Implementing interrupt driven IO

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Implementing interrupt driven IO
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Why use interrupt driven IO?

• Positive points
• Allows asynchronous operation of IO events
• Good use of resources
• Leads to modular structure of system
• Leads to better response times for interactive
  tasks

• Negative points
• It is harder to program
• Leads to more complex code and system
• Can be difficult to debug errors
• Could have security implications

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Hardware resources required

• We will need a processor which can accept
  interrupts
• A serial port module that can generate interrupts
• A priority scheme would be useful
• At least bi-modal processor at least supervisor
  and user modes

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The 68307 Serial module
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UART Mode Register 1

• RxIRQReceiver Interrupt Select
• 1 FFULL is the source that generates IRQ.
• 0 RxRDY is the source that generates IRQ.

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UART Interrupt Status Register

• RxRDYReceiver Ready or FIFO Full
• The function of this bit is programmed by UMR1
  bit 6. It is a duplicate of either the FFULL or
  RxRDY bit of USR.
• TxRDYTransmitter Ready
• This bit is the duplication of the TxRDY bit in
  USR.
• 1 The transmitter holding register is empty and
  ready to be loaded with a character.
• 0 The transmitter holding register was loaded
  by the CPU, or the transmitter is disabled.
• Characters loaded into the transmitter holding
  register when TxRDY0 are not transmitted.

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UART Interrupt Mask Register

• FFULLFIFO Full
• 1 Enable interrupt
• 0 Disable interrupt
• TxRDYTransmitter Ready
• 1 Enable interrupt
• 0 Disable interrupt

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UART Interrupt Vector Register

• IVR7IVR0Interrupt Vector Bits
• This 8-bit number indicates the offset from the
  base of the vector table where the address of the
  exception handler for the specified interrupt is
  located. The UIVR is reset to 0F, which
  indicates an uninitialized interrupt condition.

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(D)UART ISR CODE
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Library and System IO calls

• Library and system IO calls do not write to the
  IO devices. They write to internal operating
  system buffers.
• These buffers are in
• User space for library buffers fprintf etc
• In system space for system calls write, read
  etc
• This allows processes to read and write data
  off-line from peripherals

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IOCTL or IO control functions

• As there is a separation between the calls that
  handle the hardware and handle the data buffers,
  the operating system can process the data quite
  easily running a series of control functions on
  the data.
• In Unix/Linux these are called IOCTL calls. For
  example
• stty -raw

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Writing data from user space to a device
User Space
System/kernel space
Printf
Write
Sys_write
Printf data
D A T A
D A T A
D A T A
DEVICE
Buffer
Buffer
IOCTL Functions
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Connecting Processes with their IO

• In multi-tasking and multi-user systems there are
  a number of users and processes waiting to send
  or receive data.
• The operating systems must make sure that they
  get their correct data or that it goes to the
  correct resource
• This is handled by the scheduler and IO manager
  modules.