, so the processor can fetch them directly. |
The control unit does not do calculations. It decodes and controls; the ALU calculates and compares.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What does the stored program concept say?
In the lab, which register holds the address of the next instruction?
Which register is a general purpose register in the 9618 model?
What is the purpose of the system clock?
A register is a very fast storage location inside the processor that holds one value, such as an address, an instruction or a data item. The 9618 syllabus names seven.
| Register | Type | Purpose |
|---|---|---|
| Program counter (PC) | Special | Holds the address of the next instruction to be fetched. |
| Memory address register (MAR) | Special | Holds the address of the memory location about to be read from or written to. |
| Memory data register (MDR) | Special | Holds the data or instruction just read from, or about to be written to, memory. |
| Current instruction register (CIR) | Special | Holds the instruction currently being decoded and executed. |
| Index register (IX) | Special | Holds a value that is added to an address in indexed addressing, for example to step through an array. |
| Status register (SR) | Special | Holds individual bits (flags) showing the outcome of an operation, such as carry, negative, overflow and zero, and whether an interrupt has occurred. |
| Accumulator (ACC) | General | Holds the data being processed and the results of ALU operations. |
General purpose does not mean "used for everything at once". A general purpose register holds whatever data the program is working on; a special purpose register always has the same fixed job.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Which register holds a flag that is set when the result of an addition is zero?
What is the immediate access store?
Which statement about the MAR and MDR is correct?
Which register is used to form the address in indexed addressing?
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