Virtual Virtual Symbolic
IBM Device Type Add:ess
1
Name (default) Device Use
2314,2319,3310, 190 DSKO CMS System disk (read-only) 3330, 3340, 3350, 19J3 DSKI Primary disk (user files)
3370,3375,3380 cuu DSK2 Minidisk (user files)
cuu DSK3 Minidisk (user files)
192 DSK4 Minidisk (user files)
cuu DSK5 Minidisk (user files)
cuu DSK6 Minidisk (user files)
cuu DSK7 Minidisk (user files)
19E DSK8 Minidisk (user files)
cuu DSK9 Minidisk (user files)
cuu DSKH Minidisk (user files)
cuu DSKI Minidisk (user files)
cuu DSKJ Minidisk (user files)
cuu DSKK Minidisk (user files)
cuu DSKL Minidisk (user files)
cuu DSKM Minidisk (user files)
cuu DSKN Ivlinidisk (user files)
cuu DSKO Minidisk (user files)
cuu DSKP Minidisk (user files)
cuu DSKQ Minidisk (user files)
cuu DSKR Minidisk (user files)
cuu DSKT Minidisk (user files)
cuu DSKU Minidisk (user files)
cuu DSKV Minidisk (user files)
cuu DSKW Minidisk (user files)
cuu DSKX Minidisk (user files) 2540,2501,3505 OOC RDRI Virtual reader
2540,3525 OOD PCHI Virtual punch 1403, 1443, 3203, OOE PRNI Line printer
3211, 3262, 3800, 4245, 3289-4 2401, 2402, 2403, 181-4 TAPI-TAP4 Tape drives
2415, 2420, 3410, 3411, 3420, 3430, 8809 Figure 34 (Part 2 of 2). Devices Supported by a eMS Virtual Machine
Structure of eMS Storage IThe device addresses shown are those that are preassembled into the CMS resi­
dent device table. These need only be modified and a new device table made resi­
dent to change the addresses.
2The virtual address of the system console may be any valid multiplexer address.
3191 is the def aul t user-accessed A-disk unless it is dynamically changed by an
ACCESS at CMS initial program load (IPL).
Figure 35 , Figure 36 , and Figure 37 describe how CMS uses its virtual storage.
The pointers indicated (MAINSTRT, MAINHIGH, and FREELOWE) are all
found in NUCON (the nucleus constant area).
Functional Information 317

The sections of eMS storage have the following uses: I · I · • I · 318 VM/SP System Programmer's Guide DMSNUC (X'OOOOO' to X'05000'). This area contains pointers, flags, and oth­
er data updated by the various system routines.
Low-Storage DMSFREE User Free Storage Area ( X'05000' to X'OEOOO'). This
area is a free storage area, from which user requests to DMSFREE are allo­
cated.
Transient Program Area (X'OEOOO' to X'J 0000 '). 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 program 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.
Low-Storage DMSFREE Nucleus Free Storage Area (X'JOOOO' to X'20000'). This area is a free storage area from which nucleus requests to DMSFREE are
allocated. The top part of this area contains the dummy hyperblocks for the "S" and "Y" disk with each block 48 bytes long. This area may be followed
by the file status tables for the "S2" filemode files of the system disk and/or
the "Y2" filemode files of the system disk extension. Note that if the system
disk is formatted as 512, 1K, 2K, or 4K blocks, each FST is 64 bytes (X'40') long, and holds approximately 318 FSTs. If the system disk is formatted for 800 byte blocks, each FST is 40 bytes (X'28 ') long and holds approximately 509 FSTs. If there is enough room, the FREET AB table also occupies this
area, just below the file status tables, if they are there. Each entry in the
FREETAB table is one byte long and each byte represents one page (4K or 4096 bytes) of defined storage. User Program Area (X'20000' to Loader Tables or CMS Nucleus, whichever has
the lower value). User programs are loaded into this area by the LOAD com­
mand. Storage allocated by means of the GETMAIN macro instruction is tak­
en from this area, starting from the high address of the user program. In
addition, this storage area can be allocated from the top down by DMSFREE,
if there is not enough storage available in the low DMSFREE storage area.
Thus, the usable size of the user program area is reduced by the amount of free
storage that has been allocated from it by DMSFREE.
Loader Tables (top pages of storage). The top of storage is occupied by the
loader tables, which are required by the eMS loader. These tables indicate
which modules are currently loaded in the user program area (and the transient
program area after a LOAD command). The size of the loader tables can be
varied by the SET LDRTBLS command. However, to successfully change the
size of the loader tables, the SET LDRTBLS command must be issued imme­
diately after IPL. CMS Nucleus (suggested location: X'700000' to 'X'MB). Segments 29, 30 and
31 of storage contain the reentrant code for the eMS Nucleus routines, shared
copies of the system S-ST A T and Y -STAT, and the S-disk and Y -disk FST
tables. If there is not sufficient room to contain these tables in this area, they
are placed in low-storage DMSFREE Nucleus free storage area. In shared
eMS systems, these are the "protected segments," which must consist only of
reentrant code, and may not be modified under any circumstances.