javax.realtime
Class RawMemoryAccess

java.lang.Object
  javax.realtime.RawMemoryAccess

public class RawMemoryAccess

extends java.lang.Object

An instance of RawMemoryAccess models a range of physical memory as a fixed sequence of bytes. A full complement of accessor methods allow the contents of the physical area to be accessed through offsets from the base, interpreted as byte, short, int, or long data values or as arrays of these types.

Whether the offset addresses the high-order or low-order byte is normally based on the value of the RealtimeSystem.BYTE_ORDER static byte variable in class RealtimeSystem. If the type of memory used for this RawMemoryAccess region implements non-standard byte ordering, accessor methods in this class continue to select bytes starting at offset from the base address and continuing toward greater addresses. The memory type may control the mapping of these bytes into the primitive data type. The memory type could even select bytes that are not contiguous. In each case the documentation for the PhysicalMemoryTypeFilter must document any mapping other than the "normal" one specified above.

The RawMemoryAccess class allows a real-time program to implement device drivers, memory-mapped I/O, flash memory, battery-backed RAM, and similar low-level software.

A raw memory area cannot contain references to Java objects. Such a capability would be unsafe (since it could be used to defeat Java's type checking) and error-prone (since it is sensitive to the specific representational choices made by the Java compiler).

Many of the constructors and methods in this class throw OffsetOutOfBoundsException. This exception means that the value given in the offset parameter is either negative or outside the memory area.

Many of the constructors and methods in this class throw SizeOutOfBoundsException. This exception means that the value given in the size parameter is either negative, larger than an allowable range, or would cause an accessor method to access an address outside of the memory area.

Unlike other integral parameters in this chapter, negative values are valid for byte, short, int, and long values that are copied in and out of memory by the set and get methods of this class.

All offset values used in this class are measured in bytes.

Atomic loads and stores on raw memory are defined in terms of physical memory. This memory may be accessible to threads outside the JVM and to non-programmed access (e.g., DMA), consequently atomic access must be supported by hardware. This specification is written with the assumption that all suitable hardware platforms support atomic loads for aligned bytes, shorts, and ints. Atomic access beyond the specified minimum may be supported by the implementation.

Storing values into raw memory is more hardware-dependent than loading values. Many processor architectures do not support atomic stores of variables except for aligned stores of the processor's word size. For instance, storing a byte into memory might require reading a 32-bit quantity into a processor register, updating the register to reflect the new byte value, then re-storing the whole 32-bit quantity. Changes to other bytes in the 32-bit quantity that take place between the load and the store will be lost.

Some processors have mechanisms that can be used to implement an atomic store of a byte, but those mechanisms are often slow and not universally supported.

This class supports unaligned access to data, but it does not require the implementation to make such access atomic. Accesses to data aligned on its natural boundary will be atomic if the processor implements atomic loads and stores of that data size.

Except where noted, accesses to raw memory are not atomic with respect to the memory or with respect to schedulable objects. A raw memory area could be updated by another schedulable object, or even unmapped in the middle of a method.

The characteristics of raw-memory access are necessarily platform dependent. This specification provides a minimum requirement for the RTSJ platform, but it also supports optional system properties that identify a platform's level of support for atomic raw put and get. The properties represent a four-dimensional sparse array with boolean values indicating whether that combination of access attributes is atomic. The default value for array entries is false. The dimension are

Attribute	Values	Comment
Access type	read, write
Data type	byte, short, int, long, float, double
Alignment	0 to 7	For each data type, the possible alignments range from 0 == aligned to data size - 1 == only the first byte of the data is alignment bytes away from natural alignment.
Atomicity	processor, smp, memory	processor means access is atomic with respect to other schedulable objects on that processor. smp means that access is processor atomic, and atomic with respect across the processors in an SMP. memory means that access is smp atomic, and atomic with respect to all access to the memory including DMA.

The true values in the table are represented by properties of the following form. javax.realtime.atomicaccess_<access>_<type>_<alignment>_atomicity=true for example:

  javax.realtime.atomicaccess_read_byte_0_memory=true
 

Table entries with a value of false may be explicitly represented, but since false is the default value, such properties are redundant.

All raw memory access is treated as volatile, and serialized. The run-time must be forced to re-read memory or write to memory on each call to a raw memory getxxx or putxxx method, and to complete the reads and writes in the order they appear in the program order.

Constructor Summary

RawMemoryAccess(java.lang.Object type, long size)
Construct an instance of RawMemoryAccess with the given parameters, and set the object to the mapped state.

RawMemoryAccess(java.lang.Object type, long base, long size)
Construct an instance of RawMemoryAccess with the given parameters, and set the object to the mapped state.

Method Summary

byte	getByte(long offset) Gets the byte at the given offset in the memory area associated with this object.
void	getBytes(long offset, byte[] bytes, int low, int number) Gets number bytes starting at the given offset in the memory area associated with this object and assigns them to the byte array passed starting at position low.
int	getInt(long offset) Gets the int at the given offset in the memory area associated with this object.
void	getInts(long offset, int[] ints, int low, int number) Gets number integers starting at the given offset in the memory area associated with this object and assign them to the int array passed starting at position low.
long	getLong(long offset) Gets the long at the given offset in the memory area associated with this object.
void	getLongs(long offset, long[] longs, int low, int number) Gets number longs starting at the given offset in the memory area associated with this object and assign them to the long array passed starting at position low.
long	getMappedAddress() Gets the virtual memory location at which the memory region is mapped.
short	getShort(long offset) Gets the short at the given offset in the memory area associated with this object.
void	getShorts(long offset, short[] shorts, int low, int number) Gets number shorts starting at the given offset in the memory area associated with this object and assign them to the short array passed starting at position low.
long	map() Maps the physical memory range into virtual memory.
long	map(long base) Maps the physical memory range into virtual memory at the specified location.
long	map(long base, long size) Maps the physical memory range into virtual memory.
void	setByte(long offset, byte value) Sets the byte at the given offset in the memory area associated with this object.
void	setBytes(long offset, byte[] bytes, int low, int number) Sets number bytes starting at the given offset in the memory area associated with this object from the byte array passed starting at position low.
void	setInt(long offset, int value) Sets the int at the given offset in the memory area associated with this object.
void	setInts(long offset, int[] ints, int low, int number) Sets number ints starting at the given offset in the memory area associated with this object from the int array passed starting at position low.
void	setLong(long offset, long value) Sets the long at the given offset in the memory area associated with this object.
void	setLongs(long offset, long[] longs, int low, int number) Sets number longs starting at the given offset in the memory area associated with this object from the long array passed starting at position low.
void	setShort(long offset, short value) Sets the short at the given offset in the memory area associated with this object.
void	setShorts(long offset, short[] shorts, int low, int number) Sets number shorts starting at the given offset in the memory area associated with this object from the short array passed starting at position low.
void	unmap() Unmap the physical memory range from virtual memory.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

RawMemoryAccess

public RawMemoryAccess(java.lang.Object type,
                       long size)

Construct an instance of RawMemoryAccess with the given parameters, and set the object to the mapped state.

Parameters:

type - An instance of Object representing the type of memory required (e.g., dma, shared) - used to define the base address and control the mapping. If the required memory has more than one attribute, type may be an array of objects. If type is null or a reference to an array with no entries, any type of memory is acceptable. Ignored in present version.

size - The size of the area in bytes.

RawMemoryAccess

public RawMemoryAccess(java.lang.Object type,
                       long base,
                       long size)

Construct an instance of RawMemoryAccess with the given parameters, and set the object to the mapped state. If the platform supports virtual memory, map the raw memory into virtual memory. The run time environment is allowed to choose the virtual address where the raw memory area corresponding to this object will be mapped. The attributes of the mapping operation are controlled by the vMFlags and vMAttributes of the PhysicalMemoryTypeFilter objects that matched this object's type parameter. (See PhysicalMemoryTypeFilter.getVMAttributes() and PhysicalMemoryTypeFilter.getVMFlags().

Parameters:

type - An instance of Object representing the type of memory required (e.g., dma, shared) - used to define the base address and control the mapping. If the required memory has more than one attribute, type may be an array of objects. If type is null or a reference to an array with no entries, any type of memory is acceptable.

Present version requires type == null.

base - The physical memory address of the region.

size - The size of the area in bytes.

Method Detail

getByte

public byte getByte(long offset)

Gets the byte at the given offset in the memory area associated with this object. The byte is always loaded from memory in a single atomic operation.

Caching of the memory access is controlled by the memory type requested when the RawMemoryAccess instance was created. If the memory is not cached, this method guarantees serialized access (that is, the memory access at the memory occurs in the same order as in the program. Multiple writes to the same location may not be coalesced.)

Parameters:

offset - The offset in bytes from the beginning of the raw memory from which to load the byte.

Returns:

The byte from raw memory.

getBytes

public void getBytes(long offset,
                     byte[] bytes,
                     int low,
                     int number)

Gets number bytes starting at the given offset in the memory area associated with this object and assigns them to the byte array passed starting at position low. Each byte is loaded from memory in a single atomic operation. Groups of bytes may be loaded together, but this is unspecified.

Caching of the memory access is controlled by the memory type requested when the instance was created. If the memory is not cached, this method guarantees serialized access (that is, the memory access at the memory occurs in the same order as in the program. Multiple writes to the same location may not be coalesced.)

Parameters:

offset - The offset in bytes from the beginning of the raw memory from which to start loading.

bytes - The array into which the loaded items are placed.

low - The offset which is the starting point in the given array for the loaded items to be placed.

number - The number of items to load.

setByte

public void setByte(long offset,
                    byte value)

Sets the byte at the given offset in the memory area associated with this object.

This memory access may involve a load and a store, and it may have unspecified effects on surrounding bytes in the presence of concurrent access.

Caching of the memory access is controlled by the memory type requested when the RawMemoryAccess = instance was created. If the memory is not cached, this method guarantees serialized access (that is, the memory access at the memory occurs in the same order as in the program. Multiple writes to the same location may not be coalesced.)

Parameters:

offset - The offset in bytes from the beginning of the raw memory area to which to write the byte.

value - The byte to write.

setBytes

public void setBytes(long offset,
                     byte[] bytes,
                     int low,
                     int number)

Sets number bytes starting at the given offset in the memory area associated with this object from the byte array passed starting at position low.

This memory access may involve multiple load and a store operations, and it may have unspecified effects on surrounding bytes (even bytes in the range being stored) in the presence of concurrent access.

Caching of the memory access is controlled by the memory type requested when the RawMemoryAccess instance was created. If the memory is not cached, this method guarantees serialized access (that is, the memory access at the memory occurs in the same order as in the program. Multiple writes to the same location may not be coalesced.)

Parameters:

offset - The offset in bytes from the beginning of the raw memory area to which to start writing.

bytes - The array from which the items are obtained.

low - The offset which is the starting point in the given array for the items to be obtained.

number - The number of items to write.

getShort

public short getShort(long offset)

Gets the short at the given offset in the memory area associated with this object. If the short is aligned on a natural boundary it is always loaded from memory in a single atomic operation. If it is not on a natural boundary it may not be loaded atomically, and the number and order of the load operations is unspecified.

Caching of the memory access is controlled by the memory type requested when the RawMemoryAccess instance was created. If the memory is not cached, this method guarantees serialized access (that is, the memory access at the memory occurs in the same order as in the program. Multiple writes to the same location may not be coalesced.)

Parameters:

offset - The offset in bytes from the beginning of the raw memory area from which to load the short.

Returns:

The short loaded from raw memory.

getShorts

public void getShorts(long offset,
                      short[] shorts,
                      int low,
                      int number)

Gets number shorts starting at the given offset in the memory area associated with this object and assign them to the short array passed starting at position low.

If the shorts are located on natural boundaries each short is loaded from memory in a single atomic operation. Groups of shorts may be loaded together, but this is unspecified.

If the shorts are not located on natural boundaries the load may not be atomic, and the number and order of load operations is unspecified.

Caching of the memory access is controlled by the memory type requested when the RawMemoryAccess instance was created. If the memory is not cached, this method guarantees serialized access (that is, the memory access at the memory occurs in the same order as in the program. Multiple writes to the same location may not be coalesced.)

Parameters:

offset - The offset in bytes from the beginning of the raw memory area from which to start loading.

shorts - The array into which the loaded items are placed.

low - The offset which is the starting point in the given array for the loaded shorts to be placed.

number - The number of shorts to load.

setShort

public void setShort(long offset,
                     short value)

Sets the short at the given offset in the memory area associated with this object.

This memory access may involve a load and a store, and it may have unspecified effects on surrounding shorts in the presence of concurrent access.

Caching of the memory access is controlled by the memory type requested when the RawMemoryAccess instance was created. If the memory is not cached, this method guarantees serialized access (that is, the memory access at the memory occurs in the same order as in the program. Multiple writes to the same location may not be coalesced.)

Parameters:

offset - The offset in bytes from the beginning of the raw memory area at which to write the short.

value - The short to write.

setShorts

public void setShorts(long offset,
                      short[] shorts,
                      int low,
                      int number)

Sets number shorts starting at the given offset in the memory area associated with this object from the short array passed starting at position low.

Each write of a short value may involve a load and a store, and it may have unspecified effects on surrounding shorts in the presence of concurrent access - even on other shorts in the array.

Caching of the memory access is controlled by the memory type requested when the RawMemoryAccess instance was created. If the memory is not cached, this method guarantees serialized access (that is, the memory access at the memory occurs in the same order as in the program. Multiple writes to the same location may not be coalesced.)

Parameters:

offset - The offset in bytes from the beginning of the raw memory area at which to start writing.

shorts - The array from which the items are obtained.

low - The offset which is the starting point in the given array for the items to be obtained.

number - The number of items to write.

getInt

public int getInt(long offset)

Gets the int at the given offset in the memory area associated with this object. If the integer is aligned on a "natural" boundary it is always loaded from memory in a single atomic operation. If it is not on a natural boundary it may not be loaded atomically, and the number and order of the load operations is unspecified.

Caching of the memory access is controlled by the memory type requested when the RawMemoryAccess instance was created. If the memory is not cached, this method guarantees serialized access (that is, the memory access at the memory occurs in the same order as in the program. Multiple writes to the same location may not be coalesced.)

Negative integer values not currently implemented

Parameters:

offset - The offset in bytes from the beginning of the raw memory area from which to load the integer.

Returns:

The integer from raw memory.

getInts

public void getInts(long offset,
                    int[] ints,
                    int low,
                    int number)

Gets number integers starting at the given offset in the memory area associated with this object and assign them to the int array passed starting at position low.

If the integers are aligned on natural boundaries each integer is loaded from memory in a single atomic operation. Groups of integers may be loaded together, but this is unspecified.

If the integers are not aligned on natural boundaries they may not be loaded atomically and the number and order of load operations is unspecified.

Caching of the memory access is controlled by the memory type requested when the RawMemoryAccess instance was created. If the memory is not cached, this method guarantees serialized access (that is, the memory access at the memory occurs in the same order as in the program. Multiple writes to the same location may not be coalesced.)

Parameters:

offset - The offset in bytes from the beginning of the raw memory area at which to start loading.

ints - The array into which the integers read from the raw memory are placed.

low - The offset which is the starting point in the given array for the loaded items to be placed.

number - The number of integers to loaded.

setInt

public void setInt(long offset,
                   int value)

Sets the int at the given offset in the memory area associated with this object. On most processor architectures an aligned integer can be stored in an atomic operation, but this is not required.

This memory access may involve multiple load and a store operations, and it may have unspecified effects on surrounding bytes (even bytes in the range being stored) in the presence of concurrent access.

Caching of the memory access is controlled by the memory type requested when the RawMemoryAccess instance was created. If the memory is not cached, this method guarantees serialized access (that is, the memory access at the memory occurs in the same order as in the program. Multiple writes to the same location may not be coalesced.)

Parameters:

offset - The offset in bytes from the beginning of the raw memory area at which to write the integer.

value - The integer to write.

setInts

public void setInts(long offset,
                    int[] ints,
                    int low,
                    int number)

Sets number ints starting at the given offset in the memory area associated with this object from the int array passed starting at position low. On most processor architectures each aligned integer can be stored in an atomic operation, but this is not required.

This memory access may involve multiple load and a store operations, and it may have unspecified effects on surrounding bytes (even bytes in the range being stored) in the presence of concurrent access.

Caching of the memory access is controlled by the memory type requested when the RawMemoryAccess instance was created. If the memory is not cached, this method guarantees serialized access (that is, the memory access at the memory occurs in the same order as in the program. Multiple writes to the same location may not be coalesced.)

Parameters:

offset - The offset in bytes from the beginning of the raw memory area at which to start writing.

ints - The array from which the items are obtained.

low - The offset which is the starting point in the given array for the items to be obtained.

number - The number of items to write.

getLong

public long getLong(long offset)

Gets the long at the given offset in the memory area associated with this object.

The load is not required to be atomic even it is located on a natural boundary.

Caching of the memory access is controlled by the memory type requested when the RawMemoryAccess instance was created. If the memory is not cached, this method guarantees serialized access (that is, the memory access at the memory occurs in the same order as in the program. Multiple writes to the same location may not be coalesced.)

Parameters:

offset - The offset in bytes from the beginning of the raw memory area from which to load the long.

Returns:

The long from raw memory.

getLongs

public void getLongs(long offset,
                     long[] longs,
                     int low,
                     int number)

Gets number longs starting at the given offset in the memory area associated with this object and assign them to the long array passed starting at position low.

The loads are not required to be atomic even if they are located on natural boundaries.

Caching of the memory access is controlled by the memory type requested when the

setLong

public void setLong(long offset,
                    long value)

Sets the long at the given offset in the memory area associated with this object. Even if it is aligned, the long value may not be updated atomically. It is unspecified how many load and store operations will be used or in what order.

This memory access may involve multiple load and a store operations, and it may have unspecified effects on surrounding bytes (even bytes in the range being stored) in the presence of concurrent access.

Caching of the memory access is controlled by the memory type requested when the RawMemoryAccess instance was created. If the memory is not cached, this method guarantees serialized access (that is, the memory access at the memory occurs in the same order as in the program. Multiple writes to the same location may not be coalesced.)

Parameters:

offset - The offset in bytes from the beginning of the raw memory area at which to write the long.

value - The long to write.

setLongs

public void setLongs(long offset,
                     long[] longs,
                     int low,
                     int number)

Sets number longs starting at the given offset in the memory area associated with this object from the long array passed starting at position low. Even if they are aligned, the long values may not be updated atomically. It is unspecified how many load and store operations will be used or in what order.

This memory access may involve multiple load and a store operations, and it may have unspecified effects on surrounding bytes (even bytes in the range being stored) in the presence of concurrent access.

Caching of the memory access is controlled by the memory type requested when the RawMemoryAcccess instance was created. If the memory is not cached, this method guarantees serialized access (that is, the memory access at the memory occurs in the same order as in the program. Multiple writes to the same location may not be coalesced.)

Parameters:

offset - The offset in bytes from the beginning of the raw memory area at which to start writing.

longs - The array from which the items are obtained.

low - The offset which is the starting point in the given array for the items to be obtained.

number - The number of items to write.

getMappedAddress

public long getMappedAddress()

Gets the virtual memory location at which the memory region is mapped.

Not currently implemented

Returns:

The starting point of the virtual memory range.

map

public long map()

Maps the physical memory range into virtual memory. No-op if the system doesn't support virtual memory.

The run time environment is allowed to choose the virtual address where the raw memory area corresponding to this object will be mapped. The attributes of the mapping operation are controlled by the vMFlags and vMAttributes of the PhysicalMemoryTypeFilter objects that matched this object's type parameter. (See PhysicalMemoryTypeFilter.getVMAttributes() and PhysicalMemoryTypeFilter.getVMFlags().

If the object is already mapped into virtual memory, this method does not change anything.

Returns:

The starting point of the virtual memory range.

map

public long map(long base)

Maps the physical memory range into virtual memory at the specified location. No-op if the system doesn't support virtual memory. The attributes of the mapping operation are controlled by the vMFlags and vMAttributes of the PhysicalMemoryTypeFilter objects that matched this object's type parameter. (See PhysicalMemoryTypeFilter.getVMAttributes() and PhysicalMemoryTypeFilter.getVMFlags(). If the object is already mapped into virtual memory at a different address, this method remaps it to base.

If a remap is requested while another schedulable object is accessing the raw memory, the map will block until one load or store completes. It can interrupt an array operation between entries.

Parameters:

base - The location to map at the virtual memory space.

Returns:

The starting point of the virtual memory.

map

public long map(long base,
                long size)

Maps the physical memory range into virtual memory. No-op if the system doesn't support virtual memory.

The attributes of the mapping operation are controlled by the vMFlags and vMAttributes of the PhysicalMemoryTypeFilter objects that matched this object's type parameter. (See PhysicalMemoryTypeFilter.getVMAttributes() and PhysicalMemoryTypeFilter.getVMFlags().

If the object is already mapped into virtual memory at a different address, this method remaps it to base.

If a remap is requested while another schedulable object is accessing the raw memory, the map will block until one load or store completes. It can interrupt an array operation between entries.

Parameters:

base - The location to map at the virtual memory space.

size - The size of the block to map in. If the size of the raw memory area is greater than size, the object is unchanged but accesses beyond the mapped region will throw SizeOutOfBoundsException. If the size of the raw memory area is smaller than the mapped region access to the raw memory will behave as if the mapped region matched the raw memory area, but additional virtual address space will be consumed after the end of the raw memory area.

Returns:

The starting point of the virtual memory.

unmap

public void unmap()

Unmap the physical memory range from virtual memory. This changes the raw memory from the mapped state to the unmapped state. If the platform supports virtual memory, this operation frees the virtual addresses used for the raw memory region.

If the object is already in the unmapped state, this method has no effect.

While a raw memory object is unmapped all attempts to set or get values in the raw memory will throw SizeOutOfBoundsException.

An unmapped raw memory object can be returned to mapped state with any of the object's map methods.

If an unmap is requested while another schedulable object is accessing the raw memory, the unmap() will throw an IllegalStateException. The unmap() method can interrupt an array operation between entries.