Ctrl Initial
Reg Bits Name of Field Associated with Value
8 16-31 Monitor masks MONITOR CAll 0 9 0 Successful-branching-event mask Program-event recording 0 9 1 Instruction-fetching-event mask Program-event recording 0 9 2 Storage-alteration-event mask Program-event recording 0 9 3 GR-alteration-event mask Program-event recording 0 9 16-31 PER general-register masks Program-event recording 0 10 8-31 PER starting address Program-event recording 0 11 8-31 PER ending address Program-event recording 0 14 0 Check-stop control Machine-check handling 1
14 1 control Machine-check handling 1
14 2 I/O-extended-logout control I/O extended logout 0 14 4 Recovery subclass mask Machine-check handling 0 14 5 Degradation subclass mask f"lachi ne-check handling 0 14 6 External-damage subclass mask Machi ne-che.ck handling 1
14 7 Warning subclass mask Machine-check handling 0 14 8 Asynchronous-MCEl control i ne-check handling 0 14 9 Asynchronous-fixed-log control !"lach i ne-check handling 0 14 12 ASH-translation control Dual-address-space control 0 14 20-31 ASN-first-table origin Dual-address-space control 0 15 8-28 MCEL address Machine-check handling 512
2 Bits 13, 30, and 31 of control register 0, and bits 0-30 of control register 6
are assigned to functions not described in this publication. The remaining
fields not listed are unassigned. The initial value for all unlisted control-
register positions i s zero. 1 Bit 14 of control register 0, the vector-control bit, i s described in the
publication IBn Vpctor Oeerations, SA22-7125. 2 Bit 22 is set to one, with all other bits set to zeros, thus yielding a
decimal byte address of 512.
Assignment of Control-Register Fields (Part 2 of 2) DAS TRACING Three DAS instructions optionally store
32 bytes of information about the ci rcwnstances under ch the instructions are executed. This action is c<:lll<:::d DAS tracing and is perforll1od by placing information in a 32-byte block, called a tr<:lce entry, in an area c<:llied a trace table. DAS tracing assists in problem determination for privileged and scmiprivlleged programs
by providing an ongoing record in stor­ age of significant events. The trace table and the location of the last-used
entry are described by a control block
called the trace-table-entry header. The origin of the header is specified in the trace-table-designation word at
logical location 84. These relation­ ships are illustrated in the figure "DAS Tracing." DAS tracing is controlled by bit 0 of
the trace-table designation, called the
DAS-trace-control bit. When the bit is ono, a trace entry is made each time PRO G R At'1 C t\ L L , PRO G RAM T RAN 5 FER, 0 r 5 E T SECOtlDARY ASH is executed. All locations associated with DAS trac­ ing are treated as logical addresses whose handling depends on the OAT-mode bit and address-space-control bit of the PSW. For PROGRM'1 CAL L <:lnd TRANSFER, the addresses are translated
by using the old primary segment-table dcsignation. For SET SECONDARY ASH, the addresses are translated by using either the old primary segment-table desig­ nation or the old secondary segment­ table designation, depending on whether PSW bit 16 specifies the primary-space
mode or the secondary-space mode, respectively. Bits 8-28 of the trace-table designation provide the origin of the three-word
trace-table-entry header. Conceptually, the header defines a table of 32-byte
elements, called trace entries. The Chapter 4. Control 4-11

second and third words of the header designate, respectively, the beginning
and end of this table. When DAS tracing is on, the first word of the header,
called the current-entry control, is
updated in conjunction with the
execution of the instruction to be
traced. The trace entry designated by
the updated contents of the current­
entry control is used to contain the
trace information about the instruction
being traced. Updating is interlocked
to ensure that distinct entries are
produced when a common table is used for
tracing by more than one CPU. Trace-Table Designation
Logical Locations 84-87
Updating the current-entry-control word
of the header normally consists in
advancing the contents of the current­
entry-control word by 32. However, if
the advanced value equals or exceeds the
value in the last-entry-control word of
the header, the contents of the first­
entry-control word replace the contents
of the current-entry-control word.
Thus, the dynamic filling of successive
entries wraps from the last entry to the
first entry, with no special recognition
accorded this event.
Trace-Table-Entry
Header Origin A = 0: Tracing off
A = 1: Tracing on
o 31
Trace-Table-Entry Header (8-byte boundary) JFirst-Entry Ctrl ILast-Entr y Ctrl I Jcurrent-Entr y Ctrl 0 32 I 64 9 5
Trace Table (32-byte boundary) First (or Wrap) Entry Current Entry Location after the Last Entry
DAS Tracing
4-12 System/370 Principles of Operation