Upper and Lower Half Interaction

The upper half and lower half share data using a circular queue or buffer. Characters are put in the queue by the upper half for transmission by the lower half. Likewise, received characters are enqueued by the lower half of the driver for consumption by application processes.

The upper and lower halves coordinate their activities as follows:

• When an application has a character to be sent, it calls an upper half routine. This routine deposits the character in the output queue. Remember that the SLU will interrupt whenever it becomes idle. When it does so, the ISR in the lower half of the TTY driver picks up the next character from the output queue and initiates its transmission.
• Input is similar. The SLU will interrupt whenever a character has been received. At that time the ISR in the lower half will take the character from the device and deposit it in the input circular queue. Later, when an application process asks for another character, the upper half routine will take the next character from the queue.

Both input and output queues are teated as circular queues with pointers to the head and tail. Characters are always put in the queue at the head position, and characters are always taken off at the tail position.

Other coordination is also required. For example, if there is nothing to send, the upper half routines may want to stop the SLU from interrupting whenever it becomes idle. It may also want to start the lower half when it makes a character ready to be sent. Likewise, if there are no characters in the buffer, the upper half routines may want to block a process trying to read a character.