2. For a channel, a store access is prohibited whenever a key-
controlled-protection violation
exists for that access.
Change recording is always active and
takes place for all store accesses to
storage, including those made by anyCPU, channel, or operator facility. It
takes place for implicit references made
by the machine, such as thosewhich are
part of interruptions.
Change recording does not take place for
the operands of the following
instructions since they directly modify
a storage key without modifying a stor
age location:
RESET REFERENCE BIT
RESET REFERENCE BIT EXTENDED
SETSTORAGE KEY (change bit is set
to a specified value)
SETSTORAGE KEY EXTENDED (change
bit isset to a specified
value)
Change bits which have been changed from
zeros to ones are not necessarily
restored to zeros onCPU retry (see the
section"CPU Retry" in Chapter 11,
"Machine-Check Handling").See the
section "Exceptions to Nullification and
Suppression" in Chapter 5,"Program Execution," for a description of the
handling of the change bit in certain
unusual situations.PREFIXING Prefixing provides the ability to assign
the range of real addresses0-4095 (the
prefix area) to a different block in
absolute storage for eachCPU, thus
permitting more than oneCPU sharing
main storage to operate concurrently
with a minimum of interference, espe
cially in the processing of
interruptions.Prefixing is provided as
part of the multiprocessing facility.Prefixing causes real addresses in the
range0-4095 to correspond to the block
of 4K absolute addressesidentified by
the value in the prefix register for theCPU, and the block of real addresses
identified by the value in the prefix
register to correspond to absolute
addresses0-4095. The remaining real
addresses are the same as the corre
sponding absolute addresses. This
transformation allows eachCPU to access
all of main storage, including the first
4K bytes and the locations designated by
the prefix registers of otherCPUs. The relationship between real and abso
lute addresses is graphically depicted
in the figure "Relationship between Real
and Absolute Addresses."
The prefix is a 19-bit quantity
contained in bit positions 1-19 of the
prefix register. Bits 1-7 of the prefix
register are always zeros. The register
has the following format:1////////////1 018 20 31
The contents of the register can be set
and inspected by the privileged
instructions SETPREFIX and STORE PREFIX, respectively. On setting, bits
corresponding to bit positions0-7 and 20-31 of the prefix register are
ignored.On storing, zeros are provided
for these bit positions. When the
contents of the prefix register are
changed, the change is effective for the
next sequential instruction.
With the introduction of the storage
key-instruction-extension facility, the
test-block facility, and the extended
real-addressing facility, prefixing is
described in terms of 31-bit real
addresses, whether or not these facili
ties are installed. All real addresses
are considered to be 31 bits, with any
shorter address fields extended to 31
bits by appending zeros on the left.
Thus, 24-bit real addresses are extended
to 31 bits by appending zeros on the
left.
When prefixing is applied, the real
address is transformed into an absolute
address by using one of the following
rules, depending on bits 1-19 of the
real address:
1. Bits 1-19 of the address, if all
zeros, are replaced with bits 1-19
of the prefix.
2. Bits 1-19 of the address, if equal
to bits 1-19 of the prefix, are
replaced with zeros.
3. Bits 1-19 of the address,if not
all zeros and not equal to bits
1-19 of the prefix, remain
unchanged.
In all cases, bits20-31 of the address
remain unchanged.Only the address presented to storage is
translated by prefixing. The contents
of the source of the address remain
unchanged.
The distinction between real and abso
lute addresses is made even when the
prefix register contains allzeros, in
which case a real address andits corre
sponding absolute address areidentical. Chapter 3. Storage 3-11
controlled-protection violation
exists for that access.
Change recording is always active and
takes place for all store accesses to
storage, including those made by any
takes place for implicit references made
by the machine, such as those
part of interruptions.
Change recording does not take place for
the operands of the following
instructions since they directly modify
a storage key without modifying a stor
age location:
RESET REFERENCE BIT
RESET REFERENCE BIT EXTENDED
SET
to a specified value)
SET
bit is
value)
Change bits which have been changed from
zeros to ones are not necessarily
restored to zeros on
section
"Machine-Check Handling").
section "Exceptions to Nullification and
Suppression" in Chapter 5,
handling of the change bit in certain
unusual situations.
the range of real addresses
prefix area) to a different block in
absolute storage for each
permitting more than one
main storage to operate concurrently
with a minimum of interference, espe
cially in the processing of
interruptions.
part of the multiprocessing facility.
range
of 4K absolute addresses
the value in the prefix register for the
identified by the value in the prefix
register to correspond to absolute
addresses
addresses are the same as the corre
sponding absolute addresses. This
transformation allows each
all of main storage, including the first
4K bytes and the locations designated by
the prefix registers of other
lute addresses is graphically depicted
in the figure "Relationship between Real
and Absolute Addresses."
The prefix is a 19-bit quantity
contained in bit positions 1-19 of the
prefix register. Bits 1-7 of the prefix
register are always zeros. The register
has the following format:
The contents of the register can be set
and inspected by the privileged
instructions SET
corresponding to bit positions
ignored.
for these bit positions. When the
contents of the prefix register are
changed, the change is effective for the
next sequential instruction.
With the introduction of the storage
key-instruction-extension facility, the
test-block facility, and the extended
real-addressing facility, prefixing is
described in terms of 31-bit real
addresses, whether or not these facili
ties are installed. All real addresses
are considered to be 31 bits, with any
shorter address fields extended to 31
bits by appending zeros on the left.
Thus, 24-bit real addresses are extended
to 31 bits by appending zeros on the
left.
When prefixing is applied, the real
address is transformed into an absolute
address by using one of the following
rules, depending on bits 1-19 of the
real address:
1. Bits 1-19 of the address, if all
zeros, are replaced with bits 1-19
of the prefix.
2. Bits 1-19 of the address, if equal
to bits 1-19 of the prefix, are
replaced with zeros.
3. Bits 1-19 of the address,
all zeros and not equal to bits
1-19 of the prefix, remain
unchanged.
In all cases, bits
remain unchanged.
translated by prefixing. The contents
of the source of the address remain
unchanged.
The distinction between real and abso
lute addresses is made even when the
prefix register contains all
which case a real address and
sponding absolute address are