Control Address Offset No. of Field
Name Block Type (Hex) Bytes Contents MICRSEG MICBLOK Real MICCREG MtCBLOK Real EXTCRO ECBLOK Real EXTCR1 ECBLOK Real SEGPAGE SEGTABLE Real PAGCORE PAGTABLE Real Virtual Real
segment
entry SEGPAGE SEGTABLE Real PAGCORE PAGTABLE Real Vi rtual Real
page
entry SEGPAGE SEGTABLE Real PAGCORE PAGTABLE Real SEGPAGE Shadow Real
SEGTABLE PAGCORE Shadow Real PAGTABLE o
4
o
4 4SX1 4
4
4
4
4
2
4
4
2
2
4
2
4
2
Address of real
segment table
Address of ECBLOK Virtual CRO Virtual CR1 Entries to find vir­
tual segment entry
Entries to find vir­
tual segment entry
Entries to find vir­
tual page entry
Entries to find vir­
tual page entry
Entries to get real
translated address
Entries to get real
translated address Shadow segment-table
entry
Validated shadow
page-table entry
Superscripts 1, 2, 3, and 4 refer, respectively, to the seg­
ment and page indexes to translate (1) the virtual segment­
table address, (2) the virtual page-table address, (3) the
virtual address corresponding to the untranslated virtual/
virtual address, and (4) untranslated logical address of the
virtual machine.
Figure 20. Fields Used in Shadow-Table Validation 28 Virtual-Machine Assist and Shadow-Table-Bypass Assist I

Chapter 3. VM-Common-Segment Modification
ofVMA Use 6rtne-- vTrtuaf-macnTne wifh Tne --VM­ common-segment modification by the VM/370 System Extensions program product (Program No.
S748-XEl) improves the performance of virtual
machines which use the common-segment-bit
function of the System/370 extended facility.
The VM-common-segment modification alters
the virtual-machine assist so that checking for a
zero in the common-segment bit position (bit
position 30) of segment-table entries is omitted in
virtual-machine-assist functions: • For all references to real segment-table entries • For all references by virtual segment-table
entries • For reference by the shadow-table-validation
function to a shadow segment-table entry for the
purpose of locating the shadow page-table entry
to be validated. Tire VM-commurr-segment modification of VMA has no effect on the checking of shadow segment­
table entries when they are used for translating
instruction or operand addresses. Figure 21 shows
the segment-table entries that are checked in bit
position 30 for each type of reference for each
completely assisted virtual-machine-assist function.
Programming Note
If the common-segment-bit function is used only by
the virtual machine and not in the real or shadow
segment tables, the only effects of installing the
VM-common-segment modification of VMA are:
1. The load-real-address function is completed
without a program interruption when a virtual
common segment is referred to.
2. The shadow-table validation can be completed
without a program interruption when a virtual
common segment is involved.
Shadow Segment-Table Entry
Real Virtual Used for Finding:
Segment- Segment-
Table Table PTE to Be VMA Function Entry Entry Validated Instruction Operand IPK, SPKA ---X - II SK, RRB SSK, SVC Yl --X -
\lRA Y2 Yl -X - LPSW, SSM I STeTl, STNSM I I I STOSM - I - I - I X X
Shadow-table
validation Y3 Yl Yl --
Explanation:
X
Yl,Y2,Y3
No reference of this type.
Reference is made and bit 30 is ignored if System/370 extended facility is installed. One, two, or three references of this type are
made, and bit 30 is ignored if the VM-common­
segment modification IS installed.
Figure 21. Segment-Table Entries Cbeck.ed in Bit Position 30 I I VM-Common-Segment Modification of VMA 29