yoked. With a 4,096-byte page size, two keys are
associated with a page.
Programming Note
The accuracy of reference recording is such as to
allow for effective operation of paging algorithms.
The: reference bit cannot be used to establish the
usage of pages containing translation tables since,
after the initial reference, the tables may be used by
means of references to the translation-Iookaside
buffer, without any fetching of the table entries from
main storage. Address- Translation Exceptions
When the dynamic-address-translation facility is installc!d, three additional program-exception condi­
tions (llre introduced: segment-translation exception, page-translation exception, and translation­
specification exception. The CPU cannot be disabled
for translation exceptions. Tht! presence .of the dynamic-address-translation
facility also introduces new conditions that are rec­ ognized as addressing exceptions. When address
translation is invoked, an addressing exception is
recognized when an attempt is made to use a
segment-table entry or a page-table entry that is
designated at a location outside the available main
storage of the installed system. The unit of operation
is suppressed.
The handling of all exceptions associated with
dynamic address translation is summarized in the
table "Handling of Access Exceptions" in the chap­
ter "Interruptions." Summary of Dynamic Address
Translation Formats
The first table summarizes the possible combina­
tions of the page-address and byte-index fields in the
formation of a real storage address.
The eight-bit length field in control register 1
provides for a maximum length code of 255 and
permits designating a segment table of 16,384 bytes,
or 4,096 entries, which is more than can be referred
to for translation purposes by the logical address.
With 1M-byte segments, only 16 segments can be
addressed, requiring a segment table of 64 bytes. A
table of 64 bytes is specified by a length code of ° and is the smallest table that can be specified. With
64K-byte segments, up to 256 segments can be ad­
dressed, requiring at the most a segment table of
1,024 bytes and a length code of 15. These relations
are summarized in the second table.
The third table lists the maximum sizes of the
page table and the increments in which the size of
the page table can be controlled. Real Storage Address
Page Address Size of Page Bit Positions in Page-Table 2K 4K Size of Segment
(Bvtes)
64K
1M Size of
Segment Page
Entry 0-12 0-11 Segment I ndex Field Size (Bits)
8
4
Page I ndex Field (Bytes) Size (Bits)
64K 2K 5
64K 4K 4
1M 2K 9
1M 4K 8
Summary of DAT Formats
68 System/370 Principles of Operation No. of Bits
13
12
Number of Addressable
Segments
256
16
Number of Pages in
Segment
32
16
512
256
Byte Index Bit Positions in Logical Address No. of Bits
21-31 20-31 Maximum Segment Table Size Usable Length
(Bytes) Code 1,024 15
64 0 Maximum Page Table Size Usable Length
(Bytes) Code
64 15
32 15 1,024 15
512 15
11
12 Table Increment (Bytes)
64
64 Table Increment (Bytes)
4
2
64
32

I nterruption Action.
Source Identification Enabling and Disabling Instruction-Length Code Zero ILC .
Contents I LC on Instruction Fetch Exceptions. Point of Interruption Instruction Execution .
Types of Ending
Execution of I nterruptible Instructions Machine-Check Interruption Program Interruption Program I nterruption Conditions
Operation Exception Privileged-Operation Exception
Execute Exception . Protection Exception
Addressing Exception
Specification Exception
Data Exception . Fixed-Point-Overflow Exception
Fixed-Point-Divide Exception Decimal-Overflow Exception . Decimal-Divide Exception . Exponent-Overflow Exception
Exponent-Underflow Exception
Significance Exception . Floating-Point-Divide Exception
Segment-Translation Exception Page-Translation Exception
Translation-Specification Exception Special-Operation Exception
Monitor Event Program Event
Recognition of Access Exceptions Handling of. Multiple Program-I nterruption Conditions Supervisor-Call Interruption External Interruption Interval Timer Interrupt Key External Signal Malfunction Alert Emergency Signal External Call .
Time-of-Day Clock Sync Check Clock Comparator . CPU Timer Input/Output Interruption. Restart Priority of Interruptions Assigned Main-Storage Locations Real Main Storage Absolute Main Storage . Page of GA22-70004 Revised September 1, 1975
By TNL: GN22-0498
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