LASTUSER The SVC handler traces back through its system save areas for
the active user routine closest to the top of the stack, and
the storage key in effect for that routine is placed in the PSi. The old contents of the second tyte of the PSi is saved
in a stack. This option should be used only by system routines that should enter a user exit routine. NOSTACK This option may be used with any of the above options to
prevent the system from saving the second byte of the current PSi in a stack. If this is done, then no DMSKEY RESET need be
issued later. RESET The seccnd byte of the PSW is changed to the value at the top
of the PSi key stack, and removed from the stack. Thus, the
effect of the last DMSKEY NUCLEUS or USER or LASTUSER request
is reversed. This option should may not be used to reverse
the effect of a DMSKEY macro for which the NOSTACK option was
specified. A DMSKEY RESET macro must be executed for each DMSKEY NUCLEUS, USER or LASTUSER macro that was executed and
that did not specify the NOSTACK option. Failure to observe
this rule results in program abnormal termination. THE DMSEXS MACRO System commands running in user protect status use the DMSEXS macro to
execute a single instruction with a system protect key in the PSi. This
macro instruction can be used in lieu of two DMSKEY macros. The format
is:
r--------------------------------------------------- I [label] I DMSEXS I op-code,operands
The op-code and the operands of the instruction to be executed must be given as arguments to the DMSEXS macro.
For example, execution of the sequence, USING NUCON,O DMSEXS OI,OSSFLAGS,COMPSiT would cause the 01 instruction to be executed with a zero protect key in
the PSi. This sequence would turn on the COMPSiT flag in the nucleus.
It would be reset with DMSEXS NI,OSSFLAGS,255-COMPSWT The instruction to be executed may be an EX instruction.
Register 1 cannot be used in any way in the instruction being
executed.
2-112 IBM VM/370 System Logic and Program Determination--Volume 2

Simulate Non-eMS Operating Environments
The following contains descriptions for: access method support for non-eMS operating systems, eMS simulation of as functions, and implementation of DOS/VS functions.
Access Method Support for Non-eMS Operating Environments OS ACCESS METHOD SUPPORT An access method governs the manipulation of data. To make the
execution of OS generated code easier under eMS, the processing program
.ust see data as OS would present it. For instance, when the processors
expect an access method to acquire input source records sequentially, CMS invokes its sequential access method and passes data to the
processors in the format that the as access methods would have produced.
Therefore, data appears in storage as if it had been manipulated using
an as access method. For example, block descriptor words (BDW), buffer
pool management, and variable records are maintained in storage as if an
os access method had processed the data. The actual writing to and
reading from the I/O device is handled by eMS file management.
The work of the volume table of contents (VTOC) and the data set
control tlock (DSCB) is done by a master file directory (MFD) to
maintain disk contents and a file status table (PST) for each data file.
All disks are formatted in physical blocks of 800 bytes. CMS continues to maintain the OS format, within its own format, on
the auxiliary device, for files whose filemode number is 4. That is, ihe block and record descriptor words (BDi and RDW) are written along
with the data. If a data set consists of blocked records, the data is
written to and read from tbe I/O device in pbysical blocks, rather than
logical records. CMS also simulates the specific methods of
manipulating data sets.
To accomplish this simulation, CMS supports certain essential macros
for the following access methods: • BDAM (direct)--identifying a record by a key or by its relative
position within the data set. • BPAM (partitioned)--seeking a named member within an entire data set. • BDAM/QSAM (sequential)--accessing a record in a sequence relative to • VSAM (direct or sequential)--accessing a record sequentially or
directly by key or address. CMS support of as VSAM files is
based on DOS/VS access method services and the virtual storage
access method (VSAM). Therefore, the OS user is restricted to
those services available under DOS/VS A85 and VSAM. CMS Method of Operation and Program Organization 2-113