Appendix B. Functions That Differ from System/360
This appendix summarizes the areas where System/370 differs from System/360. Not included
are System/370 functions, such as block mUltiplex­
ing, which are suppressed on initialization, and new System/370 functions, such as new instructions,
which are specified in such a manner that they cause
program exceptions on System/360. Removal of USASCII-8 Mode System/360 provides for USASCII-8 by a mode
under control of PSW bit 12. USASCII-8 was a pro­
posed zoned-decimal code that has since been re­
jected. When bit 12 of the System/360 PSW is one,
the codes preferred for the USASCII-8 are generat­
ed for decimal results. When PSW bit 12 is zero, the
codes preferred for EBCDIC are generated.
In System/370, the USASCII-8 mode and the
associated meaning of PSW bit 12 are removed. In System/370, all instructions whose execution in System/360 depends on the setting of PSW bit 12
are executed generating the code preferred for
EBCDIC.
Bit 12 of the PSW is handled in System/370 as
follows: • In models that do not have the extended-control
(EC) mode installed, a one in PSW bit position
12 causes a program interruption for specifica­
tion exception. • In models that have the EC mode installed, a
one in PSW bit position 12 causes the CPU to
operate in the EC mode.
Handling Invalid Decimal Sign In System/360, an invalid decimal operand causes
the operation to be terminated. In System/370 the
operation is suppressed, instead of terminated, when
an invalid sign is detected. The action applies to all
instructions that check the validity of decimal ope­
rands: ADD DECIMAL, SUBTRACT DECIMAL,
ZERO AND ADD, COMPARE DECIMAL, MUL­ TIPL Y DECIMAL, DIVIDE DECIMAL, and
CONVERT TO BINARY. It includes also the System/370 instruction SHIFT AND ROUND DEC­
IMAL.
Recognizing Protection Exception in
Edit
In System/360, when a pattern character in an
EDIT or EDIT AND MARK operation is fetched
from a location protected against storing but remains
unchanged in the operation; it depends on the model
whether or not the protection exception is recognized.
In System/ 3 7 0, the protection exception in the above
case is recognized.
Operation Code for HALT DEVICE
In System/370, the first eight bits of the operation
code assigned to HALT DEVICE are the same as
those assigned to HALT I/O, the distinction be­
tween the two instructions being specified by bit
position 15. In System/360, bit position 15 is ig­
nored, and HALT I/O is performed in both cases.
Extent of Logout Area
In System/360, the logout area starts with location
128 and extends through as many locations as the
given model requires. Portions of this area are used
for machine-check logout, and other portions may
be used for channel logout. While no limit is set on
the size of the logout area, the extent of the area
used on most System/360 models is less than that
stored by a comparable System/370 model. On System/370, the machine-check interruption
causes information to be stored at locations 216-
239, 248-255, and 352-51l. Additionally, the mod­
el may store logout information in the fixed logout
area, locations 256-351, and the model may also
have a machine-check extended logout (MCEL),
which, on initialization, is specified to start at loca­
tion 512. Channels may place logout information in
the limited channel logout area, locations 176-179,
and in the fixed logout area, locations 256-35l.
Command Retry System/370 channels may provide command retry,
whereby the channel, in response to a signal from
the device, can retry the execution of a channel com­
mand. Since I/O devices announced prior to System/370 do not signal for command retry, no
problem of compatibility exists on these devices.
However, some new devices, which would otherwise
be compatible with former devices, do signal for
command retry.
The occurrence of command retry will usually
have no significant effect on the result produced by
a channel program; however, the following is a list
of some of the effects of command retry:
Appendix B. Functions That Differ from System/360 251

1. An immediate command specifying no chaining
may result in setting condition code ° rather
than c:ondition code 1.
2. Multiple PCI interruptions may be generated
for a single CCW with the PCI flag.
3. Since CCWs may be refetched, programs
which dynamically modify CCWs may be af­
fected.
4. The residual count in the CSW reflects only the
last execution of the command and does not
necessarily reflect the maximum storage used
in previous executions.
252 System/370 Principles of Operation
Logout on Channel Data Check
In System/360, logout is not permitted on channel
data check. System/370 permits logout to occur
when the channel causes an I/O interruption with
the channel-data-check indication.
Channel Pref etching
In System/360, on an output operation as many as
16 bytes may be prefetched and buffered; similarly,
with data chaining specified, the channel may pre­
fetch the new CCW when up to 16 bytes remain to
be transferred under control of the current CCW. In System/370, the restriction of 16 bytes is removed.