IODMACommand

Inherits from
IOCommand
Availability
Available in OS X v10.4 and later.
Declared in
IODMACommand.h

Overview

A mechanism to convert memory references to I/O bus addresses.

The IODMACommand is supersedes the IOMemoryCursor and greatly enhances the functionality and power of it. The command can be specified to output 64 bit physical addresses and also allows driver writers bypass mapping hardware or get addresses suitable for non-snooped DMA.

The command is designed to be very easily subclassable. Most driver writers need to associate some DMA operations with their memory descriptor and usually use a C structure for that purpose. This structure is often kept in a linked list. This IODMACommand has built it <kern/queue.h> linkage and can be derived and 'public:' variables added, giving the developer a structure that can associate a memory descriptor with a particular dma command but will also allow the developer to generate that command and keep the state necessary for tracking it.

It is envisaged that a pool of IODMACommands will be created at driver initialisation and each command will be kept in an IOCommandPool while not in use. However if developers wishes to maintain their own free lists that is certainly possible. See the <kern/queue.h> and <xnu/iokit/Kernel/IOCommandPool> for sample code on manipulating the command's doubly linked list entries.

The IODMACommand can be used in a 'weak-linked' manner. To do this you must avoid using any static member functions. Use the, much slower but safe, weakWithSpecification function. On success a dma command instance will be returned. This instance can then be used to clone as many commands as is needed. Remember deriving from this class can not be done weakly, that is no weak subclassing!

Tasks

Miscellaneous

Instance Methods

clearMemoryDescriptor

Clears the DMACommand's current memory descriptor

virtual IOReturn clearMemoryDescriptor( bool autoComplete = true);
Parameters
autoComplete

An optional boolean variable that will call the complete() function automatically before the memory descriptor is processed. Defaults to true.

Discussion

completes and invalidates the cache if the DMA command is currently active, copies all data from bounce buffers if necessary and releases all resources acquired during setMemoryDescriptor.

cloneCommand

Creates a new command based on the specification of the current one.

virtual IODMACommand *cloneCommand( void *refCon = 0);
Return Value

Returns a new memory cursor if successfully created and initialised, 0 otherwise.

Discussion

Factory function to create and initialise an IODMACommand in one operation. The current command's specification will be duplicated in the new object, but however none of its state will be duplicated. This means that it is safe to clone a command even if it is currently active and running, however you must be certain that the command to be duplicated does have a valid reference for the duration.

complete

Complete processing of DMA mappings after an I/O transfer is finished.

virtual IOReturn complete( bool invalidateCache = true, bool synchronize = true);
Parameters
invalidCache

Invalidate the caches for the memory descriptor. Defaults to true for kNonCoherent and is ignored by the other types.

synchronize

Copy any buffered data back to the target IOMemoryDescriptor. Defaults to true, if synchronize() is being used to explicitly copy data, passing false may avoid an unneeded copy.

Return Value

kIOReturnNotReady if not prepared, kIOReturnSuccess otherwise.

Discussion

This method should not be called unless a prepare was previously issued; the prepare() and complete() must occur in pairs, before and after an I/O transfer

gen32IOVMSegments

Helper function for a type checked call to genIOVMSegments(qv), for use with an IODMACommand set up with the output function kIODMACommandOutputHost32, kIODMACommandOutputBig32, or kIODMACommandOutputLittle32. If the output function of the IODMACommand is not a 32 bit function, results will be incorrect.

inline IOReturn gen32IOVMSegments( UInt64 *offset, Segment32 *segments, UInt32 *numSegments) ;

gen64IOVMSegments

Helper function for a type checked call to genIOVMSegments(qv), for use with an IODMACommand set up with the output function kIODMACommandOutputHost64, kIODMACommandOutputBig64, or kIODMACommandOutputLittle64. If the output function of the IODMACommand is not a 64 bit function, results will be incorrect.

inline IOReturn gen64IOVMSegments( UInt64 *offset, Segment64 *segments, UInt32 *numSegments) ;

genIOVMSegments

Generates a physical scatter/gather for the current DMA command

virtual IOReturn genIOVMSegments( UInt64 *offset, void *segments, UInt32 *numSegments);
Parameters
offset

input/output parameter, defines the starting and ending offset in the memory descriptor, relative to any offset passed to the prepare() method.

segments

Void pointer to base of output physical scatter/gather list. Always passed directly onto the SegmentFunction.

numSegments

Input/output parameter Number of segments that can fit in the segment array and returns number of segments generated.

Return Value

kIOReturnSuccess on success, kIOReturnOverrun if the memory descriptor is exhausted, kIOReturnMessageTooLarge if the output segment function's address bits has insufficient resolution for a segment, kIOReturnNotReady if the DMA command has not be prepared, kIOReturnBadArgument if the DMA command doesn't have a memory descriptor yet or some of the parameters are NULL and kIOReturnNotReady if the DMA command is not prepared.

Discussion

Generates a list of physical segments from the given memory descriptor, relative to the current position of the descriptor. The constraints that are set during initialisation will be respected. This function maintains the state across multiple calls for efficiency. However the state is discarded if the new offset is not the expected one.

getMemoryDescriptor

Get the current memory descriptor

virtual const IOMemoryDescriptor *getMemoryDescriptor() const;

getPreparedOffsetAndLength

Returns the offset and length into the target IOMemoryDescriptor of a prepared IODDMACommand.

virtual IOReturn getPreparedOffsetAndLength( UInt64 *offset, UInt64 *length);
Parameters
offset

returns the starting offset in the memory descriptor the DMA command was prepared with. Pass NULL for don't care.

length

returns the length in the memory descriptor the DMA command was prepared with. Pass NULL for don't care.

Return Value

An IOReturn code. kIOReturnNotReady if the IODMACommand is not prepared.

Discussion

If successfully prepared, returns the offset and length into the IOMemoryDescriptor. Will fail for an unprepared IODMACommand.

initWithSpecification

Primary initializer for the IODMACommand class.

virtual bool initWithSpecification( SegmentFunction outSegFunc, UInt8 numAddressBits, UInt64 maxSegmentSize, MappingOptions mappingOptions = kMapped, UInt64 maxTransferSize = 0, UInt32 alignment = 1, IOMapper *mapper = 0, void *refCon = 0);
Parameters
outSegFunc

SegmentFunction to call to output one physical segment. A set of nine commonly required segment functions are provided.

numAddressBits

Number of bits that the hardware uses on its internal address bus. Typically 32 but may be more on modern hardware. A 0 implies no-restriction other than that implied by the output segment function.

maxSegmentSize

Maximum allowable size for one segment. Defaults to 0 which means any size.

mappingOptions

is the type of mapping that is required to translate an IOMemoryDescriptor into the desired number of bits. For instance if your hardware only supports 32 bits but must run on machines with > 4G of RAM some mapping will be required. Number of bits will be specified in numAddressBits, see below.This parameter can take 3 values:- kNonCoherent - used for non-coherent hardware transfers, Mapped - Validate that all I/O bus generated addresses are within the number of addressing bits specified, Bypassed indicates that bypassed addressing is required, this is used when the hardware transferes are into coherent memory but no mapping is required. See also prepare() for failure cases.

maxTransferSize

Maximum size of an entire transfer. Defaults to 0 indicating no maximum.

alignment

Alignment restriction, in bytes, on I/O bus addresses. Defaults to single byte alignment.

mapper

For mapping types kMapped & kBypassed mapper is used to define the hardware that will perform the mapping, defaults to the system mapper.

Return Value

Can fail if the mapping type is not recognised, if one of the 3 mandatory parameters are set to 0, if a 32 bit output function is selected when more than 32 bits of address is required or, if kBypassed is requested on a machine that doesn't support bypassing. Returns true otherwise.

OutputBig32

Output big-endian Segment32 output segment function.

static bool OutputBig32( IODMACommand *target, Segment64 seg, void *segs, UInt32 ind);

OutputBig64

Output big-endian Segment64 output segment function.

static bool OutputBig64( IODMACommand *target, Segment64 seg, void *segs, UInt32 ind);

OutputHost32

Output host natural Segment32 output segment function.

static bool OutputHost32( IODMACommand *target, Segment64 seg, void *segs, UInt32 ind);

OutputHost64

Output host natural Segment64 output segment function.

static bool OutputHost64( IODMACommand *target, Segment64 seg, void *segs, UInt32 ind);

OutputLittle32

Output little-endian Segment32 output segment function.

static bool OutputLittle32( IODMACommand *target, Segment64 seg, void *segs, UInt32 ind);

OutputLittle64

Output little-endian Segment64 output segment function.

static bool OutputLittle64( IODMACommand *target, Segment64 seg, void *segs, UInt32 ind);

prepare

Prepare the memory for an I/O transfer.

virtual IOReturn prepare( UInt64 offset = 0, UInt64 length = 0, bool flushCache = true, bool synchronize = true);
Parameters
offset

defines the starting offset in the memory descriptor the DMA command will operate on. genIOVMSegments will produce its results based on the offset and length passed to the prepare method.

length

defines the ending position in the memory descriptor the DMA command will operate on. genIOVMSegments will produce its results based on the offset and length passed to the prepare method.

flushCache

Flush the caches for the memory descriptor and make certain that the memory cycles are complete. Defaults to true for kNonCoherent and is ignored by the other types.

synchronize

Copy any buffered data back from the target IOMemoryDescriptor. Defaults to true, if synchronize() is being used to explicitly copy data, passing false may avoid an unneeded copy.

Return Value

An IOReturn code.

Discussion

Allocate the mapping resources neccessary for this transfer, specifying a sub range of the IOMemoryDescriptor that will be the target of the I/O. The complete() method frees these resources. Data may be copied to buffers for kIODirectionOut memory descriptors, depending on hardware mapping resource availabilty or alignment restrictions. It should be noted that the this function may block and should only be called on the clients context, i.e never call this routine while gated; also the call itself is not thread safe though this should be an issue as each IODMACommand is independant.

prepareWithSpecification

Prepare the memory for an I/O transfer with a new specification.

virtual IOReturn prepareWithSpecification( SegmentFunction outSegFunc, UInt8 numAddressBits, UInt64 maxSegmentSize, MappingOptions mappingOptions = kMapped, UInt64 maxTransferSize = 0, UInt32 alignment = 1, IOMapper *mapper = 0, UInt64 offset = 0, UInt64 length = 0, bool flushCache = true, bool synchronize = true);
Parameters
outSegFunc

SegmentFunction to call to output one physical segment. A set of nine commonly required segment functions are provided.

numAddressBits

Number of bits that the hardware uses on its internal address bus. Typically 32 but may be more on modern hardware. A 0 implies no-restriction other than that implied by the output segment function.

maxSegmentSize

Maximum allowable size for one segment. Defaults to 0 which means any size.

mappingOptions

is the type of mapping that is required to translate an IOMemoryDescriptor into the desired number of bits. For instance if your hardware only supports 32 bits but must run on machines with > 4G of RAM some mapping will be required. Number of bits will be specified in numAddressBits, see below.This parameter can take 3 values:- kNonCoherent - used for non-coherent hardware transfers, Mapped - Validate that all I/O bus generated addresses are within the number of addressing bits specified, Bypassed indicates that bypassed addressing is required, this is used when the hardware transferes are into coherent memory but no mapping is required. See also prepare() for failure cases.

maxTransferSize

Maximum size of an entire transfer. Defaults to 0 indicating no maximum.

alignment

Alignment restriction, in bytes, on I/O bus addresses. Defaults to single byte alignment.

mapper

For mapping types kMapped & kBypassed mapper is used to define the hardware that will perform the mapping, defaults to the system mapper.

offset

defines the starting offset in the memory descriptor the DMA command will operate on. genIOVMSegments will produce its results based on the offset and length passed to the prepare method.

length

defines the ending position in the memory descriptor the DMA command will operate on. genIOVMSegments will produce its results based on the offset and length passed to the prepare method.

flushCache

Flush the caches for the memory descriptor and make certain that the memory cycles are complete. Defaults to true for kNonCoherent and is ignored by the other types.

synchronize

Copy any buffered data back from the target IOMemoryDescriptor. Defaults to true, if synchronize() is being used to explicitly copy data, passing false may avoid an unneeded copy.

Return Value

An IOReturn code. Can fail if the mapping type is not recognised, if one of the 3 mandatory parameters are set to 0, if a 32 bit output function is selected when more than 32 bits of address is required or, if kBypassed is requested on a machine that doesn't support bypassing.

Discussion

Allocate the mapping resources neccessary for this transfer, specifying a sub range of the IOMemoryDescriptor that will be the target of the I/O. The complete() method frees these resources. Data may be copied to buffers for kIODirectionOut memory descriptors, depending on hardware mapping resource availabilty or alignment restrictions. It should be noted that the this function may block and should only be called on the clients context, i.e never call this routine while gated; also the call itself is not thread safe though this should be an issue as each IODMACommand is independant.

readBytes

Copy data from the IODMACommand's buffer to the specified buffer.

UInt64 readBytes( UInt64 offset, void *bytes, UInt64 length);
Parameters
offset

A byte offset into the IODMACommand's memory, relative to the prepared offset.

bytes

The caller supplied buffer to copy the data to.

length

The length of the data to copy.

Return Value

The number of bytes copied, zero will be returned if the specified offset is beyond the prepared length of the IODMACommand.

Discussion

This method copies data from the IODMACommand's memory at the given offset, to the caller's buffer. The IODMACommand must be prepared, and the offset is relative to the prepared offset.

setMemoryDescriptor

Sets and resets the DMACommand's current memory descriptor

virtual IOReturn setMemoryDescriptor( const IOMemoryDescriptor *mem, bool autoPrepare = true);
Parameters
mem

A pointer to the current I/Os memory descriptor.

autoPrepare

An optional boolean variable that will call the prepare() function automatically after the memory descriptor is processed. Defaults to true.

Return Value

Returns kIOReturnSuccess, kIOReturnBusy if currently prepared, kIOReturnNoSpace if the length(mem) >= Maximum Transfer Size or the error codes returned by prepare() (qv).

Discussion

The DMA command will configure itself based on the information that it finds in the memory descriptor. It looks for things like the direction of the memory descriptor and whether the current memory descriptor is already mapped into some IOMMU. As a programmer convenience it can also prepare the DMA command immediately. See prepare(). Note the IODMACommand is designed to used multiple times with a succession of memory descriptors, making the pooling of commands possible. It is an error though to attempt to reset a currently prepared() DMA command. Warning: This routine may block so never try to autoprepare an IODMACommand while in a gated context, i.e. one of the WorkLoops action call outs.

synchronize

Bring IOMemoryDescriptor and IODMACommand buffers into sync.

virtual IOReturn synchronize( IOOptionBits options);
Parameters
options

Specifies the direction of the copy: kIODirectionOut copy IOMemoryDesciptor memory to any IODMACommand buffers. By default this action takes place automatically at prepare(). kIODirectionIn copy any IODMACommand buffers back to the IOMemoryDescriptor. By default this action takes place automatically at complete(). kForceDoubleBuffer copy the entire prepared range to a new page aligned buffer.

Return Value

kIOReturnNotReady if not prepared, kIOReturnBadArgument if invalid options are passed, kIOReturnSuccess otherwise.

Discussion

This method should not be called unless a prepare was previously issued. If needed a caller may synchronize any IODMACommand buffers with the original IOMemoryDescriptor buffers.

weakWithSpecification

Creates and initialises an IODMACommand in one operation if this version of the operating system supports it.

static inline IOReturn weakWithSpecification ( IODMACommand **newCommand, SegmentFunction outSegFunc, UInt8 numAddressBits, UInt64 maxSegmentSize, MappingOptions mapType = kMapped, UInt64 maxTransferSize = 0, UInt32 alignment = 1, IOMapper *mapper = 0, void *refCon = 0) __attribute__((always_inline));
Parameters
newCommand

Output reference variable of the newly created IODMACommand.

outSegFunc

SegmentFunction to call to output one physical segment. A set of nine commonly required segment functions are provided.

numAddressBits

Number of bits that the hardware uses on its internal address bus. Typically 32 but may be more on modern hardware. A 0 implies no-restriction other than that implied by the output segment function.

maxSegmentSize

Maximum allowable size for one segment. Zero is treated as an unlimited segment size.

mapType

is the type of mapping that is required to translate an IOMemoryDescriptor into the desired number of bits. For instance if your hardware only supports 32 bits but must run on machines with > 4G of RAM some mapping will be required. Number of bits will be specified in numAddressBits, see below. This parameter can take 3 values:- kNonCoherent - used for non-coherent hardware transfers, Mapped - Validate that all I/O bus generated addresses are within the number of addressing bits specified, Bypassed indicates that bypassed addressing is required, this is used when the hardware transfers are into coherent memory but no mapping is required. See also prepare() for failure cases.

maxTransferSize

Maximum size of an entire transfer. Defaults to 0 indicating no maximum.

alignment

Alignment restriction, in bytes, on I/O bus addresses. Defaults to single byte alignment.

mapper

For mapping types kMapped & kBypassed mapper is used to define the hardware that will perform the mapping, defaults to the system mapper.

Return Value

kIOReturnSuccess if everything is OK, otherwise kIOReturnBadArgument if newCommand is NULL, kIOReturnUnsupported if the kernel doesn't export IODMACommand or IOReturnError if the new command fails to init, q.v. initWithSpecification.

Discussion

Factory function to create and initialise an IODMACommand in one operation. The function allows a developer to 'weak' link with IODMACommand. This function will return kIOReturnUnsupported if the IODMACommand is unavailable. This function is actually fairly slow so it will be better to call it once then clone the successfully create command using cloneCommand (q.v.).

withSpecification

Creates and initializes an IODMACommand in one operation.

static IODMACommand * withSpecification( SegmentFunction outSegFunc, UInt8 numAddressBits, UInt64 maxSegmentSize, MappingOptions mappingOptions = kMapped, UInt64 maxTransferSize = 0, UInt32 alignment = 1, IOMapper *mapper = 0, void *refCon = 0);
Parameters
outSegFunc

SegmentFunction to call to output one physical segment. A set of nine commonly required segment functions are provided.

numAddressBits

Number of bits that the hardware uses on its internal address bus. Typically 32 but may be more on modern hardware. A 0 implies no-restriction other than that implied by the output segment function.

maxSegmentSize

Maximum allowable size for one segment. If 0 is passed the maximum segment size is unlimited.

mappingOptions

is the type of mapping that is required to translate an IOMemoryDescriptor into the desired number of bits. For instance if your hardware only supports 32 bits but must run on machines with > 4G of RAM some mapping will be required. Number of bits will be specified in numAddressBits, see below.This parameter can take 3 values:- kNonCoherent - used for non-coherent hardware transfers, Mapped - Validate that all I/O bus generated addresses are within the number of addressing bits specified, Bypassed indicates that bypassed addressing is required, this is used when the hardware transferes are into coherent memory but no mapping is required. See also prepare() for failure cases.

maxTransferSize

Maximum size of an entire transfer. Defaults to 0 indicating no maximum.

alignment

Alignment restriction, in bytes, on I/O bus addresses. Defaults to single byte alignment.

mapper

For mapping types kMapped & kBypassed mapper is used to define the hardware that will perform the mapping, defaults to the system mapper.

Return Value

Returns a new memory cursor if successfully created and initialized, 0 otherwise.

Discussion

Factory function to create and initialize an IODMACommand in one operation.

writeBytes

Copy data to the IODMACommand's buffer from the specified buffer.

UInt64 writeBytes( UInt64 offset, const void *bytes, UInt64 length);
Parameters
offset

A byte offset into the IODMACommand's memory, relative to the prepared offset.

bytes

The caller supplied buffer to copy the data from.

length

The length of the data to copy.

Return Value

The number of bytes copied, zero will be returned if the specified offset is beyond the prepared length of the IODMACommand.

Discussion

This method copies data to the IODMACommand's memory at the given offset, from the caller's buffer. The IODMACommand must be prepared, and the offset is relative to the prepared offset.

Callbacks

SegmentFunction

typedef bool ( *SegmentFunction)(
   IODMACommand *target,
   Segment64 segment,
   void *segments,
   UInt32 segmentIndex);

Parameters
segment

The 64Bit I/O bus address and length.

segments

Base of the output vector of DMA address length pairs.

segmentIndex

Index to output 'segment' in the 'segments' array.

Return Value

Returns true if segment encoding succeeded. false may be returned if the current segment does not fit in an output segment, i.e. a 38bit address wont fit into a 32 encoding.

Discussion

Pointer to a C function that translates a 64 segment and outputs a single desired segment to the array at the requested index. There are a group of pre-implemented SegmentFunctions that may be usefull to the developer below.

Availability
Declared In
IODMACommand.h

Constants

fMemory
const IOMemoryDescriptor *fMemory;

memory descriptor for current I/O.

Miscellaneous Defines

   
#define kIODMACommandOutputBig32 (IODMACommand::OutputBig32)
#define kIODMACommandOutputBig64 (IODMACommand::OutputBig64)
#define kIODMACommandOutputLittle64 (IODMACommand::OutputLittle64)
#define kIODMACommandOutputHost32 (IODMACommand::OutputHost32)
#define kIODMACommandOutputHost64 (IODMACommand::OutputHost64)
#define kIODMACommandOutputLittle32 (IODMACommand::OutputLittle32)
#define kIODMACommandOutputBig64 (IODMACommand::OutputBig64)
#define kIODMACommandOutputLittle64 (IODMACommand::OutputLittle64)
Constants
kIODMACommandOutputBig32

Output big-endian Segment32 output segment function.

Available in OS X v10.4 and later.

Declared in IODMACommand.h.

kIODMACommandOutputBig64

Output little-endian Segment64 output segment function.

Available in OS X v10.4 and later.

Declared in IODMACommand.h.

kIODMACommandOutputBig64

Output big-endian Segment64 output segment function.

Available in OS X v10.4 and later.

Declared in IODMACommand.h.

kIODMACommandOutputHost32

Output host natural Segment32 output segment function.

Available in OS X v10.4 and later.

Declared in IODMACommand.h.

kIODMACommandOutputHost64

Output host natural Segment64 output segment function.

Available in OS X v10.4 and later.

Declared in IODMACommand.h.

kIODMACommandOutputLittle32

Output little-endian Segment32 output segment function.

Available in OS X v10.4 and later.

Declared in IODMACommand.h.

kIODMACommandOutputLittle64

Output little-endian Segment64 output segment function.

Available in OS X v10.4 and later.

Declared in IODMACommand.h.

kIODMACommandOutputLittle64

Output big-endian Segment64 output segment function.

Available in OS X v10.4 and later.

Declared in IODMACommand.h.

MappingOptions

Mapping types to indicate the desired mapper type for translating memory descriptors into I/O DMA Bus addresses.

enum MappingOptions {
   kMapped = 0x00000000,
   kBypassed = 0x00000001,
   kNonCoherent = 0x00000002,
   kTypeMask = 0x0000000f,
   kNoCacheStore = 0x00000010, // Memory in descriptor
   kOnChip = 0x00000020, // Indicates DMA is on South Bridge
   kIterateOnly = 0x00000040 // DMACommand will be used as a cursor only
};
Constants
kNonCoherent

Used by drivers for non-coherent transfers, implies unmapped memmory

kMapped

Allow a driver to define addressing size

kBypassed

Allow drivers to bypass any mapper

kMaxMappingOptions

Internal use only

SynchronizeOptions

Options for the synchronize method.

enum SynchronizeOptions {
   kForceDoubleBuffer = 0x01000000
};
Constants
kForceDoubleBuffer

Copy the entire prepared range to a new page aligned buffer.

Segment32

struct Segment32 {
   UInt32 fIOVMAddr, fLength;
};
Discussion

A 32 bit I/O bus address/length pair

Segment64

struct Segment64 {
   UInt64 fIOVMAddr, fLength;
};
Discussion

A 64 bit I/O bus address/length pair

Instance Variables

fActive

UInt32 fActive;

fActive indicates that this DMA command is currently prepared and ready to go

fAlignMask

UInt32 fAlignMask;

Alignment restriction mask.

fBypassMask

UInt64 fBypassMask;

Mask to be ored into the address to bypass the given iommu's mapping.

fMapper

IOMapper *fMapper;

Client defined mapper.

fMappingOptions

MappingOptions fMappingOptions;

What type of I/O virtual address mapping is required for this command

fMaxSegmentSize

UInt64 fMaxSegmentSize;

Maximum size of one segment in a scatter/gather list

fMaxTransferSize

UInt64 fMaxTransferSize;

Maximum size of a transfer that this memory cursor is allowed to generate

fNumAddressBits

UInt32 fNumAddressBits;

Number of bits that the hardware can address

fNumSegments

UInt32 fNumSegments;

Number of contiguous segments required for the current memory descriptor and desired mapping

fOutSeg

SegmentFunction fOutSeg;

The action method called when an event has been delivered

fRefCon

void *fRefCon;

Reference Constant, client defined publicly avialable

reserved

struct IODMACommandInternal * reserved;

Reserved for future use. (Internal use only)