IBM 3745 Series, Hardware Maintenance Reference

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Mailbox Description
The NCP/PEP and the MOSS communicate through CCU storage areas called
mailboxes. These mailboxes are located in the 32-KB area reserved for MOSS
use, at the top of CCU main storage (see Table 8-1).
For a detailed description of the mailbox contents, see the 3745 Principles of Oper-
ation, GA33-0102.
Table 8-1. Mailbox Addressing in CCU Storage
CCU Storage (32K Top Area)
Address
in 4MB
Storage
Spare (192 bytes)
X'3FFFFF'
X'3FFF40'
NCP Out Mailboxes (32 bytes)
X'3FFF3F'
X'3FFF20'
NCP In Mailboxes (32 bytes)
X'3FFF1F'
X'3FFF00'
Unused
X'3FFEFF'
X'3FA400'
MOSS to Line Adapter Data Area
(8704 bytes)
X'3FA3FF'
X'3F8200'
Line Adapter Mailboxes (512 bytes)
X'3F81FF'
X'3F8000'
Top of Non-Reserved Storage X'3F7FFF'
Exchange Time Outs
The requestor fills in his mailbox and posts an interrupt to the receiver. The
receiver grants the interrupt, decodes the mailbox contents, posts a status in the
mailbox, and interrupts the requestor. Throughout this time, the mailbox is busy
with the requestor. A timer is used to provide a protection against requests that are
not answered.
On the NCP side, if a time out occurs, the MOSS is considered down and an alert
is sent to the host.
On the MOSS side, if a time out occurs, a return code is passed to the requesting
application to signal this situation. In this case, the NCP/PEP is probably down and
operator intervention may be required.
Chapter 8. Maintenance and Operator Subsystem (MOSS)
8-13