PSEUDO TIMER DIAGNOSE
The pseudo timer can also be interrogated by issuing DIAGNOSE with an
operation code of C, as described under "DIAGNOSE Instruction in a Virtual Machine." If this method is used, the virtual and total
processor times are expressed as double words in microseconds.
CP In Attached Processor Mode
In an attached processor environment, two processors share main storage.
There is special code in CP to ensure that the two processors do not
interfere in each other's operation. Most of this special code is
executed only in an attached processor environment. For information about system generation of the special code, see the glanning PSA Each processor needs its own area for processor-related information.
During CP initialization, CP obtains an area from the high-end of real
storage for prefix storage areas (PSA) for each processor. Each
processor accesses its own PSA by a process called prefixing. Prefixing is described in detail in the B! (GA22-7000) When code executing on either processor references an address from 0 to 4096, the referenced address is added to the contents of the prefix
register for that processor to produce the "absolute" address that will
be accessed. A reference to the first 4K of storage, therefore, results
in the PSA residing in high core being accessed. In this way, each
processor is given its own work area and save areas. Figure 23, a
storage map of the V=R machine, shows where PSAs are located in real
storage after CP initialization completes. However, if a processor is
varied offline and then online after CP initialization completes, the
processor's PSA may be located in any contigeous 4K byte area of free
storage. IBM VM/370 System Programmer's Guide
virtual Storage
... ;:1..:1 _____ _ auuJ.'C;::';::''C,;:) ABSOLUTE PAGE 0 4K Virtual Page 1 128K-l OK 4K-1 VIRTUAL=REAL AREA
SIZE = 128K BYTES (Minimum size is 32K bytes.) Virtual page 0 REMAINDER OF CP NUCLEUS DYNAMIC PAGING AREA
and
FREE STORAGE PSA FOR THE ATTACHED PROCESSOR PSA FOR THE MAIN PROCESSOR Figure 23. Storage in a Virtual=Real Machine I/O HANDLING
Real Storage Addresses OK 4K
128K
132K (DMKSLC) {
End of CP Nucleus (Dl!KCPE) < 1- Dl!KPSl <
512K End of
real storage
In an attached processor environment, only the main processor is capable
of handling I/O. If a command being executed on the attached processor
requires I/O processing, it is dispatched to the main processor via the SWITCH macro. This is not apparent to the user as it is handled
internally by CP. When the control program is executing, the I/O configuration tables
are protected only by the global system lock. Because the first-level
interrupt handler (FLIH) executes without the lock in some cases, there is the possibility of both processors modifying an I/O table. To
eliminate this situation, critical references to the I/O table are
preceded by forcing the system onto the main processor via a SWITCH macro. Since the unlocked I/O FLIH never executes on the attached
processor, the critical I/O code is serialized on the main processor. Part 2. Control Prograa (CP) 179
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