Dr. ClincyLecture Slide 1 CS Chapter 4 (Sec 5.1 &5.2) 1 of 5 Dr. Clincy Professor of CS

Dr. ClincyLecture2 Chapter 4 Objectives Learn the components common to every modern computer system. Be able to explain how each component contributes to program execution. Understand a simple architecture invented to illuminate these basic concepts, and how it relates to some real architectures. Know how the program assembly process works.

Dr. ClincyLecture3 Introduction In Chapter 2, we discussed how binary-coded data is stored and manipulated by various computer system components. In Chapter 3, we described how fundamental components are designed and built from digital circuits. Also from Chapter 3, we know that memory is used to store both data and program instructions in binary Having this background, we can now understand how computer components are fundamentally built The next question is, how do the various components fit together to create useful computer systems.

Dr. ClincyLecture4 Basic Structure of Computers Coded info is stored in memory for later use Input unit accepts code info from human operators, electromechanical devices (ie keyboard), other computers via networks ALU uses the coded info to perform the desired operations The output unit sends the results back out externally Program is stored in memory and determines the processing steps All actions are coordinated by the control unit Collectively called the I/O unit Collectively called the processor

Dr. ClincyLecture5 CPU Basics The next question is, how is the program EXECUTED and how is the data PROCESSED properly ? The computer ’ s CPU or Processor –Fetches the program instructions, –Decodes each instruction that is fetched, and –Perform the indicated sequence of operations on the data (execute) The two principal parts of the CPU are the Datapath and the Control unit. –Datapath - consists of an arithmetic-logic unit (ALU) and network of storage units (registers) that are interconnected by a data bus that is also connected to main memory. –Control Unit - responsible for sequencing the operations and making sure the correct data is in the correct place at the correct time.

Dr. ClincyLecture6 Registers hold data that can be readily accessed by the CPU – data like addresses, program counter, data, and control info Registers can be implemented using D flip-flops. A 32-bit register requires 32 D flip-flops. There are many different registers – –to store values, –to shift values, –to compare values, –registers that count, –registers that temporary store values, –index registers to control program looping, –stack pointer registers to manage stacks of info for processes, –status or flag registers to hold status or mode of operation, –and general purpose registers CPU Basics

Dr. ClincyLecture7 The arithmetic-logic unit (ALU) carries out –logical operations (ie. comparisons) and –arithmetic operations (ie. adding or multiplying) The ALU knows which operations to perform because it is controlled by signals from the control unit. The control unit determines which actions to carry out according to the values in a program counter register and a status register. The control unit tells the ALU which registers to use and turns on the correct circuitry in the ALU for execution of the operation. The control unit uses a program counter register to find the next instruction for execution and uses a status register to keep track of overflows, carries, and borrows. CPU Basics

Dr. ClincyLecture8 The Bus The CPU shares data with other system components by way of a data bus. A bus is a set of wires that simultaneously convey a single bit along each line. One or more devices can share the bus. The “sharing” often results in communication bottlenecks The speed of the bus is effect by its length and the number of devices sharing it

Dr. ClincyLecture9 The Bus Two types of buses are commonly found in computer systems: point- to-point, and multipoint buses. Point-to-point bus connects two specific devices Multipoint buses connects a number of devices. Because of the sharing, a bus protocol is used.

Dr. ClincyLecture10 Buses consist of data lines, control lines, and address lines. The Bus Data lines convey bits from one device to another. Moves the actual information that must be moved from one location to another. Control lines determine the direction of data flow, and when each device can access the bus. Address lines determine the location of the source or destination of the data. When sharing the bus, concurrent bus requests must be arbitrated. Four categories of bus arbitration are: –Daisy chain: Permissions are passed from the highest-priority device to the lowest. –Centralized parallel: Each device is directly connected to an arbitration circuit. –Distributed using self-detection: Devices decide which gets the bus among themselves. –Distributed using collision-detection: Any device can try to use the bus. If its data collides with the data of another device, it tries again.

Dr. ClincyLecture11 Types of Buses Processor-memory bus – short high speed bus used to transfer to and from memory I/O buses – longer buses that interface with many I/O devices other than the processor Backplane bus (or system bus) – connects the processor, I/O devices and memory. Expansion bus – connect external devices Local bus – a data bus that connect a peripheral device directly to the CPU Buses from a timing perspective: –Synchronous buses - work off clock ticks – all devices using this bus type are synchronized by the clock rate –Asynchronous buses – control lines coordinate the operations and a “handshaking protocol” is used for the timing. These types of buses can scale better and work with more devices

Dr. ClincyLecture12 Clocks Every computer contains at least one clock that: –Regulates how quickly instructions can be executed –Synchronizes the activities of its components. A fixed number of clock cycles are required to carry out each data movement or computational operation. As a result, instruction performance is measured in clock cycles. The clock frequency, measured in megahertz or gigahertz, determines the speed with which all operations are carried out. Clock cycle time is the reciprocal (or inverse) of its clock frequency. –An 800 MHz clock has a cycle time of 1.25 ns. Clock speed should not be confused with CPU performance. The CPU time required to run a program is given by the general performance equation : –We see that we can improve CPU throughput when we reduce the number of instructions in a program, reduce the number of cycles per instruction, or reduce the number of nanoseconds per clock cycle.

Dr. ClincyLecture13 The Input/Output Subsystem A computer communicates with the outside world through its input/output (I/O) subsystem. Input device examples: keyboard, mouse, card readers, scanners, voice recognition systems, touch screens Output device examples: monitors, printers, plotters, speakers, headphones I/O devices connect to the CPU through various interfaces. I/O can be memory-mapped-- where the I/O device behaves like main memory from the CPU ’ s point of view. Or I/O can be instruction-based, where the CPU has a specialized I/O instruction set.

14 Memory Organization We discussed a simple example of how memory is configured in Ch 3 – we now will cover more detail of: –How memory is laid out –How memory is addressed Envision memory as a matrix of bits – each row implemented as a register or “storage cell” – and each row being the size of a addressable Word. Each register or storage cell (typically called memory location) has a unique address. The memory addresses typically start at zero and progress upward Dr. ClincyLecture

15 Memory Organization Computer memory consists of a linear array of addressable storage cells that are similar to registers. Memory can be byte-addressable, or word-addressable, where a word typically consists of two or more bytes. Byte-addressable case: although the Word could be multiple bytes, each individual byte would have an address – with the lowest address being the “address” of the Word Memory is constructed of RAM chips, often referred to in terms of length  width. If the memory word size of the machine is 16 bits, then a 4M  16 RAM chip gives us 4 megabytes of 16-bit memory locations. Dr. ClincyLecture

16 For alignment reasons, in reading 16-bit words on a byte-addressable machine, the address should be a multiple of 2 (i.e 2 bytes) For alignment reasons, in reading 32-bit words on a byte-addressable machine, the address should be a multiple of 4 (i.e 4 bytes) For alignment reasons, in reading 64-bit words on a byte-addressable machine, the address should be a multiple of 4 (i.e 8 bytes). Memory Organization Dr. ClincyLecture

17 How does the computer access a memory location corresponds to a particular address? Memory is referred to using notation: Length x Width (L x W) We observe that 4M can be expressed as 2 2  2 20 = 2 22 words – means 4M long with each item 8 bits wide. Provided this is byte-addressable, the memory locations will be numbered 0 through Thus, the memory bus of this system requires at least 22 address lines. Memory Organization Dr. ClincyLecture

18 Physical memory usually consists of more than one RAM chip. A single memory module causes all accesses to memory to be sequential - only one memory access can be performed at a time By splitting or spreading memory across multiple memory modules (or banks), access can be performed in parallel – this is called Memory interleaving With low-order interleaving, the low order bits of the address specify which memory bank contains the address of interest. In high-order interleaving, the high order address bits specify the memory bank. Memory Organization Dr. ClincyLecture

19 Memory Organization Example: Suppose we have a memory consisting of 16 2K x 8 bit chips. – Memory is 32K = 2 5  2 10 = 2 15 –15 bits are needed for each address. –We need 4 bits to select the chip, and 11 bits for the offset into the chip that selects the byte. Dr. ClincyLecture

20 Memory Organization In high-order interleaving the high-order 4 bits select the chip. In low-order interleaving the low-order 4 bits select the chip. Dr. ClincyLecture