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The address generation unit (AGU), sometimes also called address computation unit (ACU), is an
Address generation unit in the Motorola DSP56K family
June 2003, Motorola
Address generation unit in DSP applications
September 2013, by Andreas Ehliar
Computer Science from the Bottom Up, Chapter 3. Computer Architecture
September 2013, by Ian Wienand {{Basic computer components Central processing unit Instruction processing
The address generation unit (AGU), sometimes also called address computation unit (ACU), is an execution unit
In computer engineering, an execution unit (E-unit or EU) is a part of the central processing unit (CPU) that performs the operations and calculations as instructed by the computer program. It may have its own internal control sequence unit (not ...
inside central processing unit
A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, a ...
s (CPUs) that calculates addresses used by the CPU to access main memory
Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers.
The central processing unit (CPU) of a comput ...
. By having address calculations handled by separate circuitry that operates in parallel with the rest of the CPU, the number of CPU cycles required for executing various machine instructions can be reduced, bringing performance improvements.
While performing various operations, CPUs need to calculate memory addresses required for fetching data from the memory; for example, in-memory positions of array elements must be calculated before the CPU can fetch the data from actual memory locations. Those address-generation calculations involve different integer arithmetic operations, such as addition, subtraction, modulo operation
In computing, the modulo operation returns the remainder or signed remainder of a division, after one number is divided by another (called the '' modulus'' of the operation).
Given two positive numbers and , modulo (often abbreviated as ) is ...
s, or bit shift
In computer programming, a bitwise operation operates on a bit string, a bit array or a binary numeral (considered as a bit string) at the level of its individual bits. It is a fast and simple action, basic to the higher-level arithmetic oper ...
s. Often, calculating a memory address involves more than one general-purpose machine instruction, which do not necessarily decode and execute quickly. By incorporating an AGU into a CPU design, together with introducing specialized instructions that use the AGU, various address-generation calculations can be offloaded from the rest of the CPU, and can often be executed quickly in a single CPU cycle.
Capabilities of an AGU depend on a particular CPU and its architecture
Architecture is the art and technique of designing and building, as distinguished from the skills associated with construction. It is both the process and the product of sketching, conceiving, planning, designing, and constructing buildings ...
. Thus, some AGUs implement and expose more address-calculation operations, while some also include more advanced specialized instructions that can operate on multiple operand
In mathematics, an operand is the object of a mathematical operation, i.e., it is the object or quantity that is operated on.
Example
The following arithmetic expression shows an example of operators and operands:
:3 + 6 = 9
In the above exam ...
s at a time. Furthermore, some CPU architectures include multiple AGUs so more than one address-calculation operation can be executed simultaneously, bringing further performance improvements by capitalizing on the superscalar
A superscalar processor is a CPU that implements a form of parallelism called instruction-level parallelism within a single processor. In contrast to a scalar processor, which can execute at most one single instruction per clock cycle, a sup ...
nature of advanced CPU designs. For example, Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 ser ...
incorporates multiple AGUs into its Sandy Bridge and Haswell microarchitecture
In computer engineering, microarchitecture, also called computer organization and sometimes abbreviated as µarch or uarch, is the way a given instruction set architecture (ISA) is implemented in a particular processor. A given ISA may be imp ...
s, which increase bandwidth of the CPU memory subsystem by allowing multiple memory-access instructions to be executed in parallel.
See also
*Arithmetic logic unit
In computing, an arithmetic logic unit (ALU) is a combinational digital circuit that performs arithmetic and bitwise operations on integer binary numbers. This is in contrast to a floating-point unit (FPU), which operates on floating point num ...
(ALU) a digital circuit that performs arithmetic and bitwise logical operations on integer binary numbers
* Floating-point unit
In computing, floating-point arithmetic (FP) is arithmetic that represents real numbers approximately, using an integer with a fixed precision, called the significand, scaled by an integer exponent of a fixed base. For example, 12.345 can be ...
(FPU) the same as ALU but for floating-point numbers
In computing, floating-point arithmetic (FP) is arithmetic that represents real numbers approximately, using an integer with a fixed precision, called the significand, scaled by an integer exponent of a fixed base. For example, 12.345 can ...
* Load–store unit
In computer engineering, a load–store unit (LSU) is a specialized execution unit responsible for executing all load and store instructions, generating virtual addresses of load and store operations and loading data from memory or storing it back ...
* Bulldozer (microarchitecture)
The AMD Bulldozer Family 15h is a microprocessor microarchitecture for the FX and Opteron line of processors, developed by AMD for the desktop and server markets. Bulldozer is the codename for this family of microarchitectures. It was releas ...
another CPU microarchitecture that includes multiple AGUs, developed by AMD
* Register renaming
In computer architecture, register renaming is a technique that abstracts logical registers from physical registers.
Every logical register has a set of physical registers associated with it.
When a machine language instruction refers to a partic ...
a technique that reuses CPU registers and avoids unnecessary serialization of program operations
* Reservation station
A unified reservation station, also known as unified scheduler, is a decentralized feature of the microarchitecture of a CPU that allows for register renaming
In computer architecture, register renaming is a technique that abstracts logical r ...
a CPU feature that allows results of various operations to be used while bypassing CPU registers
* Execution unit
In computer engineering, an execution unit (E-unit or EU) is a part of the central processing unit (CPU) that performs the operations and calculations as instructed by the computer program. It may have its own internal control sequence unit (not ...
References
External links
Address generation unit in the Motorola DSP56K family
June 2003, Motorola
Address generation unit in DSP applications
September 2013, by Andreas Ehliar
Computer Science from the Bottom Up, Chapter 3. Computer Architecture
September 2013, by Ian Wienand {{Basic computer components Central processing unit Instruction processing