The Stored Program Concept and the Processor

Overview

Recall questions on what makes a machine a stored-program computer, why the idea mattered, and the components of the processor.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What does it mean to say a computer is a stored-program machine?

   Model answer

   A program is not built into the wiring of the machine; it is data. Its machine code instructions are placed in main memory alongside the data they operate on, and the processor works through them serially — fetching each instruction from memory in turn, decoding it, and carrying out the arithmetic or logical operation it names before moving on to the next. Because instructions sit in ordinary memory, a different program can be loaded and run without rewiring anything, which is exactly what makes a computer general purpose rather than a single-purpose device; it is also why a program can in principle read or alter its own instructions.

2. 2

   Why was the stored program concept such an important idea?

   Model answer

   Before it, a machine was set up for a task by physically rewiring it or resetting switches, which took hours or days. Holding the program in memory means the machine's task can be changed simply by loading a different set of instructions, in a fraction of a second. That is what makes a computer general purpose rather than a single-purpose calculating machine, and it is why one processor can run a word processor, a compiler and a game.

3. 3

   In a stored program machine, how does the processor tell an instruction from a piece of data, given that both are just binary in the same memory?

   Model answer

   It cannot tell them apart by looking at them: a byte in memory has no label saying which it is. The distinction is made entirely by how the value is used. A value fetched during the fetch stage, at the address held in the program counter, is treated as an instruction; a value fetched during execution because an instruction asked for it is treated as data. This is why a program that jumps into the middle of its own data will try to execute nonsense, and why one consequence of the concept is that a program can be treated as data, which is what makes compilers and loaders possible.

4. 4

   What does 'executed serially' mean in the stored program concept?

   Model answer

   It means the processor executes one instruction at a time, in sequence, taking the next instruction from the following memory location unless the instruction just executed was a branch that changed the program counter. The order of execution is therefore determined by the order of instructions in memory, modified by branches, and not by anything in the instructions themselves running side by side.

5. 5

   Name the major components of a processor.

   Model answer

   The arithmetic logic unit, the control unit, the clock, a set of general-purpose registers, and the dedicated registers: the program counter, the current instruction register, the memory address register, the memory buffer register and the status register. The buses inside the processor connect them.

6. 6

   Explain the role and operation of the arithmetic logic unit.

   Model answer

   The arithmetic logic unit is the part of the processor that actually performs calculations and comparisons. It carries out arithmetic operations such as addition, subtraction and shifting, which is how multiplication and division by powers of two are done, and logical operations such as AND, OR, NOT, XOR and comparisons. It takes its operands from registers, performs the operation selected by the control unit, places the result in a register, and sets the flags in the status register according to what the result was.

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