APPENDIX CHANGES AFFECTING COMPATIBILITY BETWEEN SYSTEM/360 AHD SYSTEM/370 Removal of USASCII-8 Mode ••••••••••••••••••••••••••••••• H-1 Operation Codes of 110 Instructions ••••••••••••••••••••• H-1
Halt I/O ..••••.•••••••••••..•••••••••••••••••••••••••. H-1
start 1/0 ..•.•.••.•.••••••.•••.•••••.••••••••••••••.•• H-1
Test Channel ..•.•.•••••••••.••••••••.•••••••••••••.••• H-2
Logout ..•.•...••....•.•••••••••••••••••••••..••••.••••.• H-2 Command Retry ..••...•••••.••••••••••••.•••••••••..•••..• H-2 Channel Prefetching .•••..•••••.•••••••.•••••••.••••••••. H-2
Validity of Data ....••..•••..••••••••••••••••••••.•••••• H-2
This appendix summarizes those changes
included in the System/370 architecture
that may affect whether or not a program
written according to the System/360 architecture will operate on models
implementing the System/370 architecture
described in this publication. Not
included are descriptions of System/370 functions which are compatible exten­
sions, that is, (1) those that are
suppressed on initialization, such as
block multiplexing, and (2) those that
are specified in such a manner that they
cause program exceptions on System/360, such as new instructions. REMOVAL OF USASCII-8 MODE System/360 provides for USASCII-8 by a
mode under control of PSW bit 12. USAS­ CII-8 was a proposed zoned-decimal code
that has since been rejected. When bit
12 of the System/360 PSW is one, the
preferred codes for USASCII-8 are gener­
ated for decimal results. When PSW bit
12 is zero, the preferred codes 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 preferred codes for EBCDIC. Bit 12 of the PSW is System/370 as follows:
handled in • • In models that do not have the
translation facility installed, a
one in PSW bit position 12 causes a
program interruption for specifica­
tion exception.
In models that have the translation
facility installed, a one in PSW bit position 12 causes the CPU to
operate in the extended-control (EC) mode. OPERATION CODES OF I/O INSTRUCTIONS In System/360, the operation codes of
the four I/O instructions (HALT 1/0, START I/O, TEST CHANNEL, and TEST 1/0) are one byte in length, and bits 8-15 of
the I/O instructions are ignored. In System/370, the operation codes of all I/O instructions are the first two bytes
of the instruction. System/360 programs
that execute I/O instructions in which
any of bits 8-15 is not zero may perform
a different function when executed on a System/370 CPU, as explained below.
In System/370, HALT I/O (HIO) is
assigned the operation code 9EOO hex and
HALT DEVICE (HDV) the operation code 9E01. Because bits 8-14 are ignored in
both instructions, an instruction
executed as HALT I/O in System/360 will
still be executed as HALT I/O 1n System/370 if the third hex digit is any
value and the fourth hex digit is an
even value. However, in System/370, if
bit 15 of the instruction is one, the
function performed will be the HIO func­
tion or the HDV function, depending on
the design of the channel.
In System/370, START 110 is assigned the
operation code 9COO and RESUME 1/0 is
assigned the operation code 9C02. Therefore, an. instruction executed as START 110 in System/360 will be executed
as RESUME 110 in System/370 if bits 8-15
of the instruction contain the value 02 hex and the suspend-and-resume facility
is installed. When the suspend-and­
resume facility is installed, operation Changes Affecting Compatibility between System/360 and System/370 H-l

codes in the range 9C03 through 9CFF cause an operation exception to be
recognized. If the suspend-and-resume
facility is not installed, bits 8-14 of
the instruction are always ignored, and
bit 15 is ignored when the block­
multiplexing-control bit (bit 0 of
control register 0) is zero at the time
the instruction is executed.
Test Channel In System/370, TEST CHANNEL is assigned
the operation code 9FOO and CLEAR CHAN­ NEL (CLRCH) the operation code 9F01 hex. Because bits 8-14 of the instruction are
ignored in both instructions, an
instruction executed as TEST CHANNEL in System/360 will still be executed as
TEST CHANNEL in System/370 if the third hex digit is any value and the fourth hex digit is an even value. However, in System/370, if bTt15 of the instruction
is one, the CLRCH function is performed
if the recovery-extension facility is
installed; otherwise, the TCH function
is performed. LOGOUT 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 inter­
ruption causes information to be stored
at locations 216-239, 244-255, and
352-511. Additionally, the model may
store logout information in the fixed­
logout area, locations 256-351, and the
model may also have a machine-check
extended-logout (MCEL) area, which, on
initialization, is specified to start at
location 512. Channels may place logout
information in the limited channel
logout area, locations 176-179, and in
the fixed-logout area, locations
256-351.
In System/360, logout is not permitted
on data check. System/370 permits
logout to occur when the channel causes
an I/O interruption with the data-check
indication.
H-2 System/370 Principles of Operation 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 command.
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 effects of command retry usually are
not significant; however, the following
is a list of some of the differences
which command retry can cause:
1. An immediate command specifying no
chaining may result in setting
condition code 0 rather than condi­
tion 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 affected.
4. The residual count in the CSW reflects only the Ivst execution of the command and does not necessar­
ily reflect the maximum storage
used in previous executions. CHANNEL PREFETCHING 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
prefetch 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. VALIDITY OF DATA
In System/360, the contents of main
storage are preserved when power is
turned off. In System/370, because main
storage may be volatile or nonvolatile,
the program must not depend on the
validity of data in main storage after
system power has been lost or turned off
and then restored.