Control Register 1 Control Register 7 Virtual Address
PSTD I SSTD I I sx I pxl BX I I I (x4) (x2) I 4 I I ..... Effe ctive STD STO I (x64) I Segment Table
+ R PTL 01 PTa
(x8) I 2
Translation
Lookaside Buffer (TLB) Page Table + 1 PFRA 1 R PFRA G 1- 0 3 I I Real Address
R: Address is real
Translation Process (Part 1 of 2) Chapter 3. Storage 3-29

Control register 1 provides the primary segment-table designation for
translation of a primary virtual address, and, when DAS is installed, control register 7 provides the secondary segment-table designation for
translation of a secondary virtual address.
Information, which may include portions of the virtual address and the
translation parameters, is used to search the TLB.
If a match exists, the page-frame real address from the TLB is used in
forming the real address.
If no match exists, table entries in real storage are fetched. The resulting
fetched entries, in conjunction with the search information, are used to
translate the address and may be used to form an entry in the TLB.
Translation Process (Part 2 of 2)
Inspection of Control Register Q The interpretation of the virtual
address for translation purposes is
controlled by the translation format,
bits 8-12 of control register O. If
bits 8-12 contain an invalid code, a
translation-specification exception is
recognized.
Segment-Table Lookup
The segment-index portion of the virtual
address, in conjunction with the
segment-table origin contained in the
effective segment-table designation, is
used to select an entry from the segment
table.
The 24-bit real address of the segment­
table entry is obtained by appending six
zeros to the right of bits 8-25 of the
effective segment-table designation and
adding the segment index to this value,
with the rightmost bit position of the
segment index aligned with bit position
29 of the address. A carry, if any,
into bit position 7 is ignored. With
extended real addressing, this 24-bit
real address is extended on the left
with zeros; thus, the segment table can
wrap from 224 -1 to zero.
As part of the segment-table-Iookup
process, the segment index is compared
against the segment-table length, bits 0-7 of the effective segment-table
designation, to establish whether the
addressed entry is within the segment
table. With 1M-byte segments, entries
for all addressable segments are
contained in a table of minimum length
(length code of 0). With 64K-byte
segments, four zeros are appended to the
left of bits 8-11 of the virtual
address, and this extended value is
compared against the eight-bit segment- 3-30 System/370 Principles of Operation
table length. If the value in the
segment-table-length field is less than
the value in the corresponding bit posi­
tions of the virtual address, a
segment-translation exception is recog­
nized.
All four bytes of the segment-table
entry appear to be fetched concurrently
as observed by other CPUs. The fetch
access is not subject to protection.
When the storage address generated for
fetching the segment-table entry desig­
nates a location which is not available
in the configuration, an addressing
exception is recognized, and the unit of
operation is suppressed.
Bit 31 of the entry fetched from the
segment table specifies whether the
corresponding segment is available.
This bit is inspected, and, if it is
one, a segment-translation exception is
recognized. Handling of bit positions
4-7 and 29-30 of the segment-table entry
depends on the model: normally a
translation-specification exception is
indicated when they do not contain zeros; however, on some models they may
be ignored. On machines with the segment-protection
facility, bit 29 is the segment­
protection bit and does not cause a
translation-specification exception; bit
29 is retained with the entry in the
TLB. On machines with the common-segment facility, bit 30 is the common-segment
bit and does not cause a translation­
specification exception. Bit 30 may be
retained with the entry in the TLB, or
it may be ignored.
When no exceptions are recognized in the
process of segment-table lookup, the
entry fetched from the segment table
designates the beginning and specifies
the length of the corresponding page
table.