USERSECT (USER AREA)
The USERSECT CSECT defines space that is not used by CMS. A
modification or update to CMS can use the 18 full words defined for USERSECT. There is a pointpr (AUSER) in the NUCON area to the user
space. DEVTAB (DEVICE TABLE)
The DEVTAB CSECT is a table describing the devices available for the CMS syste.. The table contains the following entries: • 1 console • 10 disks • 1 reader • 1 punch • 1 pl:'in'tt?!" • 4 tapes You can change some existing entries in DEVTAB. Each device table
entry contains the following information: • Virtual device address • Device flags • Device types • Symbol device name • Address of the interrupt processing routine (for the console)
The virtual address of the console is defined at IPL time. The
virtual of the user disks can be altered dynamically with the ACCESS command. The virtual address of the tapes can te altered in the
device table. Changing the virtual address of the reader, printer, or
punch will have no effect. Figure 2 describes the devices supported by CMS. Structure of eMS Storage
Figure 3 describes how CMS uses its virtual storage. The pointers
indicated (MAIISTRT, MAINHIGH, FREELOWE, and FREEUPPR) are all found in NUCOI (the nucleus constant area). • The sections of CMS storage have the following uses: DMSNUC (X'OOOOO' 19 gEE!g!!!g!glI X'03000'). poInters,--ilags; and other data updated--by routines ..
This area contains
the various system • DMSFREE Free !!gg (!EEf2!img!eII 12 ThIs--area-Is a free storage area, from which requests from DMSFREE are allocated. The top part of this area contains the file directory for the System Disk (SSTAT). If there is enough room
(as there will be in most cases), the FREETAB table also occupies
this area, just below the SSTAT. 2-12 IBM VM/370 System Logic and Program Determination--Volume 2

Virtual IBM Device 3210, 3215, 1052, 3066, 3270 2314. 3330. 3340 3350 2314, 3330, 3340 3350 2314, 2319, 3330, 3340, 3350 2314, 2319, 3330, 3340, 3350 2314, 2319, 3330, 3340, 3350 2314, 2319, 3330, 3340, 3350 2314, 2319, 3330, 3340, 3350 2314, 2319, 3330, 3340, 3350 2314, 2319, 3330, 3340, 3350 2314, 2319, 3330, 3340, 3350 1403, 3203, 3211
1443 2540, 2501, 3505 2540, 3525
2415, 2420, 3410, 3420 Virtual I Symbolic Address11 Name
ccu 190 191 2
ccu
ccu
192
ccu
ccu
ccu
19E cell OOE OOC 000 181-4 CON1 DSKO DSK1 DSK2 DSK3 DSK4 DSK5 DSK6 DSK7 DSK8 DSK9 PRNl RDRl PCHl TAP1-TAP4
Device Type
system console System disk (read-only) Primary disk (user files)
Disk (user files)
Disk (user files)
Disk (user files)
Disk (user files)
Disk (user files)
Disk (user files)
Disk (user files)
Disk (user files)
Line printer
Card reader
Card punch
Tape drives
lThe device addresses shown are those that are preasseabled into the CMS resident device table. These need only be modified and a new
device table made resident to change the addresses.
2The virtual device address (ccu) of a disk for user files can be
any valid System/370 device address, and can be specified by the eMS user when he activates a disk. If the user does not activate
a disk immediately after loading CMS, CftS automatically activates
the primary disk at virtual address 191.
Figure 2. Devices supported by a eMS Virtual Machine • Transient 12 • Since it is not essential to keep all nucleus functions resident in storage all the
time, some of them are made "transient." This means that when they
are needed, they are loaded from the disk into the transient prograa
area. Such programs may not be longer than two pages, because that
is the size of the transient area. (A page is 4096 bytes of virtual
storage.) All transient routines must be serially reusable since
they are not read in each time they are needed. • 12 • Segment 1 of storage contains
the reentrant code for the CMS Nucleus routines. In shared eftS systems, this is the "protected segment," which must consist only of
reentrant code, and aay not be modified under any circumstances.
Thus, such functions as DEBUG breakpoints or CP address stops cannot
be placed in Segment 1 when it is a protected segment in a saved
system. CMS Introduction 2-13