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Copyright (c) 1999 Massachusetts Institute of Technology
See the COPYING file at the top-level directory of this project.
------------------------------
ITS USETS:
This file attempts to maintain up-to-date documentation on
all user variables hacked by .USET/.SUSET. Those wonderful
souls who update the information in any way (additions,
deletions, corrections) should describe their
modifications in a brief note to INFO-ITS@AI so
that interested parties can correct their copies or
conceptions without needing to print or read the
entire file again. For example:
:QMAIL INFO-ITS@AI I added more details to the .FOO user var ^C
If you want to be put on the INFO-ITS mailing list,
just say so in a message to it.
-------------------------------------------------
Each job running under ITS has a large number of variables
associated with it maintained by the system. In order to
examine and set these variables, the .USET and .SUSET
uuo's are provided. The .USET uuo allows the specification
of the job whose variables are to be accessed. The .SUSET
uuo always accesses the variables of the job which executes it
(SUSET = Self USET).
The .USET uuo is used like this: .USET CHNL,SPEC
and the .SUSET uuo like this: .SUSET SPEC
The ac field CHNL of the USET must be the number of
an input/output channel for which a job is open,
as either the USR, JOB, or BOJ device.
The .USET may modify the variables only
if it has modification rights to the job (the job
is a JOB device, or is a direct inferior).
The contents of the effective address SPEC are of this form:
4.9 0 = read variable, 1 = set variable.
4.8 Block mode (see below).
4.7-3.1 Number of user variable to access.
2.9-2.1 Address of a location within the caller which
is to receive the value of the variable, or which
contains the new value for the variable.
If bit 4.8 is 1, then the word at location SPEC is really
an AOBJN pointer to a block of specifications. This is
useful for getting several variables at a time easily.
The AOBJN pointer is updated as the operation progresses,
and so must be in a writable area. Block mode may not be nested.
If an illegal specification is given to .USET or .SUSET,
the job will receive an illegal operation word 1 interrupt
(bit 1.6 of the .PIRQC user variable).
The variables are described in this document by the names which
DDT knows for them; the first character of each name is a ".".
If you open up a location by using one of these names, DDT
treats it specially and actually accesses the corresponding
variable and not a location in the job's core.
When writing .USET's and .SUSET's in MIDAS code,
one may use the symbols defined by MIDAS for accessing them.
These are the same as the names known to DDT, except that
the "." is followed by "R" for reading or "S" for setting.
If you open a location containing a SPEC, try typing
.R$?? if it is positive, or .S$?? if it is negative; this will
use bit typeout mode to print out the spec as it might have been
assembled; for example, 400001,,500 .S$?? gives .SVAL,,500 .
Examples:
To read your own runtime in 4-microsecond ticks into FOO:
.SUSET [.RRUNT,,FOO]
To set your word 1 interrupt mask:
.SUSET [.SMASK,,[%PIDWN\%PIPDL]]
;enable "pdl overflow" and "sys going down" interrupts
To read your input/output status word for channel 13 into FOO:
.SUSET [.RIOS+13,,FOO]
To start a job (your immediate inferior!) open on channel
USRC at its location QUUX, with its option
bits indicating your readiness to supply JCL:
.USET USRC,[.SOPTION,,[%OPCMD,,]]
.USET USRC,[.SUPC,,[QUUX]]
.USET USRC,[.SUSTP,,[0]]
Another way to do the same thing is:
MOVE AC,[-3,,USBLOK]
.USET USRC,AC
. . .
USBLOK: .SOPTION,,[%OPCMD,,]
.SUPC,,[QUUX]
.SUSTP,,[0]
FORM OF DESCRIPTIONS:
The descriptions of the various variables accessible by
.USET and .SUSET are in the following form:
<DDT name> <access> <mnemonic explanation for name>
<long-winded paragraph describing variable>
The <access> descriptor contains four characters "wx yz".
The first two ("wx") refer to the access for .USET;
the last two ("yz") to the access for .SUSET.
In each case these characters are chosen from these sets:
-- May neither read nor set.
+- May read but not set.
+* May read and set.
+? May read, may sometimes set (please read on...)
That is, + means may read, * may set, - denies access,
while ? says that things are more complicated.
.40ADDR +* +* "40" address
The right half:
Initially 40 octal. Whenever the system
references locations "40" through "44" specifically,
(e.g. giving the user interrupts, or returning uuo's 50-77),
the system really references the 5-word block that
.40ADDR points to. Thus, if .40ADDR is set to 500, the
system expects location 502 to contain the address of
the interrupt routine. This is especially useful to
distinguish system-returned uuo's (0 and 50-77) from
standard user-mode uuo's (1-37); the latter always
trap through location 40, while the former trap
through .40ADDR.
The various words accessed through .40ADDR are
as follows (indexed by "normal" location #):
40 System places a uuo here when it wants
to give it to the user.
41 Should contain the address of the uuo handler.
If .40ADDR is not indeed 40, this
may be a different uuo handler from the
one which handles uuo's 1-37.
If .40ADDR is actually 40, then unless
41's index and indirect fileds are 0 and its
opcode is either 0 or JSR, all system-returned
UUOs will cause ILOPR interrupts instead.
This is to protect programs that want their
hardware user-UUOs to be handled by a PUSHJ
from being clobbered when they accidentally
execute a meaningless UUO.
42 If the job's %OPINT bit in its
.OPTION variable (q.v.) is 0, then
this is a JSR to the user's old-style
interrupt handler. The three words addressed
by the JSR are, in order, the word in
which the interrupt bits are placed, the
return address for the interrupt, and the
first instruction of the handler.
If the %OPINT bit is 1, then this
is an AOBJN pointer to a vector of
5-word blocks describing the various
interrupt handlers and their priorities.
See ITS INTRUP for details.
43 If the job's %OPLOK bit is set in
its .OPTION variable, then this is
the pointer to the list of switches
to be reset if the job is killed.
44 If the job's %OPLOK bit is set,
this is an AOBJN pointer to a block
of critical code region descriptors.
See ITS LOCKS for details.
The left half:
Initially 20 octal. This is the address of a block of
20 octal words which a job's superior may use to hack
that job. For compatibility, if LH(.40ADDR) is zero,
the default of 20 will be assumed.
.ADF1 +* +* ANDCAM into .DF1
Like .DF1 (q.v.) when reading.
Performs ANDCAM into .DF1 when writing.

.ADF2 +* +* ANDCAM into .DF2
Like .DF2 (q.v.) when reading.
Performs ANDCAM into .DF2 when writing.

.AIFPIR +* +* ANDCAM into .IFPIR
Like .IFPIR (q.v.) when reading.
Performs ANDCAM into .IFPIR when writing.

.AMASK +* +* ANDCAM into .MASK
Like .MASK (q.v.) when reading.
Performs ANDCAM into .MASK when writing.

.AMSK2 +* +* ANDCAM into .MSK2
Like .MSK2 (q.v.) when reading.
Performs ANDCAM into .MSK2 when writing.

.APIRQC +* +* ANDCAM into .PIRQ
Like .PIRQC (q.v.) when reading.
Performs ANDCAM into .PIRQC when writing.
.APRC +- +- APR CONO
4.9 Procedure is in a disowned tree.
4.7 (BUMRTL) Tree will be gunned if hasn't run for an hour
(meaningful only in top level job). Cleared by reowning or
attaching. See the DETACH and DISOWN system calls.
4.6 Core request pending for this job.
4.5 User disabled, waiting to be flushed by SYS job.
The SYS job will delete the job tree eventually.
4.4-4.1 .BCHN user variable (q.v.).
3.9 In process of deleting or logging out this job.
Bit 4.5 will probably be set eventually.
3.5-3.1 Must be zero.
2.9-1.1 CONO'ed to APR whenever job is about
to be run. Initially 447. Modified by
altering the .MASK user variable.

.BCHN +* +* bad channel number
Initially 0.
Number of channel most recently in error.
Typically after reading this variable one does a
.STATUS or a STATUS symbolic system call
to get the status word for the channel. This word
can then be fed to the ERR device.
.BCHN is actually part of the .APRC variable.
See also the .IOS variable.

.CNSL +- +- console tty number
If the job is in a console-controlled tree, this
variable contains the console's number, even if
the job doesn't own the console. If the job is
in a non-console-controlled tree, .CNSL is -1
if the job is scheduled as part of the system,
-2 if the job is scheduled as a disowned job;
for a job-device-handler job, .CNSL is the same
as it is for the job it is serving.

.DF1 +* +* defer bits, word 1
Initially 0.
Defer bits for word 1 interrupts (see .PIRQC
and .MASK). If an interrupt bit is set in both
.PIRQC and .MASK, but is also set in .DF1,
it is temporarily deferred.

.DF2 +* +* defer bits, word 2
Initially 0.
Defer bits for word 2 interrupts (see .IFPIR
and .MSK2). If an interrupt bit is set in both
.IFPIR and .MSK2, but is also set in .DF2,
it is temporarily deferred.
.EBO1 +- +- KL-10 Ebox counter
This is the low-order word of the KL-10 Ebox counter
value for the job. It contains garbage on KA-10's.
Its location is .EBOX+1, for convenience.

.EBOX +- +- KL-10 Ebox counter
This is the high-order word of the KL-10 Ebox counter
value for the job. It contains garbage on KA-10's.

.FLS +- -- flush instruction
Initially 0.
The instruction which is blocking the running of
the job. Zero if user not blocked (user can run
if .USTP is also zero).
(The scheduler tests a job for runnability by first
checking the .USTP variable, and if it is zero,
then executing the flush instruction. The job is
runnable iff the flush instruction skips.)

.FTL1 +- +- Fatal interrupt first word
After a fatal interrupt, this variable contains the
.PIRQC bits which caused the error. At any other time
it contains garbage.

.FTL2 +- +- Fatal interrupt second word
After a fatal interrupt, this variable contains the
.IFPIR bits which caused the error. At any other time
it contains garbage.

.HSNAME +* +* Home System Name
This is a word of sixbit like .SNAME (q.v.). It
is initialized to be the same as the UNAME on creation
of a not-logged-in tree, and when a job is created
as an inferior or as a job device, it's .HSNAME is
copied from that of it's creator. The LOGIN system call
sets it to be the same as the UNAME, although this should
be changed to be the XUNAME.
This is not used by ITS for anything, but is intended to
associate a directory with a specific XUNAME for the sake
of init and mail files of people without a directory of
the same name as their XUNAME. DDT will soon initialize
it by looking up information in the INQUIR database for
the particular user.

.IDF1 +* +* IORM into .DF1
Like .DF1 (q.v.) when reading.
Performs IORM into .DF1 when writing.
.IDF2 +* +* IORM into .DF2
Like .DF2 (q.v.) when reading.
Performs IORM into .DF2 when writing.

.IFPIR +* +* inferior procedure and i/o requests (?)
Initially 0.
This variable contains bits for pending word 2 interrupts.
Bit 3.n is the interrupt bit for the n'th job directly
inferior to the specified job (see the .INTB user variable).
These therefore occupy bits 3.8-3.1.
An inferior interrupt bit is set if an a class 1
or untrapped class 2 interrupt occurs in the inferior,
or if the inferior is using new-style interrupts
(see ITS INTRUP, and the %OPINT bit of the .OPTION
user variable) and any unhandled interrupt occurs.
Performing a .UCLOSE on an inferior clears the
inferior interrupt bit in the .IFPIR variable
of the job performing the .UCLOSE.
Bit 1.n is the interrupt bit for input/output
channel n-1 (actually, these occupy bits 2.7-1.1).
Thus bit 1.1 = channel 0, 1.2 = channel 1,
1.3 = channel 2, . . ., bit 2.7 = channel 17.
When an input/output interrupt is detected, the
system call WHYINT shoud be used to discover the
cause (see ITS .CALLS).
The following devices can cause channel interrupts:
TTY Input: A character was typed which according
to the job's TTYST1 and TTYST2 variables
(see ITS TTY) should be treated as an interrupt
character. If the tty is opened for input
on more than one channel, only one channel,
if any, receives the interrupt.
Output: The **MORE** condition has occurred.
See ITS TTY.
STY Input: Input is pending (i.e. the corresponding TTY
has attempted output). If the STY is open
for input on more than one channel, only
one channel, if any, receives the interrupt.
See ITS TTY.
Output: The STY's alter ego is waiting for input,
or at least was (it isn't guaranteed not to
stop waiting and do something else).
STK Input is available. Interrupts occur on all enabled channels.
USR A foreign job open on the channel has been killed.
If the job went away while the channel was pushed
on the I/O pdl, this interrupt will occur when the
channel is popped. See the .UCLOSE, .IOPUSH, and
.IOPOP uuo's.
If the job is the PDP-6, the PDP-6 has requested an interrupt.
JOB Various conditions, some programmable. See ITS JOB.
BOJ Various conditions, some programmable. See ITS JOB.
NET One of the following conditions has occurred:
The IMP has gone down.
RFC received.
After RFC sent, the connection is now open.
Input available.
Connection closed.
Net interrupt received (INR or INS).
MSP Message has been sent.
.IIFPIR +* +* IORM into .IFPIR
Like .IFPIR (q.v.) when reading.
Performs IORM into .IFPIR when writing.

.IMASK +* +* IORM into .MASK
Like .MASK (q.v.) when reading.
Performs IORM into .MASK when writing.

.IMSK2 +* +* IORM into .MSK2
Like .MSK2 (q.v.) when reading.
Performs IORM into .MSK2 when writing.

.INTB +- +- interrupt bit
Gets the interrupt bit for the procedure. This is
the word 2 interrupt bit which the procedure's
superior will see when the procedure interrupts
its superior. This variable will therefore have
exactly one of bits 3.1-3.8 set, and no others.
If the procedure is top level (has no superior),
then this variable is negative.

.IOC +<n> +- +- input/output channel
The variable .IOC+<n> is the input/output channel
word for channel <n>, for n between 0 and 17 octal.
Normally zero for a closed channel.

.IOP +<n> +- +- input/output pdl
The input/output pdl is a pdl of two-word entries
used by the .IOPUSH, .IOPOP, and .IOPDL uuo's.
The first word of each pair is the .IOC word for
the stacked channel, and the second is the .IOS word.
In addition, bits 4.9-4.6 of the .IOS word of each
pair contain the channel number which the channel was
pushed from (this is used by the .IOPDL uuo).
The pdl has eight entries; thus <n> should be between
0 and 17 (0 is the .IOC word for the least recently
pushed entry, etc.).
.IOS +<n> +- +- input/output status
The variable .IOS+<n> is the input/output status
word for channel <n>, for <n> between 0 and 17 octal.
Normally zero for a closed channel.
This word contains various bits describing the status
of the channel. The left half is a set of error
codes if non-zero; the .IOS word can be given to
the ERR device to obtain an ascii message for
the error. The right half contains various bits
describing the state of the device. (Internally to
ITS, the right half contains an access pointer;
the right half bits supplied for .IOS are the
same as those generated by the .STATUS uuo or
the STATUS symbolic system call.)
The various bits in the .IOS word are as follows:
4.9-4.6 Always zero (see the .IOP variable).
4.5-4.1 If non-zero, the number of a non-display
input/output error (see table below).
3.9-3.7 If non-zero, the number of an IDS interpreted
display input/output error (see table below).
3.6-3.1 If non-zero, the number of a standard error.
Set primarily by failing .OPEN's and
.FDELE's. These are the same as the error
codes returned by failing .CALL's (see
table below).
2.9-2.3 Device dependent.
2.2 Buffering capacity empty.
2.1 Buffering capacity full.
1.9-1.7 Mode in which device was opened.
1.9 0 = ascii, 1 = image.
1.8 0 = unit, 1 = block.
1.7 0 = input, 1 = output.
1.6-1.1 ITS internal physical device code
(see table below).
The error messages indicated by bits 4.5-4.1 are:
1 DEVICE HUNG OR REPORTING NON-DATA ERROR
2 END OF FILE
3 NON-RECOVERABLE DATA ERROR
4 NON-EXISTENT SUB-DEVICE
5 OVER IOPOP
6 OVER IOPUSH
7 USR OP CHNL DOES NOT HAVE USR OPEN
10 CHNL NOT OPEN
11 DEVICE FULL (or directory full)
12 CHNL IN ILLEGAL MODE ON IOT
13 ILLEGAL CHR AFTER CNTRL P ON TTY DISPLAY
14 DIRECTORY FULL
15 DIRECTORY'S ALLOCATION EXHAUSTED
The error messages indicated by bits 3.9-3.7 are:
1 IDS ILLEGAL SCOPE MODE
2 IDS SCOPE HUNG
3 MORE THAN 1K SCOPE BUFFER
4 IDS MEM PROTECT
5 IDS ILLEGAL SCOPE OP
6 IDS MEM PROTECT ON PDL PNTR
7 IDS ILLEGAL PARAMETER SET
The error messages indicated by bits 3.6-3.1 are:
1 NO SUCH DEVICE
2 WRONG DIRECTION
3 TOO MANY TRANSLATIONS
4 FILE NOT FOUND
5 DIRECTORY FULL
6 DEVICE FULL
7 DEVICE NOT READY
10 DEVICE NOT AVAILABLE
11 ILLEGAL FILE NAME
12 MODE NOT AVAILABLE
13 FILE ALREADY EXISTS
14 BAD CHANNEL NUMBER
15 TOO MANY ARGUMENTS (CALL)
16 PACK NOT MOUNTED
17 DIRECTORY NOT AVAIL
20 NON-EXISTENT DIRECTORY NAME
21 LOCAL DEVICE ONLY
22 SELF-CONTRADICTORY OPEN
23 FILE LOCKED
24 M.F.D. FULL
25 DEVICE NOT ASSIGNABLE TO THIS PROCESSOR
26 DEVICE WRITE-LOCKED
27 LINK DEPTH EXCEEDED
30 TOO FEW ARGUMENTS (CALL)
31 CAN'T MODIFY JOB
32 CAN'T GET THAT ACCESS TO PAGE
33 MEANINGLESS ARGS
34 WRONG TYPE DEVICE
35 NO SUCH JOB
36 VALID CLEAR OR STORED SET
37 NO CORE AVAILABLE
40 NOT TOP LEVEL
41 OTHER END OF PIPELINE GONE OR NOT OPEN
42 JOB GONE OR GOING AWAY
43 ILLEGAL SYSTEM CALL NAME
44 CHANNEL NOT OPEN
45 INPUT BUFFER EMPTY OR OUTPUT BUFFER FULL
46 UNRECOGNIZABLE FILE (LOAD)
47 LINK TO NON-EXISTENT FILE
The physical device codes from bits 1.6-1.1 are as
follows. Note that, as a half-hearted rule, bit 1.6
indicates that file names are significant, and bit 1.5
indicates a software-implemented device.
This list is subject to additions and deletions!
CODE SYMBOL DEVICE DESCRIPTION
0 TTY Console input.
1 SNTTY TTY Printing console output.
2 SNTDS TTY Display console output.
3 SNLPD LPT Data Products line printer.
4 SNVID VID Vidisector ???
5 SNBAT Vidisector ???
6 SNPLT PLT Calcomp plotter.
7 SNPTP PTP Paper tape punch.
10 SNIMPX IMX Input multiplexor.
11 SNOMPX OMX Output multiplexor.
12 SNPTR PTR Paper tape reader.
13 SN340 DIS DEC 340 display, Ascii output.
14 SN340I IDS Interpreted 340 display. ???
15 SNMTC MTn Magnetic tape.
16 SNCOD COD Morse code device.
17 SNTAB TAB Tablet. ???
21 SNNUL NUL Source of zeroes, or output sink.
22 SNJOB JOB Job device.
23 SNBOJ BOJ Inverse of JOB.
24 SNSPY SPY Spy on another console.
25 SNSTY STY Pseudo-teletype.
26 SNNET NET ARPAnet (NCP).
27 SNLPV LPT Vogue line printer (yech!)
30 SNSTK STK Stanford keyboard.
31 SNMSP MSP (DM) Interprocess message protocol.
32 SNCHA CHAOS CHAOS net.
33 SNTCP TCP TCP Internet.
34 SNTRAP TRAP Trap "device"
35 SNIPQ IPQ Internet IP Queue.
36 SNUBI UBI KS10 Unibus interrupt.
41 SNUTC UTn Microtape (DECtape).
43 SN2311 DSK 2311 disk drives or equivalent.
60 SNFUSR USR A foreign (not immediately inferior) procedure.
61 SNUSR USR An immediately inferior procedure.
62 SNCLK CLx Various core link devices (x  {AIOU})
63 SNDIR --- File directory or ERR device.
64 SNPDP USR The PDP-6.
65 SNDIRH DIRHNG Directory hang "device"
66 SNLCK LOCK Lock "device"
.IPIRQC +* +* IORM into .PIRQC
Like .PIRQC (q.v.) when reading.
Performs IORM into .PIRQC when writing.

.JNAME +* +? job name
The name of the job as a word of sixbit characters.
The uname-jname pair must be unique to a job
(there are some exceptions involving system-created
jobs or jobs not logged in).
The uname and jname are used as file names when
creating or subsequently opening the job on the USR
device. See also the .UNAME user variable.
The .JNAME variable may be set by a .SUSET only in a
top-level job, and then only if the job has no
inferiors.
Attempting to set a jname to zero causes an
illegal operation interrupt (bit 1.6 of the .PIRQC
user variable). So does attempting to set it such that
the uname-jname pair would no longer be unique.
So does attempting to illegally set one's own jname.

.JPC +* +* jump program counter
The PC as of the most recent jump instruction,
i.e. an instruction which changed the PC by other
than 1 or 2. This is actually a register in the
paging box when running.

.MARA +* +* MAR (memory address register) address
Initially 0.
The address for the MAR register in the paging box,
which gives a %PIMAR interrupt when the specified operation
is performed on the specified address.
3.3 0 = exec mode, 1 = user mode.
This is forced to 1 when set with
.SUSET or .USET; exec mode is for very
obscure system hacks only.
3.2-3.1 0 Never interrupt.
1 Interrupt on instruction fetch.
2 Interrupt on write.
3 Interrupt on any reference.
2.9-2.1 Address for MAR.
The MAR does not work well on accumulators.
See also the .PIRQC user variable, bit %PIMAR;
and ITS INTRUP and ITS USR.
.MARPC +* +* MAR program counter
The PC as of the instruction that most recently
tripped the MAR interrupt, if any. In addition,
the indirect bit will be set if that instruction
completed successfully (was not aborted by the MAR
hit). See the .MARA user variable.

.MASK +* +* word 1 interrupt mask
Initially 0.
This is a mask for word 1 interrupts which indicates
which interrupts the job is prepared to handle.
If a class 2 or class 3 word 1 interrupt tries to
occur, but the corresponding bit in .MASK is not
set, then the interrupt is converted to class 1 or
ignored, respectively. The bits in .MASK directly
correspond to those in .PIRQC (q.v.). Bits for class
1 interrupts are AND'ed out before setting the .MASK
variable.

.MBO1 +- +- KL-10 Mbox counter
This is the low-order word of the KL-10 Mbox counter
value for the job. On KA-10's, it holds garbage.
Its location is .MBOX+1, for convenience.

.MBOX +- +- KL-10 Mbox counter
This is the high-order word of the KL-10 Mbox counter
value for the job. On KA-10's, it holds garbage.

.MEMT +* +* memory top
This variable contains 1 plus the highest legal
address in the job (the accessible word with the
largest address). Setting this variables is like
performing an equivalent .CORE uuo (q.v.):
.SUSET [.SMEMT,,[FOO]]
is like doing:
.CORE <FOO+1777>_12

.MPVA +- +- memory protect violation address
The address which the last instruction to cause a memory protection
violation (or a write into read only memory violation) attempted to
reference, on KA-10's rounded down to a page boundary. Thus an
attempt to reference the non-existent location 317435 would cause
.MPVA to be set to 316000 octal on a KA-10, or 317435 on a KL-10.
On the KS-10, .MPVA also holds the non-existant IO register address
after a %PINXI.
See also the .PIRQC variable.
.MSK2 +* +* word 2 interrupt mask
Initially 0.
This is a mask for word 2 interrupts which indicates
which interrupts the job is prepared to handle.
If a class 2 or class 3 word 2 interrupt tries to
occur, but the corresponding bit in .MSK2 is not
set, then the interrupt is converted to class 1 or
ignored, respectively. The bits in .MSK2 directly
correspond to those in .IFPIR (q.v.).

.OPC +- +- old program counter
The PC just before the last instruction was executed.
This corresponds to a register in the paging box.
It doesn't exist on KL-10's.
.OPTION +* +* option bits
Initially 0.
The bits in this word correspond to various options
as follows:
4.9 %OPTRP Same as the .UTRP user variable. See ITS UUO.
4.8 %OPDET ** This bit is OBSOLETE and NO LONGER EXISTS **
It is documented for historical purposes only.
Nowadays, fatal interrupts always cause top-level
jobs to be detached. Once upon a time, top-level
jobs with consoles would be reloaded instead of
detached, unless they had set %OPDET=1.
See the DETACH and RELOAD symbolic system calls.
4.7 %OPDEC Uuo's 40, 41, and 47 (that is,
.IOT, .OPEN, and .ACCESS)
should trap to the user as uuos
via the .40ADDR user variable (q.v.).
This is useful for programs which wish to
interpret DEC TOPS-10 UUO's, since those are the
only ones which conflict with ITS UUOs.
Luckily, those ITS UUOs are not essential since
there are symbolic system calls to do the same things.
4.6 %OPCMD Superior claims it has JCL which
it will cheerfully supply in response to
the appropriate .BREAK 12, command.
4.5 %OPBRK Superior claims to handle all .BREAK's.
4.4 %OPDDT Superior claims to be DDT.
4.3 %OPINT Job desires new-style (vectored
and stacked) interrupts. See also the
.40ADDR variable. See ITS INTRUP for
information on old- and new-style
interrupts.
4.2 %OPOJB Other jobs may open this job
as the OJB device, thereby turning it
into a JOB device. See ITS JOB.
4.1 %OPLOK Job desires the switch locking
and unlocking synchronization feature.
See also the .40ADDR variable.
See ITS LOCKS for information on locks.
3.9 %OPLIV The job tree of which this is the
top-level job is permitted to survive a
system shutdown -- it should take care
of logging itself out. See the .SHUTDN uuo.
Meaningful only for top-level jobs.
Primarily useful for system demons which need to
survive system death (e.g. the statistics
demon PFTHMG DRAGON).
3.8 %OPOPC Job desires that instruction-aborting interrupts
such as MPV leave the PC pointing before the
instruction that lost, instead of the old
convention of leaving it pointing after.
The new convention is far better and all new
programs should use it.
3.7 %OPLSP Superior claims to be MacLisp.
3.6 %OPLKF Unlock locks on fatal interrupt. When a
non-disowned top-level job receives a fatal
interrupt, if it has set this bit, its locks will
be unlocked by the system job as part of the
process of detaching it. See the %OPLOK bit and
ITS LOCKS.
.PAGAHEAD +* +* page-ahead control
Normally zero, this word is set nonzero to enable
sequential paging through a part of the address space
specified by .PAGRANGE. The right half of .PAGAHEAD is
the page-ahead interval width and the left half is the page-behind
distance. Each time a page in the designated range is
touched for the first time, the next few pages, forming the
page-ahead interval begin to swap in, and a page a certain
distance behind (specified by the page-behind distance) is
swapped out or marked for swap out in the near future. The
precise treatment depends on how loaded the system is.
Exactly one of the page-ahead interval width and the
page-behind distance should be negative. The page-behind
distance should be negative, if memory is being used from low
addresses to high ones. The page-ahead width should be
negative if moving from high addresses to low ones.
Example: -2,,4 means on first reference to page n, start
reading in pages n+1 through n+4 and possibly swap out page n-2.
Page n-1 is not affected until page n+1 is touched. A
page-behind distance of 2 means that two consecutive pages are
always available.
.PAGRANGE +* +* page-ahead range
Normally zero, this word is made nonzero together with .PAGAHEAD
to enable the sequential paging feature. The two halves of
.PAGRANGE are page numbers which specify the region of the
address space in which sequential paging should go on.
The left half specifies the first page in the sequentially
paged region, and the right half specifies the first page
after the end of that region.
.PICLR +* +* priority interrupt clear
Initially -1.
If non-zero, the job may take interrupts. If zero,
interupts are deferred. This variable is cleared
when an old-style interrupt occurs (but not by new-style
interupts!), and is set to -1 by the .DISMISS uuo
and the DISMIS symbolic system call. Attempts
to set this variable will convert the value to -1 or 0
depending on bit 4.9.

.PIRQC +* +* priority interrupt request cruft (?)
This word contains bits for pending word 1 interrupts.
Setting bits in this causes the corresponding interrupts
to attempt to take place, subject to .MASK, .DF1,
.PICLR (q.v.), and the %OPINT bit of the .OPTION user
variable (see ITS INTRUP).
Interrupts are of three classes:
1 Very serious. The job is stopped and its
superior interrupted.
2 Semi-serious. The job may request to handle
such an interrupt by setting the corresponding
bit in .MASK (q.v.); otherwise it is treated
as a class 1 interrupt.
3 Trivial. The job receives the interrupt if it
has requested to handle it; otherwise the
interrupt condition is ignored. (Under the
new-style interrupt scheme, a class 3 interrupt
may become a class 1 interrupt.)
See also the .IFPIR variable for word 2 interrupts.
The interrupt bits and their classes are as follows.
The character * denotes a class 1 interrupt, and + a
class 2 interrupt.
4.9 Must be zero.
%PIRLT 4.8 Clock break (see the .REALT uuo).
%PIRUN 4.7 Run time interrupt (see the .RTMR user variable).
%PINXI 4.6 + Non-Existent IO register
A Job in User IOT mode referenced a non-existent IO
register on the KS10 Unibus. The PC is left
pointing before the guilty instruction. The address
of the non-existant register may be found in .MPVA.
%PIJST 4.5 Job Status display request.
The sequence ^_J was typed on the console owned
by this process or some inferior.
%PIDCL 4.4 * A defered call was typed while the job had the TTY.
%PIATY 4.3 The tty was given back to the job by
its superior. Indicates that the screen has
probably been written on and its contents
have changed unpredictably.
%PITTY 4.2 + Attempt to use tty when not possessing it.
%PIPAR 4.1 + Parity error.
%PIFOV 3.9 Arithmetic floating overflow.
%PIWRO 3.8 + Attempt to write into read only memory.
See the .MPVA user variable.
%PIFET 3.7 + Pure page trap (attempt to fetch an instruction from
writable memory when bit 4.2 of the PC set).
This feature doesn't exist on KL-10's, instead
this interrupt is signalled for "Illegal entry
to concealed mode" which you probably can't make
happen. (See the KI10 Processor Reference Manual.)
See bit 4.2 of the .UPC user variable.
%PITRP 3.6 * System uuo to user trap (see the .UTRP user variable).
3.5 Arm tip break 3 (OBSOLETE).
3.4 Arm tip break 2 (OBSOLETE).
3.3 Arm tip break 1 (OBSOLETE).
%PIDBG 3.2 System being debugged.
Occurs when someone uses the .SETLOC or .IFSET uuo
to alter SYSDBG and the new contents are non-zero
and the old contents non-negative.
(See also the SSTATU symbolic system call.)
%PILOS 3.1 + A .LOSE UUO or LOSE system call was executed
(their purpose is to signal the superior).
%PICLI 2.9 CLI device interrupt.
%PIPDL 2.8 Pdl overflow.
%PILTP 2.7 Light pen interrupt on 340.
Program stop or hit stop on E&S display.
%PIMAR 2.6 + MAR interrupt.
(See the .MARA and .MARPC user variables.)
%PIMPV 2.5 + Memory protection violation (attempt to reference
a page not in the job's page map).
(See the .MPVA user variable.)
%PICLK 2.4 Slow clock interrupt (every .5 second).
%PI1PR 2.3 * Single intruction proceed interrupt.
Used by DDT for ^N commands.
%PIBRK 2.2 * Breakpoint interrupt (.BREAK uuo executed).
%PIOOB 2.1 + Illegal user address. (OBSOLETE)
%PIIOC 1.9 + Input/output channel error.
(See the .BCHN and .IOS user variables.)
%PIVAL 1.8 * Value interrupt (.VALUE uuo executed).
%PIDWN 1.7 System going down or being revived.
(See the .SHUTDN, .REVIVE, and .DIETIME uuo's,
and the SSTATU symbolic call.)
%PIILO 1.6 + Illegal instruction operation.
%PIDIS 1.5 + Display memory protection violation (340 or E&S).
%PIARO 1.4 Arithmetic overflow.
%PIB42 1.3 * Bad location 42.
(See the .40ADDR user variable, and ITS INTRUP.)
%PIC.Z 1.2 * ^Z typed when this job had the TTY.
%PITYI 1.1 Character enabled for interrupt was typed on TTY.
(Semi-obsolete; see the .IFPIR user variable,
and ITS TTY.)
.PMAP +<n> +- +- page map word
This is the page map word for page <n> of the job,
for <n> between 0 and 377 octal (256.K = 400 1K pages).
The map word read has this form:
4.9 %CBWRT Page is writable.
4.8 %CBRED Page exists (if this bit is zero, then the
whole word is zero).
4.7 ??? Page is in core (as opposed to swapped out).
4.6 %CBPUB Page is public (any job can write into it
which wants to).
4.2 %CBLOK Page is locked in core
(inhibited from swapout)
3.9 %CBSLO Page is in slow memory
(Doesn't work; Moon says it never will)
3.8-3.1 Number of times the page is shared.
(See the CORTYP symbolic system call,
right half of value 4.)
2.9-2.1 Absolute page number, or page number in next
sharer in the circular list of sharers.
(See CORTYP, value 3.)
1.9-1.1 0 Page is absolute.
777 Page is not shared.
<m> Next sharer in circular list of sharers
is the job with user index <m>.

.RTMR +* +* runtime timer
Initially -1.
If non-negative, the amount of run time remaining until the job
will receive a word 1 runtime interrupt. (See bit %PIRUN of the
.PIRQC variable.) The time is measured in slightly different units
on different CPUs. On the KA in 4.069 microsecond units, on the KL
in 4 microsecond units, and on the KS in 3.9 microsecond units.

.RUNT +- +- run time
Initially 0.
This is the run time used so far by the job,
measured in units of 4 microseconds.

.SERVER +* +* server job
Initially -1.
This is the user index of a job that has been given special
permission to modify this job, or -1 if there is no such job.
This can be used to implement various oddball client/server
protocols between jobs. The client job will request some service
from the server job and will set his .SERVER variable to the server's
user index to allow the service to be performed.
If the server job is killed, all of the client jobs will have their
.SERVER variables set back to -1.
Although it is safe to set .SERVER to -1 using .SUSET or .USET,
there can be timing errors if a client loads his .SERVER variable
by simply writing the server's user index into it. Specifically,
during the time the server's user index is sitting in the client's
memory, the server job might be killed and another job started with
the same user index. This can be guarded against by handling
interrupts on the USR: channel, but this is clumsy. A better
method is to use the SSERVE system call. (See .INFO.;ITS .CALLS)

.SNAME +* +* system name
This is a word of six sixbit characters which is
the default "directory name" for various input/output
operations. It is initially the same as the job's
.UNAME (q.v.). It can be overridden by supplying the
directory name explicitly to a symbolic system call.
The other uuo's such as .OPEN and .FDELE always use .SNAME.
.SUPPRO +- +- superior
This is -1 if the job is top-level, or the user index
of the job's superior. See .UIND.

.SV40 +- +- saved 40
The contents of absolute location 40 (i.e., the last
uuo that trapped to the system for this job).
See also the .UUOH user variable.
Note that only uuo's 40-47 are really used for
communication with the system. All other uuo's are
handed back to the executing job via its .40ADDR
user variable (q.v.).

.TR1INS +? +? Trap 1 instruction.
This is the instruction to be executed when arithmetic
overflow occurs. Op-code 0 is special, and uses
its E.A. as interrupt bits to turn on in the RH of .PIRQC.
The default contents of .TR1INS are simply %PIARO to
set the overflow interrupt. On the KA-10, writing the
variable doesn't change it.

.TR2INS +? +? Trap 2 instruction.
This is the instruction to be executed when pdl
overflow occurs. Op-code 0 is special, and uses
its E.A. as interrupt bits to turn on in the RH of .PIRQC.
The default contents of .TR2INS are simply %PIPDL to
set the pdl overflow interrupt. On the KA-10, writing the
variable doesn't change it.

.TTST +- +- saved TTYSTS
In a job which does not have the TTY, this variable
holds the saved contents of TTYSTS (what would be in
TTYSTS if the job were given a TTY). This is primarily
useful for examining from DDT.
See ITS TTY for a description of the contents of TTYSTS.
See also the TTYGET and TTYSET symbolic system calls.

.TTS1 +- +- saved TTYST1
Like .TTST, but for the TTYST1 variable instead of the
TTYSTS variable.

.TTS2 +- +- saved TTYST2
Like .TTST, but for the TTYST2 variable rather than the
TTYSTS variable.
.TTY +* +* random TTY variable
This variable indicates the status of the job with
respect to the console controlling its job tree.
The %TBNVR, %TBINT, %TBWAT, %TBOUT, %TBINF, %TBOIG, and %TBIIN bits
are settable. %TBWAT may be set only by the superior.
4.9 %TBNOT Does not have TTY now.
4.8 %TBNVR If 1, an OPEN on the tty will fail
rather than hanging, unless %TBWAT is on.
4.7 %TBINT An attempt to use the console without owning
it will cause a %PITTY interrupt (LH of
the .PIRQC user variable), unless
%TBWAT is set to 1.
In particular an OPEN on the tty
will interrupt instead of failing or
hanging.
4.6 %TBWAT If 1, overrides the setting of %TBINT
and %TBNVR, and makes the system act
as if they were 0. Settable only with
.USET. DDT sets this bit when $P'ing a
job, so that even if the job loses the tty
momentarily it will not get upset.
4.4 %TBDTY If the TTY was taken from the job,
then when it gets it back, this bit says
that the TTY should stay with the job
and not be passed down to an inferior.
4.3 %TBOUT Allow this job to type out, even if it
doesn't have the TTY. Some operations,
and all input will still require the job
to have the TTY. Not effective unless
the superior enables it by setting %TBINF
in the superior's .TTY var.
4.2 %TBINF Enable this job's inferiors to take
advantage of their %TBOUT's.
4.1 %TBOIG Ignore output. Overrides %TBWAT, %TBINT.
3.9 %TBIIN Interrupt on attempt to do input. Overrides
%TBWAT.
3.6-3.1 $TBECL Number if echo lines, if the job
doesn't currently have the TTY.
2.9-1.1 If the job has the TTY, this is the TTY number.
If it doesn't, and doesn't want it, this is the
(internal) user number of the immediately
inferior job to give the TTY to.
.TVCREG +* +* TV console register
Initially -1.
This variable is placed in the console register
when the job is run if it is non-negative.
It controls which video buffer memory is used
when writing into the job's tty's TV buffer
(see the CORBLK symbolic system call).
The format of the console register is as follows:
4.9-4.2 ALU function, used when writing into TV memory:
VALUE SYMBOL FUNCTION
0 CSETC SETCAM
1 CNOR ANDCBM
2 CANDC ANDCAM
3 CSETZ SETZM
4 CNAND ORCBM
5 CCOMP SETCMM
6 CXOR XORM
7 CANCSD ANDCMM
10 CORCSD ORCAM
11 CEQV EQVM
12 CSAME SETMM/MOVES/JFCL
13 CAND ANDM
14 CSETO SETOM
15 CORSCD ORCMM
16 CIOR IORM
17 CSET SETAM/MOVEM
Note that those symbols are not predefined in MIDAS.
4.1-3.3 Video buffer number (video switch input number).
See the VIDSW symbolic system call.

.UIND +- +- user index
The unique number assigned to the job by the system
when the job was created. These numbers typically
are between 0 and 77 octal or so (the exact maximum value
is a function of the particular incarnation of the
system). When a job is killed, its user index is
freed for re-use.
Most symbolic system calls which require a job
to be specified will accept 400000+the user index
in lieu of a channel with the job open on it if the
call is only to examine the job and not to modify it.
The .GUN and DETACH commands require a user index.
Jobs 0 and 1 are special in that they always stand for
the system itself and for the CORE job, respectively.
(The CORE job manages core allocation for the system.)
This may have various implications depending on context;
for example, when sharing a page with a job via the
CORBLK symbolic system call, sharing with
job 0 means sharing with an absolute page, and "sharing"
with job 1 means getting a fresh page.
.UNAME +- +? user name
A word of sixbit characters which is the user
name of the job. All jobs in a given job
tree must all have the same user name. Furthermore,
a console-controlled tree may not log in if another
console-controlled tree is logged in under the same
uname. When a new console-controlled tree is created
by typing ^Z on a free console, the uname of the newly
created top-level job of the tree is set to "___nnn",
where "nnn" is the user index of the newly-created
job. A non-disowned non-console controlled job is also given
such a uname initially. This may subsequently be changed to
something more reasonable by using the LOGIN symbolic
system call.
The uname of a newly created inferior job is initialized
to the uname of its creator.
If a disowned job tree is re-owned, the unames of all the
jobs in the re-owned job tree are set to the uname of the
re-owning job tree.
The .UNAME user variable may be set only with a .SUSET, and
only by a top level job which has no direct inferiors;
furthermore, attempting to set the uname to zero or the left
half to -1 causes an illegal operation interrupt (bit 1.6 of
the .PIRQC user variable), as does attempting to make the
uname-jname pair of the job non-unique.
See also the .JNAME user variable.
.UPC +* +- user program counter
The PC for the job. This word, of course,
contains the PC flags in the left half;
a job may set these flags for itself only by
using JRST 2,@[<word with flags and pc>].
On KA's, the flags in the left half are as follows:
(- = .USET may not set; % = peculiar to ITS)
%PCARO==400000 Overflow.
%PCCR0==200000 Carry 0.
%PCCR1==100000 Carry 1.
%PCFOV==40000 Floating overflow.
%PCFPD==20000 First part of instruction already done.
%PCUSR==10000 - User mode.
%PCUIO==4000 - User I/O.
%PCPUR==2000 % Pure. Instructions may only be fetched from
read-only memory. See bit 3.7 of the .PIRQC user
variable. This feature is not available on
all machines -- beware!
%PCSPC==1000 - Unused. (A PDP6 feature)
%PC1PR==400 % One proceed. An interrupt will occur at the end
of the next instruction. See bit 2.3 of the .PIRQC
user variable. Used by DDT for ^N commands.
%PCX17==200 % AI KA-10 computer only. Index-off-the-PC hack.
When this bit is set, an index field of 17 means
index off the PC instead of ac 17.
%PCFXU==100 Floating underflow.
%PCDIV==40 No divide.
3.5-3.1 Always zero. May not be set non-zero.
On KL's the flags are as follows (note the "|" at the front
of lines that differ significantly from those for KA's):
| %PSPCP==400000 "Previous Context Public" - this applies only
in exec mode; in user mode this bit is the same
as in KA (%PCARO, Arithmetic overflow).
%PSCR0==200000 Carry 0.
%PSCR1==100000 Carry 1.
%PSFOV==40000 Floating overflow.
%PSFPD==20000 First part of instruction already done.
%PSUSR==10000 - User mode.
%PSUIO==4000 - User I/O.
| %PSPUB==2000 "Public Mode" - not used in ITS
| %PSINH==1000 Inhibits MAR-breaks and trap3 (one-proceed trap)
| for one instruction.
| %PSTR2==400 Set by pdl overflow; causes the "trap 2 instruction"
| to be executed. That instruction is kept in .TR2INS
| %PSTR1==200 Set by arithmetic overflow; causes the
| "trap 1 instruction" in .TR1INS to be executed.
| Both %PSTR1 and %PSTR2 set generates a one-proceed
| trap.
| %PS1PR==160 % %PSINH+%PSTR2+%PSTR1. Setting those bits
| causes a single instruction proceed.
%PSFXU==100 Floating underflow.
%PSDIV==40 No divide.
3.5-3.1 Always zero. May not be set non-zero.
The user mode (4.4) is always set to 1.
The user I/O bit (4000, bit 4.3) is set according
to whether the job is in .IOTLSR mode.
If the .UPC user variable for one job is read by another while
the first is running, an exec mode PC may be seen (bit 4.4 = 0);
this reflects the fact that the job is in the middle
of a system call or something. The .UUOH user variable
should then be examined (this is precisely what DDT does
for the evaluation of .).
.USTP +* -- user stop bits
Initially <100000,,>.
If this variable is non-zero, the job is being blocked
from running for one reason or another. If zero, the
.FLS variable controls whether the job may run.
The form of the .USTP variable is as follows:
4.9 BCSTOP Job is being moved in core.
4.8 BUCSTP Core job is stopping this job
in order to get more core for another job.
4.7 BUSRC User control bit. Only this
bit may be modified, and only by the
procedure's superior. Any attempt to
set the .USTP variable non-zero
will set this bit; any attempt to set it
to zero will clear this bit.
4.6 BSSTP Set while superior is altering the page
map for the job. (Not directly settable
by superior.)
2.9-1.1 Count of transient reasons for stopping the job.
If non-zero, inhibits relocation of the job by
the core job.
PEEK displays this variable by printing the high six bits
in octal, then a "!", then the rest in octal. This is
why one normally sees "10!0" for a stopped job.

.UTRP +* -- user trap switch
When non-zero, this switch specifies that all uuo's
which trap to the system should, instead of performing
their usual actions, should cause a word 1 class 1
interrupt to the job executing the uuo. This allows
simulators, etc., to trap all uuo's executed by a job.
An attempt to set this variable will use only bit 1.1.
This is the same as the %OPTRP bit of the .OPTION user variable.

.UUOH +- +- system uuo PC
The program counter as of the last uuo which trapped
to absolute location 40 (not location 40 in the job,
nor the location specified by .40ADDR!)
See also the variable .SV40.

.VAL +* +* value or error code of job
.VALUE instructions set this word to the contents
of the memory location they address. That is useless.
More importantly, .LOSE instructions, and the LOSE
system call, set .VAL to <error code>,,<addr of losing
instruction>, so that the superior can decode the
error.
.WHO1 +* +* user who-line control word
This variable controls the printing of .WHO2 and .WHO3
at the end of the TV who-line (see ITS TV).
4.9 If 1, suppress the who-line entirely when focused
on this job.
4.8 Suppress space between halves of .WHO2.
4.7-4.5 Mode for printing left half of .WHO2:
0 Do not print.
1 Date in packed form:
4.9-4.3 Year (mod 100.).
4.2-3.8 Month.
3.7-3.3 Day.
3.2-3.1 Unused.
See the RQDATE symbolic system call, but
note that here the date is shifted.
2 Time in fortieths of a second, printed in tenths
in the form HH:MM:SS.T.
3 Time in half-seconds, printed in the form HH:MM:SS.
4 Octal halfword.
5 Decimal halfword (no . is supplied).
6 Three sixbit characters.
7 Unused.
4.4-4.2 Mode for printing right half of .WHO2.
4.1 Print 3.9-3.3 twice (doubled character).
3.9-3.3 If non-zero, character to print after left half of .WHO2.
3.2 If 1, suppress the space between .WHO2 and .WHO3 printout.
3.1 If 1, suppress the space between halves of .WHO3.
2.9-2.7 Mode for printing left half of .WHO3.
2.6-2.4 Mode for printing right half of .WHO3.
2.3 Print 2.2-1.5 twice.
2.2-1.5 If non-zero, character to print after left half of .WHO3.
1.4-1.1 Unused.
That is, if the who-line is printed at all, what appears
at the end is these characters:
AAAAXX-BBBB=CCCCYY+DDDD
where: AAAA is the result of printing the left half of .WHO2.
BBBB right half of .WHO2.
CCCC left half of .WHO3.
DDDD right half of .WHO3.
XX one or two characters specified by .WHO1 4.1-3.3.
YY one or two characters specified by .WHO1 2.3-1.5.
- space, unless .WHO1 4.8 is 1.
= space, unless .WHO1 3.2 is 1.
+ space, unless .WHO1 3.1 is 1.
Note that the specifications fall neatly into 8-bit bytes
(for the convenience of the PDP-11); hence it is easiest
to specify this word using .BYTE 8 in MIDAS.
Example:
.SUSET [.SWHO1,,[ .BYTE 8 ? 166 ? 0 ? 144 ? 200+", ]]
causes .WHO2 to appear as a word of sixbit, and .WHO3 to
appear as octal halfword typeout in mmm,,nnn format.

.WHO2 +* +* first user who-line variable
See .WHO1 for details.

.WHO3 +* +* second user who-line variable
See .WHO1 for details.
.XJNAME +* +* "intended" job name.
This variable holds what the job's name was "intended"
to be, by its creator. Why might the job's name not
actually be what it was intended to be? Perhaps because
that name was already in use by some other job, and the
creator had to find a second choice. For example, when
:NEW T in DDT creates a job named T0 because there was
already a job named T, the job T0's .XJNAME will be "T".
In any case, when using the common technique of having
several programs be links to one file, which figures
out what name it was invoked under and behaves
accordingly, the .XJNAME is the right place to look to
find out what behavior the invoker desires.
When a job is first created, its .XJNAME is the same as it's .JNAME.

.XUNAME +* +* "real" user name.
This variable holds what says who you really are,
as opposed to what you are logged in as.
It should be a word of sixbit, just like .UNAME.
For example, if you are logged in as FOO1 then
your XUNAME will probably be FOO, because that's
what DDT will normally set it to. However, though
the .UNAME may change because of reowning or attaching,
the .XUNAME will ot change iunless requested specifically.
However, the user may alter DDT's .XUNAME by depositing in
..XUNAME, which works by informing DDT and having DDT
tell ITS.
Whenever an inferior is created, its .XUNAME is initialized
from its superior's.
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