Host-Initiated Ipl Sequence Description - IBM 1 Series Manual

Host-Initiated IPL Sequence Description
2-36 GA34-0033
Refer to Figure 2-15. The figure is divided into two basic sequence timing
diagrams: @"Initial sequence to normal end" and @ '"'Termination and
retry sequences," which assumes abnormal termination. The two parts
have a common entry point in the activation of the 'IPL' tag by the host
IPL device, as shown in the upper part of the figure. The host-initiated
IPL sequences (sometimes called remote IPL) are executed as follows:
Initial sequence to normal end (Figure 2-15)
1. The host successfully executes a command to the I/O channel device
configured as the host IPL device to IPL the system. It is important to
rememberthat a host IPL may be an asynchronous event to the
Series/1; therefore, it is to be expected that asyrchronous activity may
be in progress on the channel at the time of execution of the host IPL
command. These sequences, including normal I/O activity and resets,
may be directed to the host IPL attachment or to other devices on the
I/O channel. However, successful execution of the host IPL command
must be assumed to take precedence over Series/1 sequences.
2. After execution of the host IPL command, the device attachment must
activate the 'IPL' tag. The device attachment activates this tag only
once for each host IPL commandsuccessfully executed. After
activation of the 'IPL' tag, the host IPL device attachment must ignore
all I/O channel activity not specifically related to execution of the host
IPL sequence.
3. 'System reset' is activated at a time, CT2 less than 500 microseconds
after the IPL tag is activated. This 'system reset' is activated for a
duration, CT1, of 4.8 microseconds, minimum, and is analogous to the
second 'system reset' of the processor-initiated IPL sequence; it has a
similar unique nature. The host IPL device maintains an active 'IPL'
tag while using this 'system reset' to clear its interface signals, to reset
status not associated with host IPL, and to enable the cycle-steal
transfer for the storage load. The device should use only thetrailing
edge of this 'system reset' to accomplish this enabling. All other
devices on the channel obey this 'system reset' as if it were a normal
'system reset.' Asynchronous I/O activity on the channel is cleared, as
seen at the device interface prior to the deactivation of this 'system
reset.'
Note: The only exception to the clearing of this asynchronous I/O
activity is related to a processor-initiated IP!. sequence. This condition
would arise, for example, in a rare situation where the operator
pressed the Load key just prior to the host initiating an IPL. The host
IPL attachment should not be cognizant of such activity; for purposes
of executing the host IPL sequences, the attachment should assume
that all asynchronous I/O activity is cleared.
4. After this 'system reset' is deactivated, IPL cycle-steal requests and
transfers may begin. The time, T3, from the deactivation of the
'system reset' to the activation of the first cycle-steal request must be
greater than OQ, as seen at the device interface; the maximum timeis
device-dependent. The IPL record length can be up to a maximum of
64K bytes. Successful completion of IPL is signaled to the processor
by the device dropping the 'IPL' tag. The time, T4, from the end of
cycle-steal requests and transfers (as defined by the deactivation of the