Accumulator (computing)
Encyclopedia
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 microcontroller
s 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:
ADD memaddress
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.
Just a few of the instructions are, for example (with some modern interpretation):
No convention exists regarding the names for operations from registers to accumulator and from accumulator to registers. Tradition (e.g. Donald Knuth
's (1973) hypothetical MIX
computer), for example, uses two instructions called load accumulator from register/memory (e.g. "LDA r") and store accumulator to register/memory (e.g. "STA r"). Knuth's model has many other instructions as well.
in 1952, used a single 36-bit accumulator, along with a separate multiplier/quotient register to handle operations with longer results. The IBM 650
, a decimal machine, had one 10 digit accumulator; the IBM 7070
, a later, transistorized decimal machine had three accumulators.
The 12-bit PDP-8
was one of the first minicomputers to use accumulators, and inspired many later machines. The PDP-8 had but one accumulator. The HP 2100
and Data General Nova
had 2 and 4 accumulators. The Nova was created when this follow-on to the PDP-8 was rejected in favor of what would become the PDP-11
. The Nova provided four accumulators, AC0-AC3, although AC2 and AC3 could also be used to provide offset addresses, tending towards more generality of usage for the registers. The PDP-11
introduced a more contemporary model of general registers, numbered R0-R7 or more, adopted by most later CISC and RISC machines.
Early 4-bit and 8-bit microprocessors such as the 4004, 8008 and numerous others, typically had single accumulators. The 8051 microcontroller has two, a primary accumulator and a secondary accumulator, where the second is used by instructions only when multiplying (MUL AB) or dividing (DIV AB); the former splits the 16-bit result between the two 8-bit accumulators, whereas the latter stores the quotient on the primary accumulator A and the remainder in the secondary accumulator B. As a direct descendent of the 8008, the 8080, and the 8086, the modern ubiquitous Intel x86 processors still uses the primary accumulator EAX and the secondary accumulator EDX for multiplication and division of large numbers. For instance, MUL ECX will multiply the 32-bit registers ECX and EAX and split the 64-bit result between EAX and EDX. However, MUL and DIV are special cases, other arithmetic-logical instructions (ADD, SUB, CMP, AND, OR, XOR, TEST) may specify any of the eight registers EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI as the accumulator (i.e. left operand and destination); x86 is thus a fairly general register architecture, despite being based on an accumulator model. The 64-bit extension of x86, x86-64
, has been further generalized to 14 instead of 6 general registers.
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
's central processing unit (CPU
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
), an accumulator is a register
Processor register
In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...
in which intermediate arithmetic and logic
Arithmetic logic unit
In computing, an arithmetic logic unit is a digital circuit that performs arithmetic and logical operations.The ALU is a fundamental building block of the central processing unit of a computer, and even the simplest microprocessors contain one for purposes such as maintaining timers...
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 microcontroller
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
s 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
Computer architecture
In computer science and engineering, computer architecture is the practical art of selecting and interconnecting hardware components to create computers that meet functional, performance and cost goals and the formal modelling of those systems....
is that the accumulator (if the architecture were to have one) would be used as an implicit operand
Operand
In mathematics, an operand is the object of a mathematical operation, a quantity on which an operation is performed.-Example :The following arithmetic expression shows an example of operators and operands:3 + 6 = 9\;...
for arithmetic instructions. For instance, a CPU might have an instruction like:
ADD memaddress
Memory address
A digital computer's memory, more specifically main memory, consists of many memory locations, each having a memory address, a number, analogous to a street address, at which computer programs store and retrieve, machine code or data. Most application programs do not directly read and write to...
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.
History of the computer accumulator
Historical convention dedicates a register to "the accumulator", an "arithmetic organ" that literally accumulates its number during a sequence of arithmetic operations:- "The first part of our arithmetic organ ... should be a parallel storage organ which can receive a number and add it to the one already in it, which is also able to clear its contents and which can store what it contains. We will call such an organ an Accumulator. It is quite conventional in principle in past and present computing machines of the most varied types, e.g. desk multipliers, standard IBM counters, more modern relay machines, the ENIAC" (Goldstine and von Neumann, 1946; p. 98 in Bell and Newell 1971).
Just a few of the instructions are, for example (with some modern interpretation):
- Clear accumulator and add number from memory location X
- Clear accumulator and subtract number from memory location X
- Add number copied from memory location X to the contents of the accumulator
- Subtract number copied from memory location X from the contents of the accumulator
- Clear accumulator and shift contents of register into accumulator
No convention exists regarding the names for operations from registers to accumulator and from accumulator to registers. Tradition (e.g. Donald Knuth
Donald Knuth
Donald Ervin Knuth is a computer scientist and Professor Emeritus at Stanford University.He is the author of the seminal multi-volume work The Art of Computer Programming. Knuth has been called the "father" of the analysis of algorithms...
's (1973) hypothetical MIX
MIX
MIX is a hypothetical computer used in Donald Knuth's monograph, The Art of Computer Programming . MIX's model number is 1009, which was derived by combining the model numbers and names of several contemporaneous, commercial machines deemed significant by the author...
computer), for example, uses two instructions called load accumulator from register/memory (e.g. "LDA r") and store accumulator to register/memory (e.g. "STA r"). Knuth's model has many other instructions as well.
Notable accumulator-based computers
Most of IBM's early binary "scientific" computers, beginning with the vacuum tube IBM 701IBM 701
The IBM 701, known as the Defense Calculator while in development, was announced to the public on April 29, 1952, and was IBM’s first commercial scientific computer...
in 1952, used a single 36-bit accumulator, along with a separate multiplier/quotient register to handle operations with longer results. The IBM 650
IBM 650
The IBM 650 was one of IBM’s early computers, and the world’s first mass-produced computer. It was announced in 1953, and over 2000 systems were produced between the first shipment in 1954 and its final manufacture in 1962...
, a decimal machine, had one 10 digit accumulator; the IBM 7070
IBM 7070
IBM 7070 was a decimal architecture intermediate data processing system that was introduced by IBM in June 1960. It was part of the IBM 700/7000 series, and was based on discrete transistors rather than the vacuum tubes of the 1950s. It was the company's first transistorized stored-program...
, a later, transistorized decimal machine had three accumulators.
The 12-bit PDP-8
PDP-8
The 12-bit PDP-8 was the first successful commercial minicomputer, produced by Digital Equipment Corporation in the 1960s. DEC introduced it on 22 March 1965, and sold more than 50,000 systems, the most of any computer up to that date. It was the first widely sold computer in the DEC PDP series of...
was one of the first minicomputers to use accumulators, and inspired many later machines. The PDP-8 had but one accumulator. The HP 2100
HP 2100
The HP 2100 was a series of minicomputers produced by Hewlett-Packard from the mid-1960s to early 1990s. The 2100 was also a specific model in this series. The series was renamed HP 1000 by the 1970s and sold as real-time computers, complementing the more complex IT-oriented HP 3000, and would be...
and Data General Nova
Data General Nova
The Data General Nova was a popular 16-bit minicomputer built by the American company Data General starting in 1969. The Nova was packaged into a single rack mount case and had enough power to do most simple computing tasks. The Nova became popular in science laboratories around the world, and...
had 2 and 4 accumulators. The Nova was created when this follow-on to the PDP-8 was rejected in favor of what would become the PDP-11
PDP-11
The PDP-11 was a series of 16-bit minicomputers sold by Digital Equipment Corporation from 1970 into the 1990s, one of a succession of products in the PDP series. The PDP-11 replaced the PDP-8 in many real-time applications, although both product lines lived in parallel for more than 10 years...
. The Nova provided four accumulators, AC0-AC3, although AC2 and AC3 could also be used to provide offset addresses, tending towards more generality of usage for the registers. The PDP-11
PDP-11
The PDP-11 was a series of 16-bit minicomputers sold by Digital Equipment Corporation from 1970 into the 1990s, one of a succession of products in the PDP series. The PDP-11 replaced the PDP-8 in many real-time applications, although both product lines lived in parallel for more than 10 years...
introduced a more contemporary model of general registers, numbered R0-R7 or more, adopted by most later CISC and RISC machines.
Early 4-bit and 8-bit microprocessors such as the 4004, 8008 and numerous others, typically had single accumulators. The 8051 microcontroller has two, a primary accumulator and a secondary accumulator, where the second is used by instructions only when multiplying (MUL AB) or dividing (DIV AB); the former splits the 16-bit result between the two 8-bit accumulators, whereas the latter stores the quotient on the primary accumulator A and the remainder in the secondary accumulator B. As a direct descendent of the 8008, the 8080, and the 8086, the modern ubiquitous Intel x86 processors still uses the primary accumulator EAX and the secondary accumulator EDX for multiplication and division of large numbers. For instance, MUL ECX will multiply the 32-bit registers ECX and EAX and split the 64-bit result between EAX and EDX. However, MUL and DIV are special cases, other arithmetic-logical instructions (ADD, SUB, CMP, AND, OR, XOR, TEST) may specify any of the eight registers EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI as the accumulator (i.e. left operand and destination); x86 is thus a fairly general register architecture, despite being based on an accumulator model. The 64-bit extension of x86, x86-64
X86-64
x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other...
, has been further generalized to 14 instead of 6 general registers.