Virtual CTCA Terminology
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Virtual CTCA Terminology
The following terms are used extensively in the material
that describes the Channel-to-Channel Adapter (CTCA):
These terms are also used to document the code that
handles Virtual CTCA simulation.
X-SIDE and Y-SIDE terminology
The Channel-to-Channel adapter is designed to link
two systems, giving each system a channel interface
that behaves like an I/O device. The specifications
for this I/O interface describe the "device" connected
to the initiating an event as the "X side of the
adapter" (or X-SIDE) and the "device" connected to
the opposite system as the "Y side of the adapter" (or
the Y-SIDE). It is important to note that these
labels (X-SIDE verses Y-SIDE) are NOT static. The
perspective can change depending on which side of
the adapter initiated the subject operation. For
example:
- System A initiates a control command to an idle
adapter. The control command is accepted on the
X-SIDE (System A) and suspended.
- Next, System B initiates a sense-command-byte
command. The sense-command-byte command is
accepted on the X-SIDE (System B) and completes
the Y-SIDE (System A) control command.
X-SIDE of the Adapter
In general, you can think of the X-SIDE of the adapter as
"this" virtual device, and the Y-SIDE as the "other" device.
The two sides of the adapter are symmetric, so the terms are
interchangeable. They are used in the hardware specifications
and the CP commentary to express the side in the context of a
specific operation.
In CP code, the X-SIDE of the adapter is referenced using
the CACXSIDE fields of the CACBK
control block.
See X-SIDE and Y-SIDE for more information.
Y-SIDE of the Adapter
In general, you can think of the Y-SIDE of the adapter as
the "other" virtual device, and the X-SIDE as "this" device.
The two sides of the adapter are symmetric, so the terms are
interchangeable. They are used in the hardware specifications
and the CP commentary to express the side in the context of a
specific operation.
In CP code, the Y-SIDE of the adapter is referenced using
the CACYSIDE fields of the CACBK
control block.
See X-SIDE and Y-SIDE for more information.
Basic Mode and Extended Mode terminology
The terms
"Basic Mode" and
"Extended Mode"
refer to the protocol that is in effect for
the adapter.
The Virtual CTCA (like the 3088 hardware) is in Basic Mode
while both sides of the adapter are set to Basic Mode.
If either side of the adapter is set to Extended Mode,
the active protocol for both sides becomes Extended Mode.
Basic Mode
"Basic Mode" uses a limited set of commands to
provide basic data transfer. Significant features
of "Basic Mode" are:
- Write End of File (WEOF) CCW is treated as a
traditional Write CCW.
- Unit Check device status is not used.
When a data transfer fails, the residual length
must be used to determine the number of bytes that
were actually transferred.
- Sense Adapter State (X'04') CCW is treated as a
Sense Command Byte function.
- READY and NOTREADY states are not defined.
When the other side of the adapter is inactive,
any "dependent" command initiated on this side
will be suspended until the partner responds.
The original CTCA hardware only operated in "Basic Mode."
The 3088 hardware operates in "Basic Mode" or
"Extended Mode" under control
of the application programs.
Extended Mode
"Extended Mode" uses additional commands and status information
to provide data transfer. Significant features of "Extended Mode"
are:
- Write End of File (WEOF) CCW is supported.
This allows an application to signal the partner
(who might be executing a long chain of Read commands)
that there is no more data to transmit.
This is more efficient than writing a series of
empty buffers to match the partner's Read chain.
- Unit Check device status is used to reflect a device
error (e.g. NOTREADY).
- Sense Adapter State (X'04') CCW is supported to
retrieve sense data pertaining to the current
failure (e.g. Intervention Required).
- READY and NOTREADY states are defined.
Standard Mode
"Standard Mode" is the default operational mode for
the ESCON device. When the ESCON control device is
an "SCTC" the adapter operates in Standard Mode.
The ESCON Standard Mode is fundamentally the same
as the 3088 Extended Mode with the following exceptions:
- The ESCON SCTC device can never enter Basic Mode.
- The ESCON CNC device can only be switched to Basic Mode
by being COUPLEd to an ESCON BCTC device.
- The 3088 "Set Basic Mode" CCW (x'C3') is treated as
a NOP (the adapter remains in Standard Mode).
- Reset operations (Halt, Clear, or System Reset)
leave the adapter in Standard Mode.
Adapter States
System/390 Channel Architecture defines the state of a
given device as Available, Interruption Pending, or Working.
The behavior of the CTCA working on a Dependent command is
significantly different than the traditional Working state,
so both are included in the list of Adapter States:
A (Available)
W(D) (Working on a Dependent command)
W(C) (Working on an Independent command, or command chaining)
I (Interruption Pending)
Since the operation of either side may also be affected
by the state of the other side, the adapter function is
generally described based on the following CTCA cases:
CASE 1: x = A, y = W(D)
CASE 2: x = A, y = A, I, or W(C)
CASE 3: x = W, y = any state
CASE 4: x = I, y = any state
CACBK Extension for VCTC
One CACBK is created for a VCTC device when it is defined. The
CACBK definition maps two symmetrical structures (CACXSIDE and
CACYSIDE) which MUST be interchangeable. Since the VCTC is NOT
initially connected (or coupled) at that point, the CACBK is
defined with enough space to map both parts. The CACYSIDE part
of the CACBK storage is only used when the the adapter is uncoupled.
The rest of the time, the CACYSIDE fields are used to map the
CACXSIDE portion of THIS CACBK or the OTHER CACBK.
Serialization for a specific adapter is maintained using:
Refer to VCTC Serialization
for more information about these locks.
Each side of the CACBK contains status flags and buffers to
represent the equivalent hardware latches for one side of the
Channel-to-Channel Adapter.
UNCOUPLED Adapter Linkage
An uncoupled adapter is represented by a single virtual
device. It behaves as if the "other" side of the adapter
is NOT connected to any system.
Owner VDEV:
VDEVCTCA = A(CACBK(X)) for the primary CACBK
VDEVINTV = 1 (Intervention Required)
CACBK(X) at CACXSIDE using CACXSIDE:
CACADISC = 0 (This side is NOT disconnected)
CACXVDEV = A(VDEV) for owner
CACXLINK = A(CACYSIDE) (Using the Y-SIDE definition)
CACXLOCK = A(CACXLKWD) (Using the internal Adapter Lock)
CACXBUFF = 0
CACBK(X) at CACYSIDE using CACYSIDE:
CACADISC = 1 (This side IS disconnected)
CACYVDEV = 0 (No device)
CACYLINK = A(CACXSIDE) for the primary CACBK
CACYLOCK = A(CACXLKWD) (Using the internal Adapter Lock)
CACYBUFF = 0
COUPLED Adapter Linkage
A coupled adapter is represented by two virtual
devices. It behaves as if both sides of the adapter
are connected to live systems (represented by the
VM users).
X-SIDE VDEV:
VDEVCTCA = A(CACBK(X))
VDEVINTV = 0 (coupled)
CACBK(X) at CACXSIDE using CACXSIDE:
CACADISC = 0 (This side is NOT disconnected)
CACXVDEV = A(VDEV) for X-SIDE device
CACXLINK = A(CACBK(Y)) for the partner CACBK
CACXLOCK = A(LKWRD) (shared Adapter Lock)
CACXBUFF = A(CABFR) (shared data exchange buffer)
Y-SIDE VDEV:
VDEVCTCA = A(CACBK(Y))
VDEVINTV = 0 (coupled)
CACBK(Y) at CACXSIDE using CACYSIDE:
CACADISC = 0 (This side is NOT disconnected)
CACYVDEV = A(VDEV) for Y-SIDE device
CACYLINK = A(CACXSIDE) for the partner CACBK
CACYLOCK = A(LKWRD) (shared Adapter Lock)
CACYBUFF = A(CABFR) (shared data exchange buffer)
VCTC Serialization
The VCTC Global Lock
serializes configuration changes
(DEFINE, COUPLE, and DETACH) for VCTC devices. It
must be held in exclusive mode whenever an operation
might affect multiple VCTC devices, or whenever an
operation might conflict with normal I/O simulation.
The Adapter Lock
is defined for each adapter (which
could be represented by one or two CACBK structures).
The Adapter Lock must be held in exclusive mode to
reference either CACBK. Normal I/O simulation only
requires the Adapter Lock. The Adapter Lock is not
sufficient for configuration changes because an
operation like COUPLE could involve as many as three
different devices belonging to three different users.
CP also maintains a VDEV Lock for each virtual device,
but the VDEV Lock management is generally left to the
Virtual I/O subsystem.
If the VDEV lock is required for an operation, it
MUST NOT be requested while holding the VCTC Global
Lock or the Adapter Lock.
VCTC Global Lock
The VCTC Global Lock is a lock word as defined by HCPLKWRD.
It resides in HCPIOLCT, and is never destroyed. Refer to
VCTC Serialization for related information.
The VCTC Global Lock is required for:
- DEFINE operations (to avoid conflict with DETACH)
- COUPLE operations (to avoid conflict with DETACH)
- DETACH operations (to avoid conflict with DEFINE or COUPLE).
All other operations can be initiated and performed
without obtaining the VCTC Global Lock.
VCTC Adapter Lock
The Adapter Lock is a lock word as defined by HCPLKWRD.
When the CACBK is uncoupled,
the Adapter Lock resides within the CACBK
(a section is reserved for the LKWRD).
When the CACBK is coupled, the internal LKWRD is destroyed
(by calling HCPLCKDX) and a shared LKWRD is acquired from
free storage. Both sides of the adapter (both CACBKs)
point to this common Adapter Lock while they are coupled.
When a coupled adapter becomes uncoupled (e.g. one
side is detached), then the shared LKWRD is destroyed
and each CACBK is reset to use its own internal LKWRD
as the Adapter Lock.
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