IBM 1 Series - Page 24

Poll
Poll
prime
Figure 2-6.
Poll and Poll Prime. The ‘poll’ tag is a serially propagated tag generated
by the channel to resolve contention between multiple devices requesting
interruptions on the same level and cycle-steal requests. Each I/O device
receives the ‘poll’ tag and redrives it to the next device on the I/O channel
via its ‘poll propagate’ output line.
The ‘poll’ and ‘poll prime’ tags are always recognizable by an I/O device
that is capable of presenting interrupts or cycle-stealing. An I/O device
recognizes a poll as the leading edge of the logical AND of ‘poll’ and ‘poll
prime.’ The device does a logical compare between the poll ID bits on the
interface and the ID bits in its prepare register. An equal compare of the
poll ID Bits ANDed with ‘poll’ and ‘poll prime’ is called poll capture. Once
the poll is captured, an I/O device can then respond with ‘poll return’ or
‘burst return.’ If the poll capture does not occur, the ‘poll’ tag is
propagated to the next device on the I/O channel.
Because of the serial nature of the poll mechanism, the relative physical
position of I/O attachments on the channel is a major determinant of the
priority for servicing contending cycle-steal requests and for servicing
contending interrupt requests on the same interrupt level. Those
attachments located in positions nearer to the processor are the first
elements on the serial poll chain and are effectively of higher priority.
To facilitate the removal of an I/O attachment card from the I/O channel
without interrupting the poll propagation, a bypass mechanism is provided
(refer to Figure 2-6). The ‘poll prime’ tag received by I/O attachment
card N+2 is the same line that I/O attachment card N+1 receives as its
‘poll’ tag. If I/O attachment card N is removed, the ‘poll’ tag input to
attachment card N+1 and the ‘poll prime’ input to attachment card N+2
appear as a logical 1. The ‘poll prime’ input to attachment card N+1 then
follows the ‘poll propagate’ output of attachment card N-1.
1/O I/O I/O
attachment attachment attachment
card card card
N N+t+1 N+2.
‘Poll Poll Poll
propagate Poll propagate Poll propagate
M12 M11 M12 Mill M12 M11
Poll Poll
prime prime
M13 M13 M13
Y
Poll bypass mechanism
Processor I/O Channel 2-13