Microprocessors are central components of almost all digital systems, because combinations of hardware and software are used to solve design problems. A computer is formed by combining a microprocessor
with a mix of certain basic elements and customized logic. Software runs on a microprocessor and provides a flexible framework that orchestrates the behavior of hardware that has been customized to fit the application. When many people think about computers, images of desktop PCs and laptops come to their minds. Computers are much more diverse than the stereotypical image and permeate everyday life in increasing numbers. Small computers control microwave ovens, telephones, and CD players.
Computer architecture is fundamental to the design of digital systems. Understanding how a basic computer is designed enables a digital system to take shape by using a microprocessor as a central
control element. The microprocessor becomes a programmable platform upon which the major components of an algorithm can be implemented. Digital logic can then be designed to surround the microprocessor
and assist the software in carrying out a specific set of tasks.
The first portion of this chapter explains the basic elements of a computer, including the microprocessor, memory, and input/output devices. Basic microprocessor operation is presented from a
hardware perspective to show how instructions are executed and how interaction with other system components is handled. Interrupts, registers, and stacks are introduced as well to provide an overall
picture of how computers function. Following this basic introduction is a complete example of how an actual eight-bit computer might be designed, with detailed descriptions of bus operation and address
decoding. Once basic computer architecture has been discussed, common techniques for improving and augmenting microprocessor capabilities are covered, including direct memory access and bus expansion. These techniques are not relegated to high-end computing but are found in many smaller digital systems in which it is more economical to add a little extra hardware to achieve feature and performance goals instead of having to use a microprocessor that may be too complex and more expensive than desired.
The chapter closes with an introduction to assembly language and microprocessor addressing modes. Writing software is not a primary topic of this book, but basic software design is an inseparable
part of digital systems design. Without software, a computer performs no useful function. Assembly language basics are presented in a general manner, because each microprocessor has its own instruction set and assembly language, requiring specific reading focused on that particular device.
Basic concepts, however, are universal across different microprocessor implementations and serve to further explain how microprocessors actually function. A digital computer is a collection of logic elements that can execute arbitrary algorithms to perform data calculation and manipulation functions. A computer is composed of a microprocessor, memory, and some input/output (I/O) elements as shown in Fig. 3.1. The microprocessor, often called a microprocessor unit (MPU) or central processing unit (CPU), contains logic to step through an algorithm, called a program , that has been stored in the computer’s program memory. The data used and manipulated by that program is held in the computer’s data memory. Memory is a repository for data
that is usually organized as a linear array of individually accessible locations. The microprocessor can access a particular location in memory by presenting a memory address (the index of the desired
location) to the memory element. I/O elements enable the icroprocessor to communicate with the outside world to acquire new data and present the results of its programmed computations. Such elements can include a keyboard or display controller.
Programs are composed of many very simple individual operations, called instructions, that specify in exact detail how the microprocessor should carry out an algorithm. A simple program may have dozens of instructions, whereas a complex program can have tens of millions of instructions. Collectively, the programs that run on microprocessors are called software , in contrast to the hardware on which they run. Each type of microprocessor has its own instruction set that defines the full set of unique, discrete operations that it is capable of executing. These instructions perform very narrow tasks that, on their own, may seem insignificant. However, when thousands or millions of these tiny instructions are strung together, they may create a video game or a word processor. A microprocessor possesses no inherent intelligence or capability to spontaneously begin performing
useful work. Each microprocessor is constructed with an instruction set that can be invoked in arbitrary sequences. Therefore, a microprocessor has the potential to perform useful work but will
do nothing of the sort on its own. To make the microprocessor perform useful work, it requires explicit guidance in the form of software programming. A task of even moderate complexity must be broken down into many tiny steps to be implemented on a microprocessor. These steps include basic arithmetic, Boolean operations, loading data from memory or an input element such as a keyboard, and storing data back to memory or an output element such as a printer.
Memory structure is one of a computer’s key characteristics, because the microprocessor is almost constantly accessing it to retrieve a new instruction, load new data to operate on, or store a calculated
result. While program and data memory are logically distinct classifications, they may share the same physical memory resource.
Random access memory (RAM) is the term used to describe a
generic memory resource whose locations can be accessed, or
addressed , in an arbitrary order and either read or written. A
read is the process of retrieving data from a memory address and loading it into the microprocessor. A write is the process of storing data to a memory address from the microprocessor. Both programs and data can occupy RAM. Consider your desktop computer. When you
Computer Architecture
execute a program that is located on the disk drive, that program is first loaded into the computer’s RAM and then executed from a region set aside for program memory. As on a desktop computer, RAM is most often volatile— meaning that it loses its contents when the power is turned off. Some software cannot be stored in volatile memory, because basic initialization instructions, or boot code , must be present when the computer is turned on. Remember that a icroprocessor can do nothing useful without software being readily available. When power is first applied to a computer, the microprocessor must be able to quickly locate boot code so that it can get itself ready to accept input from a user or load a program from an input device. This startup sequence is called booting , hence the term
boot code . When you turn your computer on, the first messages that it displays on the monitor are a product of its boot code. Eventually, the computer is able to access its disk drive and begins loading software into RAM as part of its normal operation. To ensure that boot code is ready at power-up, nonvolatile memory called read only memory
(ROM) exists. ROM can be used to store both programs as well as any data that must be present at power-up and immediately accessible.
Software contained in ROM is also known as firmware . As its name implies, ROM can only be read but not written. More complex computers contain a relatively small quantity of ROM to hold basic
boot code that then loads main operating software from another device into RAM. Small computers may contain all of their software in ROM. Figure 3.2 shows how ROM and RAM complement each
other in a typical computer architecture. A microprocessor connects to devices such as memory and I/O via data and address buses . Collectively, these two buses can be referred to as the microprocessor bus . A bus is a collection of wires that serve a common purpose. The data bus is a bit array of sufficient size to communicate one complete
data unit at a time. Most often, the data bus is one or more bytes in width. An eight-bit microprocessor, operating on one byte at time, almost always has an eight-bit data bus. A 32-bit microprocessor, capable of operating on up to 4 bytes at a time, can have a data bus that is 32, 16, or 8 bits wide. The exact data bus width is implementation specific and varies according to the intended
application of the microprocessor. A narrower bus width means that it will take more time to communicate a quantity of data as compared to a wider bus. Common notation for a data bus is D[7:0] for an 8-bit bus and D[31:0] for a 32-bit bus, where 0 is the least-significant bit.
The address bus is a bit array of sufficient size to fully express the microprocessor’s address space . Address space refers to the maximum amount of memory and I/O that a microprocessor can directly address. If a microprocessor has a 16-bit address bus, it can address up to 2
16 = 65,536 bytes. Therefore, it has a 64 kB address space. The entire address space does not have to be used; it simply establishes a maximum limit on memory size. Common notation for a 16-bit address bus is A[15:0] , where 0 is the least-sigificant bit. Figure 3.3 shows a typical microprocessor bus configuration in a computer. Note that the address bus is unidirectional (the microprocessor asserts requested
addresses to the various devices), and the data bus is bidirectional (the microprocessor asserts data on a write and the devices assert data on reads).
Basic ROM-RAM memory complement.
Keyword :
computer architecture ,computer ,memory ,registers ,processor ,instruction set ,instruction ,implementation ,www computer architecture ,technology ,research laboratory ,project ,programmer ,processors ,pipelining ,patterson ,operating system ,microprocessors ,instruction set architecture ,embedded systems ,digital systems ,design ,courses ,computer system ,computer science ,computer organization ,shimon schocken ,screen memory ,power consumption ,physical keyboard ,operating systems ,microarchitecture ,machine language ,latency ,instruction memory ,high performance ,hack computer ,elements ,digital design ,computing systems ,computer engineering ,computer design ,computer architecture research ,computer architecture laboratory ,computer architecture group ,capabilities ,architectures ,architecture courses ,architecture.
