OS Macro Simulation Under eMS When a language processor or a user-written program is executing in the CMS environment dnd using Os-type functions, it is not executing os
code. Instead, CMS provides routines that simulate the os functions
required to support as language processors and their generated object
code. CMS functionally simulates the os macros in a way that presents
equivalent results to programs executing under CMS. The as macros are
supported cnly to the extent stated in the publications for the
supported language processors, and then only to the extent necessary to
successfully satisfy the specific requirement of the supervisory
function.
The restrictions for COBOL and PL/I program execution listed in
"Executing a program that Uses as Macros" in the exist because of the limited CMS simulation of
the as macros.
Figure 7 shows the as macro functions that are partially or
completely simulated, as defined by SVC number.
as Data Management Simulation The disk format and data base organization of CMS are different froll
those of as. A CMS file produced by an as program running under eMS and
written on a CMS disk, has a different format from that of an as data
set produced by the same as program running under os and written on an
as disk. The data is exactly the same, but its format is different. (An
as disk is one that has been formatted by an as program, such as
IBCDASDI. )
HANDLING FILES THAT RESIDE ON CMS DISKS CMS can read, write, or update any as data that resides on a eMS disk.
By simulating as macros, CMS simulates the following access methods so
that as data organized by these access methods can reside on eMS disks:
direct
partitioned
sequential
identifying a record by a key or by its relative
position within the data set.
seeking a named member within the data set.
accessing a record in a sequence in relation to
preceding or following ite.s in the data set.
Refer to Figure 7 and the "Simulation Notes," then read "Access Method Support" to see how CMS handles these access methods. Since CMS does not simulate the indexed sequential access aethod (ISAM), no as program that uses 1SAM can execute under CMS. Therefore,
no program can write an indexed sequential data set on a CftS disk. CMS Introduction 2-35

HANDLING FILES THAT RESIDE ON OS OR DOS DISKS
By simulating
sequential and same simulated
on DOS disks.
Thus, a DOS running under OS macros, CMS can read, but not write or update, CS pactitioned data sets that reside on OS disks. Using the OS macros, eMS can read DOS sequential files that reside
The OS macros handle the DOS data as if it were OS data.
sequential file can be used as input to an as program CMS. However, an as s9quential or partitioned data set that resides on an
as disk can be written or updated only by an as program running in a
real OS machine. eMS can execute programs that read and write VSAE files from OS programs written in the VS BASIC, COBOL, or PL/I programming languages.
This CMS support is based on the DOS/VS Access Method Services and Virtual Storage Access Method (VSAM) and, therefore, the as user is
li.ited to those VSAM functions that are available under DOS/VS. 2-36 IBM VM/370 System Logic and Program Determination--Volume 2