Types of Processor
i. CISC Processors
One of the earlier goals of CPU designers was to provide more and more instructions in the instruction set of a CPU to ensure that the CPU supports more functions directly. This makes it easier to translate high-level language programs to machine language and ensures that the machine language programs run more effectively. Of course, every additional instruction in the instruction set of a CPU requires the necessary hardware circuitry to handle that instruction, adding more complexity to the CPU’s hardware circuitry. Another goal of CPU designers was to optimize the usage of expensive memory. To achieve this, the designers tried to pack more instructions in memory by introducing the concept of variable -length instructions such as half-word, one-and-half-word, ‘etc. For example, an operand in an immediate instruction needs fewer bits and can be designed as a half-word instruction. Additionally, CPUs were designed to support a variety of addressing modes (discussed later in this chapter during the discussion of memory). CPUs with large instruction set, variable-length instructions, and a variety of addressing modes are said to employ CISC (Complex Instruction Set Computer) architecture. Since CISC processors possess so many processing features, they make the job of machine language programmers easier. However, they are complex and expensive to produce. Most personal computers of today use CISC processors.
ii. RISC Processor
In early 1980s, some CPU designers realized that many instructions supported by a CISC based CPU are rarely used. Hence, an idea evolved that the design complexity of CPU can be reduced greatly by implemented only bare minimum basics of instructions and some of the more frequently used instructions in the hardware circuitry of the CPU. Other complex instructions need not be supported in the instruction set of the CPU because they can always be implemented in software by using the basic set of instructions. While working on simpler CPU design, the designers also came up with the idea of making all the instructions of uniform length so that the decoding and execution of all instructions becomes simple and fast. Furthermore, to speed up computation and to reduce the complexity of handling a number of addressing modes they decided to design all the instructions in such a way that they retrieve operands stored in registers in CPU rather than from memory. These design ideas resulted in producing faster and less expensive processors. CPUs with a small instruction set, fixed-length instructions, and reduced references to memory to retrieve operands are said to employ RISC (Reduced Instruction Set Computer) architecture. Since RISC processors have a small instruction set, they place extra demand on programmers who must consider how to implement complex computations by combining simple instructions. However, RISC processors are faster for most applications, less complex, and less expensive to produce than CISC processors because of simpler design.
iii. EPIC Processors
The Explicitly Parallel Instruction Computing (EPIC) technology breaks through the sequential nature of conventional processor architectures by allowing the software to communicate explicitly to the processor when operations can be done in parallel. For this, it uses tighter coupling between the compiler and the processor. It enables the compiler to extract maximum parallelism in the original code and explicitly describe it to the processor. Processors based on EPIC architecture are simpler and more powerful than traditional CISC or RISC processors. These processors are mainly targeted to next-generation, 64-bit, high-end server and workstation market (not for personal computer market).
iv. Multicore Processor
Till recently, the approach used for building faster processors was to keep. reducing the size of chips while increasing the number of transistors they contain. Although, this trend has driven the computing industry for several years; it has now been realized that transistors cannot shrink forever. Current transistor technology limits the ability to continue making single-core processors more powerful due to following reasons:
i. As a transistor gets smaller, the gate, which switches the electricity ON and OFF, gets thinner and less able to block the flow of electrons. Thus, small transistors tend to use electricity all the time, even when they are not switching, This wastes power.
ii. Increasing clock speeds causes transistors to switch faster and generate more heat and consume more power. These and other challenges have forced processor manufacturers to research for a new approach for building faster processors. In the new architecture, a processor chip has multiple cooler-running, more energy-efficient processing cores instead of one increasingly powerful core. The multicore chips do not necessarily run as fast as the highest performing single-core models, but they improve overall performance by handling more work in parallel. For instance, a dual-core chip running multiple applications is about 1.5 times faster than a chip with just one comparable core.
iii. The operating system (OS) controls the overall assignment of tasks in a multicore processor. In a multicore processor, each core has its independent cache (though in some designs all cores share the same cache) thus providing. the OS with sufficient resources to handle multiple applications in parallel. When a single-core chip runs multiple programs, the OS assigns a time slice to work on one program and then assigns different time slices for other programs. This can cause conflicts, errors, or slowdowns when the processor must perform multiple tasks simultaneously. However, multiple programs can be run at the same time on a multicore chip with each core handling a separate program. The same logic holds for running multiple threads of a multithreaded application at the same time on a multicore chip with each core handling a separate thread. Based on this, either the OS or a multithreaded application parcels out work to the multiple cores.
Advantages of Multicore Processors
i. They enable building of computers with better overall system performance by handling more work in parallel.
ii. For comparable performance, multicore chips consume less power and generate less heat than single- core chips. Hence, multicore technology is also referred to as energy-efficient or power-aware processor technology.
iii. Because the chips’ cores are on the same die in case of multicore processors architecture, they can share architectural components, such as memory elements and memory management. They thus have fewer components and lower costs than systems- running multiple chips ‘(each a single-core processor).
iv. Also, the signaling between cores can be faster and use less electricity than on multichip systems.
Limitations of Multicore Processors
i. To take advantage of multicore chips, applications must be redesigned so that the processor can run them as multiple threads. Note that it is more challenging to create software that is multithreaded.
ii. To redesign applications, programmers must find good places to break up the applications, divide the work into roughly equal pieces that can run at the same time, and determine the best times for the threads to communicate with one another. All these add to extra work for programmers.
iii. Software vendors often charge customers for each processor that will run the software (one software license per processor). A customer running an application on an 8-processor machine (multiprocessor computer) with single-core processors would thus pay for 8 licenses. A key issue with multicore chips is whether software vendors should consider a processor to be a single core or an entire chip. Currently, different vendors have different views regarding this issue. Some consider a processor as a unit that plugs into a single socket on the motherboard, regardless of whether it has one or more cores. Hence, a single software license is sufficient for a multicore chip. On the other hand, others charge more to use their software on multicore chips for per-processor licensing. They are of the opinion that customers get added performance benefit by running the software on a chip with multiple cores, so they should pay more. Multicore-chip makers are concerned that this o/pe of non-uniform policy will hurt their products sales.
Chip makers like Intel, AMD, IBM, and Sun have already introduced multicore chips for servers, desktops, and laptops. The current multicore chips are dual-core (2 cores per chip), quad-core (4 cores per chip), 8 cores per chip, and 16 cores per chip. Industry experts predict that multicore processors will be useful immediately in server class machines but won’t be very useful on the desktop systems until software vendors develop considerably more multithreaded software. Until this occurs, single-core chips will continue to be used. Also, since single-core chips are inexpensive to manufacture, they will continue to be popular for low-priced PCs for a while.
