Chapter 1: Computer Systems Overview (Lect. Slides 1-14) ● A computer consists of 4 main structural elements: ○ 1. Processor ■ It is also termed as the Central Processing Unit (CPU). ■ It controls the operation of the computer and performs its data processing functions. ○ 2. Main Memory ■ It is also termed as the Random Access Memory (RAM). ■ This memory is volatile, meaning it erases its storage once the computer power turns off. ■ It stores the current data and programs. ○ 3. I/O Modules ■ These modules move data between the computer and its external environment. ■ This external environment consists of external disks and other terminals. ○ 4. System Bus ■ Provides for communication among processors, main memory, and I/O modules. ■ The Bus is a subsystem that is used to transfer data and other information between devices. ○ Processor in Detail: ■ Every processor has 3 parts to it: ● Control Unit (CU) - It determines the sequence of operations including branch instructions that transfer control from one part of a program to another. ● Arithmetic Logic Unit (ALU) - Carries out basic arithmetic and logical operations. ● Registers - Registers usually consist of a small amount of fast storage and is a quickly accessible location available to a computer’s processor. ■ The CPU contains the following Registers: ● 1. Program Counter (PC) - holds the address of the next instruction to be executed. ● 2. Instruction Registry (IR) - holds the instruction currently being executed. ● 3. Accumulator (AC) - intermediate arithmetic logic unit results are stored in this register. ● 4. Memory Address Register (MAR) - stores the memory address from which data will be fetched to the CPU or the address to which data will be sent and stored. ● 5. Memory Buffer Register (MBR) - Contains the data to be written into memory or receives the data read from memory. ● 6. I/O Address Register (I/O AR) - holds the address of a particular I/O device. ● 7. I/O Buffer Register (I/O BR) - holds the data to be exchanged between the I/O module and the processor. ● Instruction Cycle is a program to be executed by a processor consists of a set of instructions stored in the memory. ○ Instruction processing consists of 2 steps: ■ Read instructions from memory (fetch stage) ■ Execute each instruction (execute stage) ○ Program Execution consists of a repeating process of instruction fetch and instruction execution. The processing required for a single instruction is called an Instruction Cycle. ■ Program Counter (PC) holds the address of the next instruction to be fetched. ■ After each instruction, the Program Counter (PC) is incremented. ○ The processor interprets the instruction and performs the required actions. These actions fall into 4 categories: ■ The processor to memory (and vice versa) [Sending Data] ■ Processor to I/O device (and vice versa) [Sending Data] ■ Data processing. ■ Control: This alters the sequence of execution. Instruction Cycle Fetch Start Execute Stop ● Most modern processors include instructions that contain more than one address. Thus the execution stage for a particular instruction may involve more than one reference to memory. ○ However, we will restrict ourselves to instructions with one address only. ● Instruction format: Op-code 0 Address 34 Operation to be performed 13 Address of the operand ● Interrupts are provided primarily as a way to improve processor utilization. I/O modules interrupt normal processing of the CPU. With interrupts, the processor can be engaged in executing other instructions while an I/O operations is in progress.
