Word
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
2
8
9
9
9
9
9
10
11
14
14
14
14
14
14
14
14
14
15
Bits 0 2
8-9
10
11-12
16
17
18
19
20
21
24
25
26
0-7
8-25
0-31
16-31 0 2
3
16-31
8-31
8-31 0 2
4
5
6
7
8
9
8-28
Explanation:
Name of field
Block-Multiplexing Control SSM-Suppression Control TOD Clock Sync Control
Page-Size Control
Unassigned, must be zero Segment-Size Control
Malfunction-Alert Mask
Emergency-Signal Mask 1 "',' , External-Call Mask .. TOD-Clock-Sync-Check Mask
Clock-Comparator Mask
CPU-Timer Mask Interval-Timer Mask
Interrupt-Key Mask
External-Signal Mask Se£ment-Table Length Segment"Table Address
Channel Masks
Monitor Masks
Successful-Branching Event Mask
Instruction-fetching-Event Mask
Storage-Alteration-Event Mask
GR-Alteration-Event Mask
PER 1 General Register Masks
PER Starting Address
PER Ending Address
Check-Stop Control
Synchronous-MCE L
2
Control
I/O-Extended-Logout Control
Recovery-Report Mask
Degradation-Report Mask
External-Damage-Report Mask
Warning Mask
Asynchronous-MCEL Control
Asynchronous-Fixed-Log Control
MCEL Address
The fields not listed are unassigned.
Associated With
Block-Multiplexing SSM Suppression
Multiprocessing
Dynamic Addr. Translation
Dynamic Addr. Translation
Dynamic Addr. Translation
Multiprocessing
Multiprocessing
Multiprocessing
Multiprocessing
Clock Comparator
CPU Timer
Interval Timer
Interrupt Key
External Signal Dynamic Addr. Translation
Dynamic Addr. Translation
Channels
Monitoring
Program-Event Recording
Program-Event Recording
Program-Event Recording
Program-Event Recording
Program-Event Recording
Program-Event Recording
Program-Event Recording
Machine-Check Handling
Machine-Check Handling I/O Extended Logout
Machine-Check Handling
Machine-Check Handling
Machine-Check Handling
Machine-Check Handling
Machine-Check Handling
Machine-Check Handling
Machine-Check Handling
Except for bit 10 of control register 0, the initial value of unassigned register positions is unpredictable.
1 PER means program-event recording.
2 MCEL means machine-check extended logout.
3 Bit 22 is set to one, with all other bits set to zero, thus yielding a decimal byte address of 512.
Assignment of Control Registel' Fields Initial Value
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
1
o
o
o
o
o
o
512
3
System Control 37
Key ill Storage
For purposes of protection and recording of refer­
ences and changes, main storage is divided into
blocks of 2,048 bytes, each block having an address
that is a multiple of 2,048. A control field, called "key in storage," is associated with each block of
storage.
The key in storage has the following format: o 4 6
The bit positions in the key are allocated as fol­
lows: Access-Control Bits (ACC): Bits 0-3 are matched
against the four-bit protection key whenever infor­
mation is stored, or whenever information is fetched
from a location that is protected against fetching.
Fetch-P,rotection Bit (F): Bit 4 controls whether
protection applies to fetch-type references: a zero
indicates that only store-type references are moni­
tored and that fetching with any protection key is a one indicates that protection applies to
both fetching and storing. No distinction is made
between the fetching of instructions and of oper­
ands. Reference Bit (R): Bit 5 normally is set to one each
time a location in the corresponding storage block is
referred to either for storing or for fetching of in­
formation. This bit is associated with dynamic ad­
dress translation.
Change Bit (C): Bit 6 is set to one each time in­
formation is stored into the corresponding storage
block. This bit is associated with dynamic address translatilon. The key in storage is not part of addressable stor­
age. program can explictly place information in
all seven bits of the key by SET STORAGE KEY,
and the contents of the key can be inspected by IN­
sERT STORAGE KEY. Additionally, the instruc­
tion RESET REFERENCE BIT provides a means of
inspecting the reference and change bits and of set­
ting the reference bit to zero.
Protection
The protection facility is provided to protect. the
contents of main storage from destruction or misuse
caused by erroneous or unauthorized storing or
fetching by the program. It provides protection
38 System/370 Principles of Operation
against improper storing or against both improper
storing and fetching, but not against improper fetch-
ing alone.
Protection Action
When protection applies to a storage access, the key
in storage is compared with the protection key asso­
ciated with the request for storage access. A store is
permitted only when the key in storage matches the
protection key. The keys are said to match when the
four high-order bits of the key in storage are equal
to t.he protection key or when the protection key is
zero. A fetch is permitted when the keys match or
when bit 4 of the key in storage is zero. The protec­
tion action is summarized in the table "Summary of
Protection Action. " Conditions
Bit 4 of Key Key
in Storage Relation 0 Match 0 Mismatch
Match
Mismatch Explanation: I s Access to Storage
Permitted?
Fetch Store
Yes Yes
Yes No
Yes Yes
No No
Match The four high-order bits of the key in storage are equal to the protection key, or the protection key
is zero.
Yes Access is permitted.
No Access is not permitted; on fetching, the information
is not made available to the program, and, on storing,
the contents of the storage location are not changed.
Summary of Protection Action
When the access to storage is initiated by the CPU, and protection applies, the protection key of
the current PSW is used as the comparand. The pro­
tection key of the CPU occupies bit positions 8-11
of the PSW. When the reference is made by a chan­
nel, and protection applies, the protection key asso­
ciated with the I/O operation is used as the compa­
rand. The protection key for an I/O operation is
specified in bit positions 0-3 of the channel address
word ( CAW) and is recorded in bit positions 0-3 of
the channel status word (CSW) stored as a result of
an I/O operation.
When a CPU access is prohibited because of pro­
tection, the operation is suppressed or terminated,
and a program interruption for a protection excep­
tion takes place. When a channel access is prohibit­
ed, a protection-check condition is indicated in the
channel status word (CSW) stored as a result of the
operation.
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