Q.What is an accumulator?

Every microprocessor or microcontroller is organised into a small set of cooperating internal blocks, and understanding each one is the foundation of the whole chapter.

The Arithmetic and Logic Unit (ALU) performs arithmetic (add, subtract) and takes logical decisions. The accumulator is the special working register tied to the ALU: it holds one number, and when a second number arrives it can add the two and store the result; it can also be cleared or complemented. General working registers provide fast short-term storage for a single word of data.

Two address-holding registers control the flow of a program. The Program Counter (PC) holds the memory address of the next instruction to be fetched and executed, so the CPU always knows where it is in the program. The Stack Pointer (SP) holds an address in internal RAM that marks the top of the stack, a region used to save and restore data quickly during calls and interrupts.

Two support circuits keep everything working together. The clock circuit generates a steady train of pulses that synchronise all internal operations, so each step happens in the correct order. The interrupt circuit can break the normal program flow to respond to an urgent event and then let the program resume from exactly the point it left off. Together these blocks let the processor fetch, decode, execute and sequence instructions reliably.