Editing Accumulator (computing) (section)

== Basic concept ==
Mathematical operations often take place in a stepwise fashion, using the results from one operation as the input to the next. For instance, a manual calculation of a worker's weekly payroll might look something like:

# look up the number of hours worked from the employee's time card
# look up the pay rate for that employee from a table
# multiply the hours by the pay rate to get their basic weekly pay
# multiply their basic pay by a fixed percentage to account for income tax
# subtract that number from their basic pay to get their weekly pay after tax
# multiply that result by another fixed percentage to account for retirement plans
# subtract that number from their basic pay to get their weekly pay after all deductions

A computer program carrying out the same task would follow the same basic sequence of operations, although the values being looked up would all be stored in computer memory. In early computers, the number of hours would likely be held on a [[punched card|punch card]] and the pay rate in some other form of memory, perhaps a [[magnetic drum]]. Once the multiplication is complete, the result needs to be placed somewhere. On a "drum machine" this would likely be back to the drum, an operation that takes considerable time. Then the very next operation has to read that value back in, which introduces another considerable delay.

Accumulators dramatically improve performance in systems like these by providing a scratchpad area where the results of one operation can be fed to the next one for little or no performance penalty. In the example above, the basic weekly pay would be calculated and placed in the accumulator, which could then immediately be used by the income tax calculation. This removes one save and one read operation from the sequence, operations that generally took tens to hundreds of times as long as the multiplication itself.