Accumulator (computing)

Accumulator (computing)

In a computer's central processing unit (CPU), an accumulator is a register in which intermediate arithmetic and logic results are stored. Without a register like an accumulator, it would be necessary to write the result of each calculation (addition, multiplication, shift, etc.) to main memory, perhaps only to be read right back again for use in the next operation. Access to main memory is slower than access to a register like the accumulator because the technology used for the large main memory is slower (but cheaper) than that used for a register.

The canonical example for accumulator use is summing a list of numbers. The accumulator is initially set to zero, then each number in turn is read and added to the value in the accumulator. Only when all numbers have been added is the result held in the accumulator written to main memory or to another, non-accumulator, CPU register.

An accumulator machine, also called a 1-operand machine, or a CPU with accumulator-based architecture, is a kind of CPU where, although it may have several registers, the CPU mostly stores the results of calculations in one special register, typically called "the accumulator". Historically almost all early computers were accumulator machines; and many microcontrollers still popular as of 2010 (such as the 68HC12, the PICmicro, the 8051 and several others) are basically accumulator machines.

Modern CPUs are typically 2-operand or 3-operand machines—the additional operands specify which one of many general purpose registers (also called "general purpose accumulators") are used as the source and destination for calculations. These CPUs are not considered "accumulator machines".

The characteristic which distinguishes one register as being the accumulator of a computer architecture is that the accumulator (if the architecture were to have one) would be used as an implicit operand for arithmetic instructions. For instance, a CPU might have an instruction like:

This instruction would add the value read from the memory location at memaddress to the value from the accumulator, placing the result in the accumulator. The accumulator is not identified in the instruction by a register number; it is implicit in the instruction and no other register can be specified in the instruction. Some architectures use a particular register as an accumulator in some instructions, but other instructions use register numbers for explicit operand specification.