IBM System/370 145 Manual page 77

Table 15.10.1. Number and size of segments and pages for a 16-million-
byte virtual storage
Number of
CR 0 Bits Segment Size segments in the
Page
~ize Number of Pages
11,12 8,9
in Bytes
Virtual Storage in Bytes
in a Se9!!!ent
10 01 1,048,576 '16
2048 512
10
10 1,048,576 116 4096 256
00 01 65,536 2S6
2048 32
00 10 65,536
2!i6 4096 16
For the purpose of translation,
it
virtual storage address is divided
into three fields: (1) a segment field, which identifies a segment
within the virtual storage, (2) a page field, which identifies a page
within the segment addressed, and
(3)
a byte displacement field, which
identifies a byte within the page addressed. The number of bits in each
field varies depending on the segment and page sizes used. Virtual
storage address fields for a
segment~
size of 64K and a specific example
of how the fields are used to address a location in virtual storage are
shown in Figure 15.10.1.
OPERATION OF DYNAMIC ADDRESS TRANSLP..TION HARDWARE
Address Translation Tables
One segment table is required to describe one virtual storage. If
more tha~ one virtual storage is supported by a single computing system,
there is a segment
t~ble
for each vi.rtual storage implemented.
A
segment table contains one four-byte: entry for each segment in the
virtual storage the table describes, up to a maximum of 256 entries for
the maximum size virtual storage of 16 million bytes (using 64K
segments). The real storage address of the segment table (or of the
currently active segment table if multiple virtual storages are
implemented) is contained in control register 1. The current length of
the segment table is also indicated in control register 1. The length
value is used by the hardware during tra.nslation to ensure that the
segment entry being referenced falls within the segment table.
The segment table entries point to the real storage locations of the
page tables. There is one page table for each segment in the virtual
storage defined (or. in OS/VS2, currently allocated), up to a maximum of
256 page tables for a 16-million-byte virtual storage with 64K segments.
A
segment table entry contains an indication of the length of the page
table, the high-order 21 bits of the real storage, address of the page
table, and an indication of whether .or not the entry itself is valid and
can be used for translation purposes (invalid bit). If the invalid bit
is on in a segment table entry, a translation exception occurs during
the translation process.
A
Guide to the IBM System/370 Model '145 67