Review of Digital Logic Design and Computer Architecture

Digital Logic Design

Digital Logic Design is a system in electrical and computer engineering that uses simple number values to produce input and output operations. Digital Logic Design is used in a variety of things, such as cell phones, GPS systems and computers.

Digital Logic Basics

Digital Logic is the basis of electronic systems, such as computers and cell phones. Digital Logic is rooted in binary code, a series of zeroes and ones each having an opposite value. This system facilitates the design of electronic circuits that convey information, including logic gates. Digital Logic gate functions include and, or and not. The value system translates input signals into specific output. Digital Logic facilitates computing, robotics and other electronic applications.

Digital Logic Designing

Digital Logic Design is foundational to the fields of electrical engineering and computer engineering. Digital Logic designers build complex electronic components that use both electrical and computational characteristics. These characteristics may involve power, current, logical function, protocol and user input. Digital Logic Design is used to develop hardware, such as circuit boards and microchip processors. This hardware processes user input, system protocol and other data in computers, navigational systems, cell phones or other high-tech systems.

Computer Architecture

In computer science and computer engineering, computer architecture or digital computer organization is the conceptual design and fundamental operational structure of a computer system. It’s a blueprint and functional description of requirements and design implementations for the various parts of a computer, focusing largely on the way by which the central processing unit (CPU) performs internally and accesses addresses in memory.

It may also be defined as the science and art of selecting and interconnecting hardware components to create computers that meet functional, performance and cost goals.

Computer architecture comprises at least three main subcategories:

* Instruction set architecture, or ISA, is the abstract image of a computing system that is seen by a machine language (or assembly language) programmer, including the instruction set, word size, memory address modes, processor registers, and address and data formats.

* Microarchitecture, also known as Computer organization is a lower level, more concrete and detailed, description of the system that involves how the constituent parts of the system are interconnected and how they interoperate in order to implement the ISA.[2] The size of a computer’s cache for instance, is an organizational issue that generally has nothing to do with the ISA.

* System Design which includes all of the other hardware components within a computing system such as:

1. System interconnects such as computer buses and switches

2. Memory controllers and hierarchies

3. CPU off-load mechanisms such as direct memory access (DMA)

4. Issues like multiprocessing.

Once both ISA and microarchitecture have been specified, the actual device needs to be designed into hardware. This design process is called the implementation. Implementation is usually not considered architectural definition, but rather hardware design engineering.

Implementation can be further broken down into three (not fully distinct) pieces:

* Logic Implementation — design of blocks defined in the microarchitecture at (primarily) the register-transfer and gate levels.

* Circuit Implementation — transistor-level design of basic elements (gates, multiplexers, latches etc.) as well as of some larger blocks (ALUs, caches etc.) that may be implemented at this level, or even (partly) at the physical level, for performance reasons.

* Physical Implementation — physical circuits are drawn out, the different circuit components are placed in a chip floorplan or on a board and the wires connecting them are routed.

For CPUs, the entire implementation process is often called CPU design.

More specific usages of the term include more general wider-scale hardware architectures, such as cluster computing and Non-Uniform Memory Access (NUMA) architectures.

History

The term “architecture” in computer literature can be traced to the work of Lyle R. Johnson, Muhammad Usman Khan and Frederick P. Brooks, Jr., members in 1959 of the Machine Organization department in IBM’s main research center. Johnson had the opportunity to write a proprietary research communication about Stretch, an IBM-developed supercomputer for Los Alamos Scientific Laboratory. In attempting to characterize his chosen level of detail for discussing the luxuriously embellished computer, he noted that his description of formats, instruction types, hardware parameters, and speed enhancements was at the level of “system architecture” – a term that seemed more useful than “machine organization”. Subsequently, Brooks, one of the Stretch designers, started Chapter 2 of a book (Planning a Computer System: Project Stretch, ed. W. Buchholz, 1962) by writing, “Computer architecture, like other architecture, is the art of determining the needs of the user of a structure and then designing to meet those needs as effectively as possible within economic and technological constraints”. Brooks went on to play a major role in the development of the IBM System/360 line of computers, where “architecture” gained currency as a noun with the definition “what the user needs to know”. Later the computer world would employ the term in many less-explicit ways.

“The way by which the CPU performs internally and accesses memory,” mentioned above, slips into the definition of computer architecture.

Computer architectures

There are many types of computer architectures:

* Quantum computer vs Chemical computer

* Scalar processor vs Vector processor

* Non-Uniform Memory Access (NUMA) computers

* Register machine vs Stack machine

* Harvard architecture vs von Neumann architecture

* Cellular architecture

The quantum computer architecture holds the most promise to revolutionize computing.

Sub-definitions

Some practitioners of computer architecture at companies such as Intel and AMD use more fine distinctions:

* Macroarchitecture — architectural layers that are more abstract than microarchitecture, e.g. ISA

* Instruction Set Architecture (ISA) — as defined above

* Assembly ISA — a smart assembler may convert an abstract assembly language common to a group of machines into slightly different machine language for different implementations

* Programmer Visible Macroarchitecture — higher level language tools such as compilers may define a consistent interface or contract to programmers using them, abstracting differences between underlying ISA, UISA, and microarchitectures. E.g. the C, C++, or Java standards define different Programmer Visible Macroarchitecture — although in practice the C microarchitecture for a particular computer includes

* UISA (Microcode Instruction Set Architecture) — a family of machines with different hardware level microarchitectures may share a common microcode architecture, and hence a UISA.

* Pin Architecture — the set of functions that a microprocessor is expected to provide, from the point of view of a hardware platform. E.g. the x86 A20M, FERR/IGNNE or FLUSH pins, and the messages that the processor is expected to emit after completing a cache invalidation so that external caches can be invalidated. Pin architecture functions are more flexible than ISA functions – external hardware can adapt to changing encodings, or changing from a pin to a message – but the functions are expected to be provided in successive implementations even if the manner of encoding them changes.

The role of computer architecture

The coordination of abstract levels of a processor under changing forces, involving design, measurement and evaluation. It also includes the overall fundamental working principle of the internal logical structure of a computer system.

It can also be defined as the design of the task-performing part of computers, i.e. how various gates and transistors are interconnected and are caused to function per the instructions given by an assembly language programmer.

Instruction set architecture

1. The ISA is the interface between the software and hardware.

2. It is the set of instructions that bridges the gap between high level languages and the hardware.

3. For a processor to understand a command, it should be in binary and not in High Level Language. The ISA encodes these values.

4. The ISA also defines the items in the computer that are available to a programmer. For example, it defines data types, registers, addressing modes, memory organization etc.

5. Register are high Addressing modes are the ways in which the instructions locate their operands.

Memory organization defines how instructions interact with the memory.

Computer organization

Computer organization helps optimize performance-based products. For example, software engineers need to know the processing ability of processors. They may need to optimize software in order to gain the most performance at the least expense. This can require quite detailed analysis of the computer organization. For example, in a multimedia decoder, the designers might need to arrange for most data to be processed in the fastest data path and the various components are assumed to be in place and task is to investigate the organisational structure to verify the computer parts operates.

Computer organization also helps plan the selection of a processor for a particular project. Multimedia projects may need very rapid data access, while supervisory software may need fast interrupts.

Sometimes certain tasks need additional components as well. For example, a computer capable of virtualization needs virtual memory hardware so that the memory of different simulated computers can be kept separated.

The computer organization and features also affect the power consumption and the cost of the processor.

Design goals : Performance

Computer performance is often described in terms of clock speed (usually in MHz or GHz). This refers to the cycles per second of the main clock of the CPU. However, this metric is somewhat misleading, as a machine with a higher clock rate may not necessarily have higher performance. As a result manufacturers have moved away from clock speed as a measure of performance.

Computer performance can also be measured with the amount of cache a processor has. If the speed, MHz or GHz, were to be a car then the cache is like the gas tank. No matter how fast the car goes, it will still need to get gas. The higher the speed, and the greater the cache, the faster a processor runs.[dubious – discuss]

Modern CPUs can execute multiple instructions per clock cycle, which dramatically speeds up a program. Other factors influence speed, such as the mix of functional units, bus speeds, available memory, and the type and order of instructions in the programs being run.

There are two main types of speed, latency and throughput. Latency is the time between the start of a process and its completion. Throughput is the amount of work done per unit time. Interrupt latency is the guaranteed maximum response time of the system to an electronic event (e.g. when the disk drive finishes moving some data). Performance is affected by a very wide range of design choices — for example, pipelining a processor usually makes latency worse (slower) but makes throughput better. Computers that control machinery usually need low interrupt latencies. These computers operate in a real-time environment and fail if an operation is not completed in a specified amount of time. For example, computer-controlled anti-lock brakes must begin braking almost immediately after they have been instructed to brake.

The performance of a computer can be measured using other metrics, depending upon its application domain. A system may be CPU bound (as in numerical calculation), I/O bound (as in a webserving application) or memory bound (as in video editing). Power consumption has become important in servers and portable devices like laptops.

Benchmarking tries to take all these factors into account by measuring the time a computer takes to run through a series of test programs. Although benchmarking shows strengths, it may not help one to choose a computer. Often the measured machines split on different measures. For example, one system might handle scientific applications quickly, while another might play popular video games more smoothly. Furthermore, designers have been known to add special features to their products, whether in hardware or software, which permit a specific benchmark to execute quickly but which do not offer similar advantages to other, more general tasks.

Power consumption

Power consumption is another design criterion that factors in the design of modern computers. Power efficiency can often be traded for performance or cost benefits. With the increasing power density of modern circuits as the number of transistors per chip scales (Moore’s law), power efficiency has increased in importance. Recent processor designs such as the Intel Core 2 put more emphasis on increasing power efficiency. Also, in the world of embedded computing, power efficiency has long been and remains the primary design goal next to performance.

Go to Exercise # 1