Editing Accumulator (computing) (section)

==Accumulator machines==
An '''accumulator machine''', also called a [[Instruction_set_architecture#Number_of_operands|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". Almost all {{clarify|text=early|reason=To what years does "early" refer|date=July 2021}} computers were accumulator machines with only the high-performance "[[supercomputer]]s" having multiple registers. Then as [[mainframe computer|mainframe]] systems gave way to [[microcomputer]]s, accumulator architectures were again popular with the [[MOS Technology 6502|MOS 6502]] being a notable example. Many [[8-bit computing|8-bit]] [[microcontroller]]s that are still popular {{as of|2014|lc=y}}, such as the [[PIC microcontrollers|PICmicro]] and [[Intel 8051|8051]], are accumulator-based machines.

Modern CPUs are typically 2-operand or 3-operand machines. The additional operands specify which one of many [[general-purpose register]]s (also called "general-purpose accumulators"<ref>{{cite web|url=http://www.freescale.com/webapp/sps/site/overview.jsp?code=DRMCRHC16OV&srch=1 |title=HC16 Overview |publisher=Freescale.com |access-date=2008-09-22 |url-status=dead |archive-url=https://web.archive.org/web/20070928031937/http://www.freescale.com/webapp/sps/site/overview.jsp?code=DRMCRHC16OV&srch=1 |archive-date=28 September 2007 }}</ref>) are used as the source and destination for calculations. These CPUs are not considered "accumulator machines".

The characteristic that 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 [[instruction (computer science)|instruction]]s. For instance, a CPU might have an instruction like: <code>ADD ''[[memory address|memaddress]]''</code> that adds the value read from memory location ''memaddress'' to the value in the accumulator, placing the result back in the accumulator. The accumulator is not identified in the instruction by a register number; it is [[addressing_mode#Implicit|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.