An Address is a pattern of 0’s and 1 ’s that represents a specific location in memorynor a particular I/O device. Typical 8-bit microprocessors have 16 address lines, and, these 16 lines can produce 216 unique 16-bit patterns from 0000000000000000 to 11 11 1 1 11 1 11 11 11 1, representing 65,536 different address combinations.
Addressing mode is the manner in which the microprocessor determines the operand (data) and destination addresses during execution of an instruction.
An Arithmetic-logic unit (ALU) is a digital circuit that performs arithmetic and logic operations on two n-bit digital words. The value of n can be 4, 8, 16, 32, or 64.
Typical operations performed by an ALU are addition, subtraction, ANDing, ORing, and comparison of two n-bit digital words. The size of the ALU defines the size of the microprocessor. For example, a 32-bit microprocessor contains a 32-bit ALU.
Bit is an abbreviation for the term binary digit. A binary digit can have only two values, which are represented by the symbols 0 and 1, whereas a decimal digit can have 10 values, represented by the symbols 0 through 9. The bit values are easily implemented in electronic and magnetic media by two-state devices whose states portray either of the binary digits 0 and 1. Examples of such two-state devices are a transistor that is conducting or not conducting, a capacitor that is charged or discharged, and a magnetic material that is magnetized north to south or south to north.
Bit size refers to the number of bits that can be processed simultaneously by the basic arithmetic circuits of a microprocessor. A number of bits taken as a group in this manner is called a word. For example, a 32-bit microprocessor can process a 32-bit word. An 8-bit word is referred to as a byte , and a 4-bit word is known as a nibble.
A bus consists of a number of conductors (wires) organized to provide a means of communication among different elements in a microprocessor system. The conductors in a bus can be grouped in terms of their functions. A microprocessor normally has an address bus, a data bus, and a control bus. Address bits are sent to memory or to an external device on the address bus. Instructions from memory, and data to/from memory or external devices, normally travel on the data bus. Control signals for the other buses and among system elements are transmitted on the control bus. Buses are sometimes bidirectional; that is, information can be transmitted in either direction on the bus, but normally in only one direction at a time.
Cache Memory is a high-speed, directly accessible, relatively small, semiconductor readwrite memory block used to store datdinstructions that the microprocessor may need in the immediate future. It increases speed by reducing the number of external memory reads required by the microprocessor. Typical 32-bit microprocessors such as the Intel Pentium are provided with on-chip cache memory. Pentium I1 supports two
levels of cache. These are L1 (Level 1 cache) and L2 (Level 2 cache) cache memories.
The L1 cache (smaller in size) is contained inside the microprocessor while L2 cache (larger in size) is interfaced to the microprocessor. This two level cache enhances the performance of the microprocessor.
A Complex Instruction Set Computer (CISC) contains a large instruction set. It isdifficult to pipeline compared to RISC. Motorola 68020 is a CISC microprocessor.
Clock is analogous to human heart beats. The microprocessor requires synchronization among its components, and this is provided by a clock or timing circuits.
The instruction set of a microprocessor is a list of commands that the microprocessor is designed to execute. Typical instructions are ADD, SUBTRACT, and STORE.
Individual instructions are coded as unique bit patterns which are recognized and executed by the microprocessor. If a microprocessor has 3 bits allocated to the representation of instructions, the microprocessor will recognize a maximum of 23, or eight, different instructions. The microprocessor will then have a maximum of eight instructions in its instruction set. It is obvious that some instructions will be more
suitable than others to a particular application. For example, if a microprocessor is to be used in a calculating mode, instructions such as ADD, SUBTRACT, MULTIPLY, and DIVIDE would be desirable. In a control application, instructions inputting digitized signals to the processor and outputting digital control variables to external circuits are essential. The number of instructions necessary in an application will directly influence the amount of hardware in the chip set and the number and organization of the interconnecting bus lines.
Memoly Management Unit ( M N ) allows programmers to write programs much larger than could fit in the main memory space available to the microprocessor. The programs are simply stored in a secondary device such as a hard disk and portions of the programs are swapped into the main memory as needed for execution by the microprocessor. The MMU is implemented as on-chip hardware in typical microprocessors such as the Pentium.
A microprocessor is the CPU of a microcomputer contained on a single chip, and must be intefaced with peripheral support chips in order to function. In general, a CPU contains several registers (memory elements), an ALU, and a control unit. Note that the control unit translates instructions and performs the desired task. The number of peripheral devices depends on the particular application involved and may even vary within an application. As the microprocessor industry matures, more of these functions are being integrated onto chips, to reduce the system package count. In general, a microcomputer typically consists of a microprocessor (CPU) chip, input and output chips, and memory chips in which programs (instructions and data) are stored. Note that a microcontroller, on the other hand, is implemented on a single chip containing typically a CPU, memory, I/O, a timer, and A/D and D/A converter circuits.
Pipelining is a technique that overlaps instruction fetch (instruction read) with execution. This allows a microprocessor’s processing operation to be broken down into several steps (dictated by the number of pipeline levels or stages) so that the individual step outputs can be handled by the microprocessor in parallel. Pipelining is often used to fetch the microprocessor’s next instruction while executing the current instruction,
which speeds up the overall operation of the microprocessor considerably.
Random-access memory (RAM) is a storage medium for groups of bits or words whose contents cannot only be read but can also be altered at specific addresses. A RAM normally provides volatile storage, which means that its contents are lost in case power is turned off. RAMs are fabricated on chips and have typical densities of 4096 bits to 1 megabit per chip. These bits can be organized in many ways: for example, as 4096-by-1-bit words or as 2048-by-%bit words. RAMs are normally used for the storage of temporary data and intermediate results as well as programs that can be reloaded from a backup nonvolatile source. RAMs are capable of providing large storage capacity, in the megabit range.
Read-only memory (ROM) is a storage medium for the groups of bits called words, and its contents cannot normally be altered once programmed. A typical ROM is fabricated on a chip and can store, for example, 2048 eight-bit words, which can be accessed individually by presenting to it one of 2048 addresses. This ROM is referred to as a 2K by 8-bit ROM. 101 101 11 is an example of an 8-bit word that might be stored
in one location in this memory. A ROM is a nonvolatile storage device, which means that its contents are retained in case power is turned off. Because of this characteristic, ROMs are used to store programs (instructions and data) that must always be available to the microprocessor.
A register can be considered as volatile storage for a number of bits. These bits may be entered into the register simultaneously (in parallel) or sequentially (serially) from right to left or from left to right, 1 bit at a time. An 8-bit register storing the bits 1 1 1 10000 is represented as follows:
A reduced instruction set computer (RISC) contains a simple instruction set. The RISC architecture maximizes speed by reducing clock cycles per instruction and makes it easier to implement pipelining. A Power PC is a RISC microprocessor.
A Superscalar microprocessor is provided with more than one pipeline and can execute more than one instruction per clock cycle. The Pentium is a superscalar microprocessor.
sourced by: Microprocessor Theory and Applications with 68000/68020 and Pentium, M. Rafiquzzaman, Ph.D.
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