Microprocessors
Functions performed and overall operation of a microprocessor; the fetch–decode–execute cycle, the register set, interrupts and clocking. Examined for Category B2.
Microprocessors — summary notes
Free- A microprocessor executes a stored program by repeating the FETCH → DECODE → EXECUTE cycle, driven by the system clock. One pass through the cycle handles one instruction.
- FETCH: the program counter (PC) puts the address of the next instruction on the address bus; the instruction returns on the data bus into the instruction register (IR); the PC increments.
- DECODE: the control unit interprets the opcode in the IR and sets up the internal data paths. EXECUTE: the ALU performs the operation and the result goes to a register or memory.
- Key registers: the accumulator (holds ALU operands and results), the program counter (address of the NEXT instruction), the instruction register (the current instruction), the stack pointer (top of the stack) and the status/flag register (zero, carry, negative, overflow).
- Interrupts break into the cycle: the current state is pushed onto the stack, the processor services the interrupt, then restores state and resumes — this is how a processor responds to time-critical aircraft events without polling.
- Clock speed sets the instruction rate; word length (8, 16, 32 bit) sets how much data is handled per operation.
- ⚠ Exam trap: the program counter holds the address of the NEXT instruction, not the current one — the instruction register holds what is being executed now.
- Instruction cycle
- FETCH → DECODE → EXECUTE (repeat)
- Clock period
- T = 1 ÷ f
During a fetch, which bus carries the address and which carries the instruction?
The address bus carries the address from the program counter to memory (unidirectional, CPU outward); the instruction itself returns to the CPU on the bidirectional data bus, and the control bus asserts the read line that makes it happen.
Why must the processor save its registers when an interrupt occurs?
So the interrupted program can be resumed exactly where it stopped. The current PC and status are pushed onto the stack before the interrupt service routine runs, and popped back afterwards — without that, the return would resume with corrupted state.
Microprocessors concept map
FreeMicroprocessors
Microprocessors quiz
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