Chapter 3 Top Level View of Computer Function and Interconnection

1. Chapter 3Top Level View of Computer Function and Interconnection Prepared by: Dr. Bahjat Qazzaz Reference: Text Book

2. Von Newmann architecture • It is based on three key concepts: • Data and instructions are stored in a single read-write memory. • The contents of this memory are addressable by location, without regard to the type of data contained there. • Execution occurs in a sequential fashion from one instruction to the next.

3. Sequece of arithmetic and logic functions Data Results Programming in hardwired • Hardwired program: • A set of basic logic components can be combined in a way to perform arithmetic and logical operations on the data • (e.g. a configuration of logic components can be designed to do specific calculation. • The process of connecting these components produce a program in the form of hardware

4. Instruction codes Instruction Interpreter Control signals General-purpose aritmetic and logic functions Data Results Programming in software Software Program • Software: Instead of rewiring the hardware for each new program, all we need to do is • provide a new sequence of codes. • Each code is, in effect, an instruction, and • part of the hardware interprets each instruction and generates control signals.

5. Programming in software: CPU Components • So, we can say, the CPU consists of • an instruction interpreter • a module for general purpose arithmetic and logic functions Other components are needed

6. Programming in software: CPU Components • I/O component for accepting data and instruction in some form and converting them into an internal form of signals usable by the system, and for reporting results in the form of an output module. • Memory module (main memory) is a place where to store temporarily both data and instructions

7. Computer Component: Top Level View • CPU exchanges data with memory using MBR for data & MAR for addresses. • I/O AR specifies a particular I/O device, and the I/O BR is used to exchange data bwn an I/O module and the CPU • A Memory module consists of locations addressable by numbers. • I/O module transfers data from external devices to CPU and memory, and vice versa.

8. Computer Basic Function • The basic function performed by a computer is execution of a program, which consists of a set of instructions stored in memory. • The instruction is in the form of a binary code that specifies what action the CPU is to take. Instruction processing consists of two steps:

9. Fetch/Execute Cycle • Instruction fetching and • Instruction execution

10. Fetch cycle • Fetch cycle • Program Counter (PC) holds address of next instruction to fetch • Processor fetches instruction from memory location pointed to by PC • Increment PC (Unless told otherwise (branch)) • Instruction loaded into Instruction Register (IR) • Processor interprets instruction and performs required actions (see execute cycle below)

11. Execute Cycle • Execute Cycle • Processor-memory (data transfer between CPU and main memory) • Processor I/O (Data transfer between CPU and I/O module) • Data processing (Some arithmetic or logical operation on data) • Control • Alteration of sequence of operations (e.g. jump) • Combination of above

12. Example of Program Execution • Consider a machine with the following characteristics: Instruction format Integer format • The CPU contains an accumulator (AC) to temporarily store data • The notation used is hexadecimal (each digit represents 4 bits)

13. Example of Program Execution (cont.) Program fragment: LOAD 940 ADD 941 STORE941

14. Instruction Cycle state diagram • iac: determines the address of the next instruction to be executed • if: fetch (read) instruction from memory location into the processor

15. Instruction Cycle state diagram • iod: decode (analyze) instruction to determine type of operation to be performed and operand/s to be used • oac: (operand address calculation) determine the address of the operand (in memory or I/O) if the instruction has a reference to an operand. • of: fetch the operand from memory or read it in from I/O • do: (data Operation) perform the operation indicated in the instruction • os (operand store): Write the result into memory or out to I/O

16. Notice that: The upper part of the diagram involve data exchanging bwn the CPU and either the memory or an I/O module, while • the lower part involve only internal processor operations

17. Interrupts • Interrupt is a mechanism by which other modules (e.g. I/O, memory) may interrupt the normal processing of the processor. • Its main goal is to improve processing efficiency since the external devices (e.g. I/O modules) are very slow. CPU may waste time waiting for these slow devices to perform their tasks • With interrupts, the processor can be engaged in executing other instructions while an I/O operation is in progress.

18. Interrupts • Example: • Without interrupts, the processor will set idle after each write operation until the printer is catching up. • The length of the pause may be in order of thousands of instructions cycles that don’t involve memory. It is wasteful!

19. Interrupts • Classes of interrupts • Program: Generated by some condition that occurs as a result of an instruction execution, such as arithmetic overflow, divide by zero, illegal memory use. • Timer: Generated by a timer within the processor. It allows the OS to perform certain functions on a regular basis.

20. Interrupts • Classes of interrupts • I/O: Generated by an I/O controller, to signal normal completion of an operation or to signal a variety of error conditions. 4. Hardware failure: Generated by a failure such as power failure or mem parity error.

21. Interrupts

22. Interrupts • a) No interrupts • User program performs a series of WRITE calls. code segments 1,2, and 3 don’t involve I/O • The WRITE calls are to an I/O program that is a system utility that performs the actual I/O operation. • The I/O program consists of the following:

23. Interrupts • a) No interrupts • A sequence of instructions to prepare for the actual I/O operation (e.g. copy data to buffer, preparing the parameters for device command). code 4 • The actual I/O command: once this command is issued, the program will wait until the I/O device is finished if we don’t use interrupts. • A sequence of instructions to complete the operation. This may include setting flags indicating the success or failure of the operation. code 5. • The user program stopped at the write operation for some period of time

24. Interrupts • b) Interrupts, short I/O wait • with interrupts, the cpu can be engaged in executing other instructions while an I/O operation is in progress. • When the user’s program executes write call • The I/O program is invoked, and the preparation commands are executed • then, control returns to the users’ program, and the external device is busy accepting data from memory and printing it.

25. Interrupts • b) Interrupts, short I/O wait • This I/O operation is conducted concurrently with the execution of instructions in the user program. • When the external device becomes ready to accept more data from the processor, the I/O module for the external device sends an interrupt request signal to the processor. • The processor suspends execution of the current program, service the interrupt, and go back to proceed executing the user’s program • The processor and the OS are responsible for handling the interrupts

26. Interrupts: Instruction cycle with interrupts

27. Interrupts • If an interrupt is pending, the processor does the following: 1. It suspends the execution of the current program and saves its context (saves the address of the next instruction to be executed). 2. It sets the program counter to the starting address of an interrupt handler routine (part of the OS), and fetches the instructions of the interrupt handler for execution 3. When the interrupt handler routine is completed, the processor can resumes execution of the user’s program.

28. gain in efficiency with short I/O wait

29. Interrupts • c) Interrupts, long I/O wait • The idea is to show long time execution by an I/O (e.g printer) so, the user’s program makes another write call before the first write call is completed. What will happen? • The user program is hung at this point. • when the preceding I/O operation is completed, the new write call may be proceed, and a new I/O operation may be started

30. The figure shows that there is still a gain in efficiency

31. The revised instruction cycle state diagram that includes interrupt cycle processing

32. Interrupts • Multiple interrupts Example: • A program may be receiving data from a communication line and printing results. So, • the printer will generates an interrupt every time that it completes a print operation, and • the communication line controller will generate an interrupt every time a unit of data arrives.

33. Interrupts • It is possible for a communication interrupt to occur while a printer interrupt is being processed. So what will happen: • Two approaches: • Disable interrupts • Define priorities

34. Interrupts • Disable interrupts • Processor will ignore further interrupts whilst processing one interrupt • Interrupts remain pending and are checked after first interrupt has been processed • Interrupts are handled in sequence as they occur

35. Interrupts: sequential interrupt processing

36. Interrupts • Define priorities • Low priority interrupts can be interrupted by higher priority interrupts • When higher priority interrupt has been processed, processor returns to previous interrupt

37. Interrupts: nested interrupts processing

38. Interrupts: Example

39. Interrupts

40. Interrupts

41. Interrupts