accumulator

processor In a central processing unit, a register in

which intermediate results are stored. Without an

accumulator, it would be necessary to write the result of each

calculation (addition, multiplication, shift, etc.) to mainmemory and read them back. Access to main memory is slower

than access to the accumulator which usually has direct paths

to and from the arithmetic and logic unit (ALU).

The canonical example is summing a list of numbers. The

accumulator is set to zero initially, each number in turn is

added to the value in the accumulator and only when all

numbers have been added is the result written to main memory.

Modern CPUs usually have many registers, all or many of which

can be used as accumulators. For this reason, the term

"accumulator" is somewhat archaic. Use of it as a synonym for

"register" is a fairly reliable indication that the user has

been around for quite a while and/or that the architecture

under discussion is quite old. The term in full is almost

never used of microprocessor registers, for example, though

symbolic names for arithmetic registers beginning in "A"

derive from historical use of the term "accumulator" (and not,

actually, from "arithmetic"). Confusingly, though, an "A"

register name prefix may also stand for "address", as for

example on the Motorola 680x0 family.

2. programming A register, memory location or variable being

used for arithmetic or logic (as opposed to addressing or a

loop index), especially one being used to accumulate a sum or

count of many items. This use is in context of a particular

routine or stretch of code. "The FOOBAZ routine uses A3 as an

accumulator."

[Jargon File]

(1999-04-20)