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README.IOARCH
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README.IOARCH
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HERCULES INPUT/OUTPUT ARCHITECTURE
Hercules is composed of two related but distinct architectures:
- the Central Processing Unit architecture and
- the Input/Output architecture.
The two architectures intersect and influence each other during input/output
operations and when managing the input/output system. This intersection
becomes visible to the programmer in four ways:
- available input/output related instructions,
- input/output related interruptions,
- allocation of control register uses for input/output,
- structures utilized by the input/output system.
Traditionally the input/output architecture has been treated as a subset of the
CPU architecture. System/370 uses channels. ESA/390 and z/Architecture use
subchannels. Each has its related CPU instructions. Additionally, the input/
output architecture is cast in concrete with the CPU architecture when the
Hercules system is built.
Motivation for I/O Architecture Enhancements
--------------------------------------------
Many users of Hercules are constrained by the System/370 architecture supported
by the readily available legacy operating systems. These constraints fall into
two categories: not enough memory and no modern CPU instructions. The most
straight forward albeit burdensome answer would be to migrate the legacy
operating system to one of the modern architectures. One of the major hurdles
seen by the Hercules community for this approach is the input/output
architecture change that occurred with System/370-XA when the subchannel-based
input/output architecture was introduced. The Hercules developers' answer to
this hurdle is to make the System/370 channel-based input/output architecture
available to ESA/390 and z/Architecture CPU architectures. This approach does
not eliminate the need for operating system adoption of a more modern CPU
architecture, but reduces the effort. Other approaches can be taken to solving
these operating system hurdles. Nevertheless, without a Hercules supported
approach, the operating system changes will never occur, lacking a platform on
which they could be used.
Architecture Feasibility
------------------------
In the System/370 era before the release of 370-XA, the two input/output
architectures were completely capable of co-existing on a single system. This
would in fact likely be necessary as the System/370 operating systems were being
modified to support 370-XA.
With the advent of Access Register addressing mode in ESA/390, the System/370
control register previouly used for channel masks, control register 2, was
repurposed, having no longer been used for years, for access register mode.
The only decision today for implementation of this "forward-porting" strategy
became how to accomodate in ESA/390 or z/Architecture channel masks.
Polling of the Hercules community revealed that channel masks were only disabled
when hardware problems made it necessary to do so. Under Hercules software
emulation such problems can not occur. The Hercules developers concluded that
channel masks could be safely eliminated when the CPU architecture requires
control register 2 for access register mode. This is the normal case with a
default build of Hercules that includes ESA/390 or z/Architecture. A
non-default build where access register mode is not enabled, for example a
build targeted for ESA/370 instead of ESA/390, control register 2 would still
be used for channel masks.
Channel Input/Output Architecture in ESA/390 or z/Architecture
--------------------------------------------------------------
The input/ouput architecture continues to be fixed to the CPU architecture
during the Hercules build process. Coexistence of both input/output
architectures is not possible at this time, although the subchannel-based
input/output architecture standard with ESA/390 or z/Architecture can now be
replaced with the channel-based input/output architecture expected in
System/370.
When either ESA/390 or z/Architecture or both are built with the channel-
based input/output architecture, the following changes occur:
- I/O instructions available: CLRIO, HIO, HDV, SIO, SIOF, STIDC, TCH, TIO
- If separate enabled, I/O instructions available: CONCS, DISCS
- Only Format 0 CCW's are enabled, restricting I/O data areas to addresses
0x0-0xFFFFFF.
- Real storage addresses 0x40-0x47 used for storing of the Channel Status
Word on an interruption.
- Real storage addresses 0x48-0x4B used for identifying the location of the
first Format 0 CCW of the I/O operation. This is the Channel Address Word.
- Real storage addresses 0xB8-0xB9 used for storing zeros on interruptions.
From the perspective of ESA/390 or z/Architecture this has the appearance
of a zero being stored instead of a 1 in bit 15 of the Subsystem-
Identification Word in these same real storage location.
- Real storage addresses 0xBA-0xBB used for storing of the I/O device address
on interruptions instead of the subchannel address.
- Program interrupt operation exceptions occur for instructions: CSCH, HSCH,
MSCH, RCHP, RSCH, SAL, SCHM, SSCH, STCPS, STRCW
- No machine-check interruptions occur for I/O devices related to CRW's.
- I/O related SIE interceptions that occur under System/370 will occur under
ESA/390 or z/Architecture.
Enabling Channel-based I/O in ESA/390 or z/Architecture
------------------------------------------------------
Identical changes are made to enable the channel-based I/O architecture and
disable the subchannel-based I/O architecture under ESA/390 or z/Architecture.
The only difference are the Hercules source files in which the changes are
made. The changes described below should be made in
- feat390.h for ESA/390 and/or
- feat900.h for z/Architecture.
In either or both of the above files, change from:
#define FEATURE_CANCEL_IO_FACILITY /* comment if FEATURE_S370_CHANNEL used */
#define FEATURE_CHANNEL_SUBSYSTEM /* comment if FEATURE_S370_CHANNEL used */
// #define FEATURE_CHANNEL_SWITCHING /* comment if FEATURE_CHANNEL_SUBSYSTEM used */
// #define FEATURE_S370_CHANNEL /* comment if FEATURE_CHANNEL_SUBSYSTEM used */
to:
// #define FEATURE_CANCEL_IO_FACILITY /* comment if FEATURE_S370_CHANNEL used */
// #define FEATURE_CHANNEL_SUBSYSTEM /* comment if FEATURE_S370_CHANNEL used */
#define FEATURE_CHANNEL_SWITCHING /* comment if FEATURE_CHANNEL_SUBSYSTEM used */
#define FEATURE_S370_CHANNEL /* comment if FEATURE_CHANNEL_SUBSYSTEM used */
The features are found in collating sequence in the source file and are not
adjacent as identified above.
Following these change(s), locally build the Hercules system. Errors will be
reported if the wrong combination of features are enabled during the build
process.
Harold Grovesteen