Features To Support Demand Paging - IBM System/370 145 Manual

Expanded A1ter/Oisplay for Console Printer-Keyboard
The alter/display function of the console printer-keyboard is
expanded in the Model 145 to accept virtual storage addresses in
alter/display requests. When the operator indicates that a virtual
storage address has been entered, the address is translated into a real
storage address under microprogram control, without reference to the
TLB, using the existing contents of control registers 0 and
1
to define
the segment and page sizes and the location of the segment table.
Translation mode need not be specified.
The virtual storage address entered and the translated real storage
address are printed on the console for both alter and display
operations. If a translation exception occurs, an error message is
printed instead of the real storage address.
An
exception occurs, for
example, when a page frame is not currently allocated to the virtual
storage page referenced.
The STORE and DISPLAY console panel functions still use real storage
addresses only. The ADDRESS COMPARE and SET IC functions assume the
storage address indicated to be virtual or real, depending on the
setting of the select switch.
FEATURES TO SUPPORT DEMAND PAGING
Reference and Change Recording Facility, for Real Storaqe Blocks
A hardware recording facility is standard in the Model 145.
This
facility provides continuous recording of the activity of all 2K real
storage blocks via reference and change bits,. The settings of these
recording bits can be used by control program routines to support a
demand paging environment. This hardware facility is always active. It
does not depend on EC or translation mode being operative.
The seven-bit key associated with each 2K real storage block in the
Model 145 has four storage-protect bits, one fetch-protect bit, one
reference bit, and one change bit. During system operation, the
activity of each 2K real storage block is monitored by hardware.
Whenever a fetch is made by either a CPU or a channel to a real storage
address, the reference bit in the key associated with the 2K storage
block that contains ·that real storage address is turned on by the
hardware. A store. into any real storage address causes .the hardware to
turn on both the change bit and the reference bit for the affected 2K
block.
Alter/display operations initiated from the console printer-keyboard
and store/display operations initiated from the console panel also cause
appropriate changing of the reference and change bits. The RESET
REFERENCE BIT instruction is provided to allow the reference bit of any
2K real storage block to be reset by programming without altering the
contents of the other six bits in.the protect key.
The hardware reference and change recording facility is used by the
page replacement algorithm of a virtual storage operating system. When
a page is loaded into a page frame, the reference and change bits for
that page frame are set to zero. (When a 4K page size is used, the
reference and change bits for both of the 2K storage blocks involved are
reset.) Thereafter, the reference bit is used to determine the activity
of a page. The change bit is inspected to determine whether a page must
be paged out when its page frame is reassigned. The SET STORAGE KEY
instruction must be used to reset the change bit.
14
A Guide to the IBM System/310 Model 145