IBM 1 Series - Page 21

2-10
GA34-0033
Status Bus. This is an outbound four-bit bus used by the channel on a
cycle-steal service sequence. The bus signals the device being serviced of
any errors that the channel has detected. The bus is bit-significant, as
follows:
Status
bus bit Meaning IPL meaning
0 Storage data check Primary device
1 Invalid storage address Alternate device
2 Protect check
3 Interface data check
If an error is indicated on cycle-steal service sequences that are not a part
of IPL, the device retains this information for presentation to the software
via ISB bits 4—7 at interruption time. Cycle-steal operations are terminated
and an exception interrupt is presented. If an error is indicated on IPL
cycle-steal sequences, the device terminates cycle-steal operations, but
remains in an IPL state with the ‘IPL’ line active and does not present an
end interrupt. Therefore, it is not necessary that status bus bits be
recorded during IPL for later presentation. DPC devices supporting IPL
should especially note this.
Bits O and 1 of the status bus are also used to select the primary and
alternate IPL source, respectively. The appropriate bit of the status bus is
activated with the ‘initiate IPL’ signal to accomplish this selection. Only
one device can be configured as a primary IPL source and only one device
can be configured as an alternate IPL source at any one time on the I/O
channel attached to the processor.
Data Strobe. This is an outbound tag to the I/O device on: DPC,
interrupt, or cycle-steal service sequences. This tag can be used by I/O
devices to: (1) accomplish control actions, (2) register data on outbound
transfers, and (3) accomplish appropriate data resets on inbound transfers.
‘Data strobe’ always occurs on normal DPC write, cycle-steal, and
interrupt-service sequences. On DPC read sequences, ‘data strobe’ is not
activated if the channel detects a parity error.
Initiate IPL. This is an outbound tag from the channel to the IPL source
when the IPL is initiated by the processor Load key. The ‘initiate IPL’ tag
is singular in nature and meaning, and signals the IPL source that the
processor requires an IPL. The receiver for this tag is always enabled.
Bits O and 1 of the status bus are used for selecting the primary and
alternate sources, respectively; see “‘Status Bus”’ in this section.
IPL. This is an inbound tag (1) from the IPL source, activated in response
to the ‘initiate IPL’ tag or (2) from a host system, to signal the processor
that the host is initiating an IPL action. The storage load itself takes place
via the cycle-steal mechanism from the selected device or from the host
system.