Transmitter Interrupts

1
Transmitter Interrupts

• Reference on Critical RegionsText Tanenbaum
  2.3.2

2
How to Output Character Strings

• Details on programming UART registers
• slide 14 of http//www.cs.umb.edu/bobw/CS341/Data
  Book/c8652.pdf
• Use Programmed I/O Set up a loop for the number
  of characters in the string check the
  transmit-ready(THRE) bit of the LSR Register if
  on, write the char to UART_TX register otherwise
  , loop
• Use Interrupt-driven I/O

3
Transmitter Interrupts

• Each interrupt signals readiness to output
  another byte of data
• The transmitter ISR gives another byte to output
• Issues to consider
• How to distinguish the Tx from Rx interrupts
  since they share one interrupt line(e.g. IRQ3 for
  COM2)?
• what to do when data runs out?
• How to start/restart the Tx interrupt?

4
How to Distinguish the Interrupts?

• One interrupt service routine to perform both Tx
  and Rx functions
• Check the UARTs LSR to see if Data Ready (DR)
  bit or the Transmit Holding Register(THRE) bit is
  set. Then execute the corresponding function.
• Any one of the 3 situations can happen
• Both Tx and Rx interrupts are ready
• Only Tx interrupt is ready
• Only Rx interrupt is ready

5
What to Do when Tx Data Runs Out ?

• THRE bit set to 1 when the UART hardware is ready
  to accept a new character for transmission.
• THRE bit set to 0 after UART_Tx register is
  loaded with a new character.
• If there is a Tx interrupt, dequeue a character
  and output it.
• Shut down the Tx interrupt when queue is empty.
• Rx interrupts need to be on continuously. Tx
  interrupts go on and off depending on output
  availability.

6
The Transmitter Strategy

• Set up interrupt gate in IDT, but leave Tx
  interrupt off in the UART.
• To load a string (n chars) into queue
• 1. Load the first part of the string into the
  output buffer (i.e. enqueue the chars until the
  queue is full (Qmax)).
• 2. Turn Tx interrupt on in the UART_IER.
• 3. While the interrupt is ongoing, loop over the
  enqueuing of the rest of the chars.
• 4. Return when all chars have been enqueued.
• To output the chars in the queue
• 1. Check if the queue is empty.
• 2. If there are chars, dequeue one and output it
  to UART_TX.
• 3. If there none, shut down Tx interrupt in the
  UART_IER.

7
An Example

• write(TTY1, 123456789, 9) with nchar 9 and
  Qmax 6
• 1. 123456 put in queue during the setup in
  ttywrite
• 2. 123 output ,one by one, by ISR
• 3. 789 get loaded in the queue in ttywrite
• 4. ttywrite returns after all nchar in queue
• 5. ttywrite has returned, but 6 chars 456789
  are still in the queue. They are outputted one by
  one by ISR

8
Timeline Representation
Note 1. Still have a busy loop in ttywrite,
i.e. loop of enqs 2. OK for hw1. Fix it later
using a scheduler 3. Could have a race condition
due to concurrent activities on modifying the
queues
9
The Race Condition

• 2 concurrent activities occur when executing
  ttywrite
• ttywrite looping on enqueues
• intermittent ISR executing a dequeue
• working with the same queue (tty-gttbuf)
• Interrupt can occur between 2 machine
  instructions.
• in ttywrite if (enqueue( (tty-gttbuf),
  bufi)) i
• during enqueue function, ISR calling dequeue
  function can mess up the queue data structure

10
How to Avoid the Race Condition

• This part of program is called critical region or
  critical section when it accesses shared memory.
• Use mutual exclusion to make sure one part of
  program is using the shared variable while the
  other part is excluded from doing the same thing.
• Four conditions to provide mutual exclusion
• No two processes simultaneously in critical
  region
• No assumptions made about speeds or numbers of
  CPUs
• No process running outside its critical region
  may block another process
• No process must wait forever to enter its
  critical region

11
Mutual Exclusion Using Critical Regions
12
Mutual Exclusion with Busy Waiting

• Disabling Interrupts
• Locked Variables
• Strict Alternation
• Petersons Solution
• Test and Set Lock (TSL)

13
Turning off Interrupts in ttywrite

• We can turn off interrupts during the execution
  of enqueue and turn them on when enqueue is done
• How to do itcli() / disable interrupts in
  CPU by setting IF0 in EFLAGS/enqueue() /
  critical region code /sti() / or
  set_eflags(save_eflags) to re-enable interrupts /

14
Turning off Interrupts in printf

• Important to turn off interrupts before using
  printf
• Prevent part of the code to do programmed I/O and
  another one doing interrupt-driven I/O using the
  Tx of the UART
• kprintf() is the printf routine that turns off
  interrupts int saved_eflags save_eflags
  get_eflags() cli() printf() /
  SAPC-library printf uses programmed I/O /
  set_eflags(saved_eflags)

15
Echoes

• Characters to be echoed are generated in the
  input interrupt handler
• One copy is enqueued in the input queue and
  another in an output echo queue
• When the Tx ISR runs, it should look first in the
  echo queue. The user sees the echo as soon as
  possible