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Section 50: Virtual Storage and Address Translation
When a 4381 Processor is operating in System/370 and EC modes or in
System/370-XA mode, dynamic address translation (DAT) hardware is made
operative by turning on the translation mode bit in the current PSW. When DAT is
operative, virtual storage addresses in programs referring to instructions and data
are translated into real storage addresses after instructions are fetched during
program execution. The address in the instruction counter is translated also. When
DAT is not operative and System/370 or System/370-XA mode is in effect,
storage addresses in programs are used as real storage addresses.
When DAT is operative, the storage addresses in CCW lists are not translated by
channel hardware during channel program operation. The channel indirect data
addressing feature, also standard in 4381 Processors, and programmed channel
program translation are used for address translation for channel programs for both
System/370 and System/370-XA modes.
The following privileged instructions are associated with the dynamic address
translation facility: LOAD REAL ADDRESS (LRA), RESET REFERENCE BIT
(RRB) for System/370 mode only, RESET REFERENCE BIT EXTENDED
(RRBE), INVALIDATE PAGE TABLE ENTRY (IPTE), and PURGE TLB
(PTLB).
Virtual Storage Organization
The 4381 Processors operating in System/370 mode support a virtual storage size
of 16Mb (16,777,216 bytes) using a 24-bit address. A virtual storage segment size
of either 64K or 1024K bytes, is implemented. With either segment size, the page
size can be 2K or 4K for all 4381 models except 14 and 3 for which page size is
always 4K bytes. A segment size of 64K bytes is supported by DOS/VSE,
OS/VSl, MVS/370, and VM/370. For 4381 Processors with more than
16M-bytes installed, only 4K-byte pages can be used.
While a 16Mb virtual storage is always available for System/370 mode operations
as far as the DAT hardware is concerned, the actual amount of virtual storage to be
supported for a given 4381 Processor is defined by the installation when a
DOS/VSE or VS 1 operating system is generated. DOS/VSE with VSE/ Advanced
Functions Release 2.1.0 supports up to three virtual storages. Each can be up to
16Mb in size with a maximum of 40Mb supported for the three virtual storages.
MVS/370 supports multiple 16Mb virtual storages and VM/370 supports a virtual
storage up to 16Mb for each virtual machine.
The 4381 Processor operating in System/370-XA mode supports a virtual storage
size of 2Gb (2,147,483,648 bytes) using a 31-bit address. A virtual storage
segment size of 1024K bytes and a page size of4K bytes are implemented for
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A Guide to the IBM 4381 Processor
.._,, I
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