April 1, 1981
To attempt to this inaccuracy at the cost of some additional VM/370 overhead, an installation may wish to add the following dummy DIAGNOSE instruction: 83000000 at the following locations in the DOS/VS supervisor source statements: • In the SVC 24 routine, before the L R3,SYSTIMER statement • In the SVC 52 routine, before the L R3,SYSTIMER statement • In the STCLOCK routine, before the STCK CLOCK statement • In the timer interruption handler R2,R3,SYSTIMER statement
rou tine, before the LM • In the job accounting initialization routine (JATIMER), before the
statement that references SYSTIMER NQte: To prevent a possible specification exception to DOS/VS, ensure
that general register zero contains zeros before issuing the DIAGNOSE instruct ion.
If VM/370 is running on a processor model that has ECPS: VM/370 (as
defined in and enable virtual
interval timer assist. This action lets the hardware, rather than VM/370, update the virtual interval timer. Hardware update frequency is 300 times per second and results in accurate and repeatable time
measurements. DOS RELEASE 27 IN A V=R VIRTUAL MACHINE
To avoid compromising the CP real page zero, the DOS Release 27
supervisor must be modified and reassembled to prevent DOS from reading
sense information into page zero. The recommended supervisor changes
are:
1. In the FOPT macro instruction, remove the label SSKADR from the SSKADR DC statement.
2. Following the SGTCON macro instruction, add the following 5-byte DC
statement: SSKADR DC CL5
3. Assemble and link edit the DOS supervisor again.
This supervisor now works correctly in a V=R machine as well as on a
virtual (or real) machine.
Generating DOS/VS Under VM/370 This topic presents the major steps in a DOS/VS system generation
procedure that is performed under VM/370. This virtual machine is
assumed to have a VM/370 directory entry as shown in Figure The
link CMSSYS provides the user with CMS facilities. Alternate procedures to CMS are discussed wherever applicable. 74 VM/370 Operatinq Systems in a virtual Machine

The virtual machine console is assumed to be a 3270 display terminal. USER DOSVSYS PASSWORD 512K ACCOUNT ACCTNO BIN4 OPTION ECMODE CONSOLE 009 3210 SPOOL OOC 2540 R SPOOL OOD 2540 P SPOOL OOE 1403 LINK CMSSYS 190 190 RR MDISK 191 3330 50 5 UDISK3 MR MDISK 250 3330 0 404 DOSSYS MR Figure 15. virtual Machine for DOS/VS System Generation
In addition to the devices specified in the directory entry for user DOSVSYS, this user needs a dedicated magnetic tape unit at
address 180 on which to mount the distribution tape. the attachment of
the magnetic tape drive is discussed later in this topic. BUILDING SYSTEM GENERATION JOB STREAMS During the entire system generation process, the user supplies the
virtual machine with job streams to perform the individual steps. On a
stand-alene system, these job streams are usually kept in card form. Under VM/370, the CMS editor can be used to create CMS files containing
these job streams. Once created, use the VM/37C spooling functions to Flace these jcb streams into the virtual machine's card reader whenever
they are needed. This technique is especially helpful when the virtual
ccnsole is seme distance from the real reader.
Te generate a tape and disk configuration, unload the IBM-supplied
tase system from a distribution tape to an initialized DASD volume.
Create two small CMS files. These files contain the card reader input
data for the two utility programs that occupy the first two files on the
distribution tape. The file assumes: (1) the user logs onto VM/370 as
user DOSVSYS, and (2) the user is in the CMS environment. Section 3. DOS/VS in a Virtual Machine 75