OS Macro Simulation under eMS When a language processor or a user-written program is executing in the CMS environment and using OS-type functions, it is not executing os
code. Instead, CMS provides routines that simulate the as functions
required to support as language processors and their generated object
code. CMS functionally simulates the as macros in a way that presents
equivalent results to programs executing under eMS. The as macros are
supported only to the extent stated in the publications for the sUFPorted 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 glanning Qene!g!i2n exist because of the limited CMS simulation of
the as macros.
Figure 31 shows the as macro functions that are partially or
completely simulated, as defined by SVC number. OS Data Management Simulation The disk format and data base organization of CMS are different fre.
those of as. A CMS file produced by an as program running under CMS 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 as and written on an OS 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 CMS disk.
By simulating as macros, CMS si.ulates the following access methods so
that as data organized by these access methods can reside on CMS 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 items in the data set.
Refer to Figure 31 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 method (ISAM), no as program that uses ISAM can execute under eMS. Therefore,
no program can write an indexed sequential data set on a CMS disk. 270 IBM VM/370 System Programmer's Guide

HAIDLING FILES THAT RESIDE ON 05 OR DOS DISKS By simulating
sequential and
saae simulated
on DOS disks.
Thus, a DOS running under 05 macros, CMS can read, but not write or update, 05 partitioned data sets that reside on OS disks. Using the
os macros, CMS 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 05 progra. CMS. However, an 05 sequential or partitioned 05 disk can be written or updated only by
real 05 machine.
data set that resides on an
an OS program running in a CMS can execute programs that read and write VSAe files fro. CS programs written in the vs BASIC, COBOL, or PL/I programming languages.
This CMS support is based on the DOS/iS Access Services and Virtual Storage Access Method (VSAM) and, therefore, the 05 user is
li.ited to those VSAM functions that are available under DOS/VS. Part 3. Conversational Monitor System (CMS) 211