![Interrupt
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• Software can execute the software instruction (SWI) or Interrupt n
(INT n) to signal execution of ISR (interrupt service routine). The n
is as per the handler address.
• Signal interrupt [The signal differs from the function in the sense
that execution of signal handler (ISR) can be masked and till mask
is reset, the handler will not execute on interrupt. Function on the
other hand always executes on the call after a call-instruction.]](https://image.slidesharecdn.com/devicedriversandinterruptsservicemechanism-240412163817-7c00f782/85/DEVICE-DRIVERS-AND-INTERRUPTS-SERVICE-MECHANISM-pdf-33-320.jpg)
DEVICE DRIVERS AND INTERRUPTS SERVICE MECHANISM.pdf
- 1. DEVICE DRIVERS AND INTERRUPTS SERVICE MECHANISM Programmed I/O (busy and wait) method for ports and devices, and the need for interrupt driven IOs 1
- 2. An embedded system uses its input/output devices to interact with the external world. Input devices allow the computer to gather information, and output devices can display information. Output devices also allow the computer to manipulate its environment. The tight-coupling between the computer and external world distinguishes an embedded system from a regular computer system. The challenge is under most situations the software executes much faster than the hardware. E.g., it might take the software only 1 us to ask the hardware to clear the LCD, but the hardware might take 1 ms to complete the command. During this time, the software could execute tens of thousands of instructions. Therefore, the synchronization between the executing software and its external environment is critical for the success of an embedded system.
- 4. Interrupt Concepts An interrupt is the automatic transfer of software execution in response to a hardware event that is asynchronous with the current software execution. This hardware event is called a trigger. The hardware event can either be a busy to ready transition in an external I/O device (like the UART input/output) or an internal event (like bus fault, memory fault, or a periodic timer). When the hardware needs service, signified by a busy to ready state transition, it will request an interrupt by setting its trigger flag.
- 5. A thread is defined as the path of action of software as it executes. The execution of the interrupt service routine is called a background thread. This thread is created by the hardware interrupt request and is killed when the interrupt service routine returns from interrupt (e.g., by executing a BX LR). A new thread is created for each interrupt request. It is important to consider each individual request as a separate thread because local variables and registers used in the interrupt service routine are unique and separate from one interrupt event to the next interrupt.
- 6. In a multi-threaded system, we consider the threads as cooperating to perform an overall task. Consequently, we will develop ways for the threads to communicate (e.g., FIFO) and to synchronize with each other. Most embedded systems have a single common overall goal. On the other hand, general-purpose computers can have multiple unrelated functions to perform.
- 7. A process is also defined as the action of software as it executes. Processes do not necessarily cooperate towards a common shared goal. Threads share access to I/O devices, system resources, and global variables, while processes have separate global variables and system resources. Processes do not share I/O devices.
- 8. Programmed IOs approach from ports and devices • Processor is continuously busy in executing the program related to input or output from the port and waits for the input ready or output completion • Processor is continuously busy in executing the program related to device functions and waits for the device status ready or function completions 8
- 9. Example─ A 64-kbps UART Input •When a UART transmits in format of 11-bit per character format, the network transmits at most 64 kbps 11 = 5818 characters per second, which means every 171.9 s a character is expected. •Before 171.9 s, the receiver port must be checked to find and read another character assuming that all the received characters are in succession without any in-between time- gap 9
- 10. Format of bits at UART protocol 10
- 11. Ports A and B with no interrupt generation and interrupt service (handling) mechanism •Port A be in a PC, and port B be its modem input which puts the characters on the telephone line. •Let In_A_Out_B be a routine that receives an input- character from Port A and re-transmits the character to Port B output. 11
- 12. In_A_Out_B routine 12 • Has to cyclically call the following steps a to e and executes the cycles of functions i to v, thus ensuring the modem port A does not miss reading the character
- 13. In_A_Out_B routine •Call function i •Call function ii •Call function iii •Call function iv •Call function v •Loop back to step 1 13
- 14. Steps a, b and c 14 • Step a: Function i─ Check for a character at port A, if not available, then wait • Step b: Function ii─ Read Port A byte (character for message) and return to step a instruction, which will call function iii • Step c: Function iii─ Decrypt the Message and return to step a instruction, which will call function iv
- 15. Steps d and e 15 • Step d: Function iv─ Encode the Message and return to step a instruction, which will call function v. • Step e: Function v ─ Transmit the encoded message to Port B and return to step a last instruction, which will start step a from beginning
- 17. Step a 17 • Does Polling─ Polling a port means to find the status of the port─ whether ready with a character (byte) at input or not. • Polling must start before 171.9 s because characters are expected at 64 kbps/11 bit format.
- 18. Condition in which no character misses 18 • If the program instructions in four steps b, c, d and e (functions ii to v) take total running time of less than 171.9 s then the above programmed IO method works.
- 19. Problems with Programmed IOs approach from ports and devices 1. (a) The program must switch to execute the In_A_Out_B cycle of steps a to e within a period less than 171.9 s. (b) Programmer must ensure that steps of In_A_Out_B and any other device program steps never exceed this time. 19
- 20. Problems with Programmed IOs approach from port as and devices 2. When the characters are not received at Port A in regular succession, the waiting period during step a for polling the port can be very significant. 3. Wastage of processor times for the waiting periods is the most significant disadvantage of the present approach 20
- 21. Problem with Programmed IOs approach from ports and devices 21 4. When the other ports and devices are also present, then programming problem is to poll each port and device, and ensure that program switches to execute the In_A_Out_B step a as well as switches to poll each port or device on time and then execute each service routines related to the functions of other ports and devices within specific time intervals such that each one is polled on time.
- 22. Problems with Programmed IOs approach from ports and devices 5. The program and functions are processor and device specific in the above busy-wait approach, and all system functions must execute in synchronization and timings are completely dependent on periods of software execution 22
- 23. IO based on an interrupt from Port A 23 • Instead of continuously checking for characters at the port A by executing function (i) we can first call step a when a modem receives an input character, sets a status bit in its status register and interrupts Port A. The interrupt should be generated by port hardware. • In response to the interrupt, an interrupt service routine ISR_ PortA _Character executes─ an efficient solution in place the wait at step a (poll for input character)
- 24. Application of programmed IOs 24 • In case of single purpose processor and dedicated IOs or device functions with continuous device status polling
- 25. Interrupt and Interrupt Service Routine Concept 25
- 26. • Interrupt means event, which invites attention of the processor on occurrence of some action at hardware or software interruptinstruction event. 26 Interrupt Concept
- 27. Action on Interrupt 27 • In response to the interrupt, the routine or program, which is running presently interrupts and an interrupt service routine (ISR) executes.
- 28. Interrupt Service Routine 28 • ISR is also called device driver in case of the devices and called exception or signal or trap handler in case of software interrupts
- 29. Interrupt approach for the port or device functions • Processor executes the program, called interrupt service routine or signal handler or trap handler or exception handler or device driver, related to input or output from the port or device or related to a device function on an interrupt and does not wait and look for the input ready or output completion or device-status ready or set 29
- 30. Hardware interrupt 30 • Examples─ When a device or port is ready, a device or port generates an interrupt, or when it completes the assigned action or when a timer overflows or when a time at the timer equals a preset time in a compare register or on setting a status flag (for example, on timer overflow or compare or capture of time) or on click of mice in a computer • Hardware interrupt generates call to an ISR
- 31. Software Interrupt 31 Examples •When software run-time exception condition (for examples, division by 0 or overflow or illegal opcode detected) the processor- hardware generates an interrupt, called trap, which calls an ISR • When software run-time exception condition defined in a program occurs, then a software instruction (SWI) is executed─ called software interrupt or exception or signal, which calls an ISR .
- 32. Software Interrupt 32 • When a device function is to be invoked, for example, open (initialize/configure) or read or write or close , then a software instruction (SWI) is executed─ called software interrupt to execute the required device driver function for open or read or write or close operations.
- 33. Interrupt 33 • Software can execute the software instruction (SWI) or Interrupt n (INT n) to signal execution of ISR (interrupt service routine). The n is as per the handler address. • Signal interrupt [The signal differs from the function in the sense that execution of signal handler (ISR) can be masked and till mask is reset, the handler will not execute on interrupt. Function on the other hand always executes on the call after a call-instruction.]
- 34. Interrupt Driven Actions On Interrupt Interrupt Service Routine (ISR) Running Program Return No continuous checks of device status 34
- 35. How does call to ISR differ from a function (routine) call? 35
- 36. Routine (function) and ISR call features 36 • On a function call, the instructions are executed from a new address • Execute like in as function in the ‘C’ or in a method in Java. • ISR also the instructions are executed from a new address like in as function in the ‘C’ or in a method in Java
- 37. Routine (function) call and ISR call features 37 • A function call is after executing present instruction in any program and is a planned (user programmed) diversion from the present instruction in sequence of instructions to another sequence of instructions; this sequence of instructions executes till the return from that. • An ISR call is after executing present instruction in any program and is interrupt related diversion from the current sequence of instructions to another sequence of instructions; this sequence of instructions executes till the return from that or till another interrupt of higher priority
- 38. Nesting of function calls and ISRs 38 • Nesting the function-calls ─ When a function 1 calls another function 2 and that call another function 3, on return from 3, the return is to function 2 and return from 2 is to function. Functions are said to be nested • The ISR calls in case of multiple interrupt occurrences can be nested or can be as per priority of the interrupts
- 39. Using the Interrupt(s) and ISR(s) for each device function (for example, read, write) - Use of ISRs (Interrupt Service Routine) by SWIs is the main mechanism used for the device accesses and actions 39
- 40. Interrupt driven IO and device accesses feature 1. There is no continuous monitoring of the status bits or polling for status by the application. 2. Between two interrupt calls the program task(s) continue. There are many device- functions, and each of which executes on a device interrupt. 40
- 41. Example─ A 64-kbps UART Input •When a UART transmits in format of 11-bit per character format, the network transmits at most 64 kbps 11 = 5818 characters per second, which means every 171.9 s a character is expected. •Before 171.9 s, the receiver port is not checked. Only on interrupt from the port the character is read •There is no in-between time-gap for polling 41
- 42. Ports A and B with interrupt generation and interrupt service (handling) mechanism • Port A be at in a PC, and port B be its modem input which puts the characters on the telephone line. • Let ISR_ PortA_Character be a routine that receives an input character from Port A on interrupt and Out_B routine re-transmits an output character to Port B. 42
- 43. ISR_ PortA _Character 43 • Step f function (vi): Read Port A character. Reset the status bit so that modem is ready for next character input (generally resetting of status bit is automatic without need of specific instruction for it). Put it in memory buffer. Memory buffer is a set of memory addresses where the bytes (characters) are queued for processing later. • Return from the interrupt service routine.
- 44. 2008 Chapter-4 L02: "Embedded Systems - " , Raj Kamal, Publs.: McGraw-Hill Education 20 ISR_ PortA _Character On interrupt call ISR_ PortA _Character Save PC, status word and registers (current program context) on stack Step f Execute function vi codes (read character, reset port status bit, character save in memory buffer) Return (retrieve the saved context) Current program
- 45. Out_B routine 45 • Step g: Call function (vii) to decrypt the message characters at the memory buffer and return for next instruction step h. • Step h: Call function (viii) to encode the message character and return for next instruction step k • Step k: Call function (ix) to transmit the encoded character to Port B • Return from the function.
- 46. Out_B Routine Out_B Routine Step g Step h Step k Each step has three parts, save context, execute codes, and retrieve the context for return 46
- 47. Application of Interrupt based IOs and device functions • In case of multiple processes and device functions with no device status polling or wait 47
- 48. Use of ISR in the mobile phone reset key interrupt example Mobile Keypad events input processor at mobile Event: Reset Key pressed to reset the mobile ISR_ResetKey Read key state at Port bits P0-P7 Reset key state status bit Sent message for tasks Signal timer start for enabling display off after 15 s (preset time) P0-P7 Reset event 1 2 Call 4 Signal timer Tasks 4 Data bus 5 R et ur n Interrupt 3 Sent messag e for Tasks 48
- 49. Software Interrupts and Interrupt Service routines 4 9
- 50. Software Interrupt (Throw an Exception) Concept • A program needs to detect error condition or run time exceptional condition encountered during the running. • In a program either the hardware detects this condition or in a program detects this condition, then an instruction SWI (software interrupt) is used 5 0
- 51. Detection of exceptional run-time condition •Called throwing an exception by the program. •An interrupt service routine (exceptional handler routine) executes, which is called catch function as it executes on catching the exception thrown. 5 1
- 52. SWI •Executes on detecting the exceptional run- time condition during computations or communication. •For example, on detecting that the square root of a negative number is being calculated or detecting illegal argument in the function or detecting that connection to network not found. 5 2
- 53. Example: SWI a1 and SWI a2 5 3 • The SWI (software interrupt) instructions, SWI a1 and SWI a2 will be inserted for trapping (A- B) as –ve number and trapping y > 100 or less than 0
- 54. Software instruction SWI a1 5 4 • Causes processor interrupt. • In response, the software ISR function ‘catch (Exception_1 a1) { }’ executes on throwing of the exception a1 during try block execution. • SWI a1 is used after catching the exception a1 whenever it is thrown
- 55. Software instruction SWI a2 5 5 • Causes processor interrupt. • In response, the software ISR function ‘catch (Exception_2 a2) { }’ executes on throwing of the exception a1 during try block execution. • SWI a2 is used after catching the exception a2 whenever it is thrown
- 56. 2008 Chapter-4 L03: "Embedded Systems - " , Raj 8 Use of SWI software interrupt-instruction for calling an ISR in the software on throwing and catching the exceptional run- time conditions a1 and a2 encountered during computations Exceptional handler Find exception source a1 Run codes for actions to be taken on event a1 in catch (Exception_1 a1) { } Find exception source a2 Run codes for actions to be taken on event a2 in catch (Exception_1 a2{ } Call function to execute at end Call function to execute end on detecting an exception condition or error condition in try block End of Function Run Codes at finally { …} codes, which should execute when the exceptions or try bl ock instructions finished Status Flags for the Exception sources in the memory throw event a1 Software interrupt throw event a2 Software interrupt on detecting another exception condition or error condition in try block 1 2
- 57. SWI a3 57 • Software ISR function ‘finally { }’ executes either at the end of the try or at the end of catch function codes. • SWI a3 is used after the try and catch functions finish, then finally function will perform final task, for example, exit from the program or call another function
- 58. Signal from a thread for Signal handler Interrupt Service Routine • ISR is also called signal handler in case of a routine or program thread, or task sends a signal using an SWI • Signals are used to notify error conditions or notifying end of an action to enable signal handler thread or task to initiate action on that. 58
- 59. Action on Signal generated by SWI and signal handling A program or task or routine 1 Run Codes Signal (event e1) for run routine 2 Status Flags for the Signal sources in the memory interrupt A program or task or routine 2 Run Codes Signal (event e2) for run routine 3 Software interrupt 1 Software 2 Signal handler Find signal a1 Run codes for actions to be taken on signal a1 from program Find signal a2 Run codes for actions to be taken on signal a2 from program to execute at end 59
- 60. Device driver 60 • Does the interrupt service for any event related to the device and use the system and IO buses required for the device service. • Device driver can be considered software layer between an application program and the device
- 61. Interrupt service routines 61 • An Interrupt service routine (ISR) accesses a device for service (configuring, initializing, activating, opening, attaching, reading, writing, resetting, deactivating or closing). • Interrupt service routines thus implements the device functions of the device driver
- 62. First level ISR and second level IST approach to handle the device hardware interrupts followed by software interrupt Command c1 Device driver command 1 Run short code Signal ISR 1 Device driver command 2 Run short code Signal ISR 2 Command c2 Interrupt Interrupt (signal) Interrupt (signal) ISR 1 Run codes of device function 1 ISR 2 Run codes of device function 2 device event e3 Interrupt Interrupt 62