Page of GC20-1821-3 As Updated April 1, 1981 by TNL GN25-0840 When generating DOS/VS under VM/370, an installation must restore the DOS/VS PID tape onto a full disk pack; a minidisk cannot be used.
Create an operational system residence pack on a full pack, and then
use CORGZ to copy the DOS/VS SYSRES to a minidisk. Kake sure that ECMODE is set 'ON'. In the FOPT macro instruction, specify enough second level directory (SLD) entries to reduce repetitious reading of the directory. Use the system directory list (SDL) in the shared virtual area (SVA) for all job control, disk and tape open and close transients, and for
the attention routine.
If minidisks are used, although permitted on DOS/VS not use the same DOS/VS file name for more than one VM/370 may select the wrong minidisk.
running native, do
disk. Otherwise, If a program in the DOS/VS core image library may be executed in either
a DOS/VS virtual machine or under CMS/DOS, link edit the program with ACTION REL linkage editor control statement so that it uses the DOS relocating loader. DOS/VS ACCOUNTING This topic only applies when running DOS/VS Release 34 (or
earlier). It does nQ! apply when running either DOS/VS Release 34 with
the Advanced Functions-DOS/VS Program Product (5746-XE2) or DOS/VSE. Except with processors that have ECPS: VM/370 and virtual interval
timer assist, DOS/VS accounting gives inaccurate and inconsistent
elapsed processor times when operating under VM/370 with virtual machine
assist. This inconsistency occurs because the interval timer (located
at virtual storaqe lecation i'SO') used by the DOS/VS accounting
routine is only updated when VM/370 gets control. Therefore, when DOS/VS accesses the interval timer data, a variable amount of time may
have elapsed since VM/370 last updated the interval timer, and thus DOS/VS records an inaccurate processor time.
section 3. DOS/VS in a virtual Machine 73

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