Memory Organisation

Memory Hierarchy

  • The memory unit is used for storing programs and data. It fulfills the need of storage of the information.
  • The additional storage with main memory capacity enhance the performance of the general purpose computers and make them efficient.
  • Only those programs and data, which is currently needed by the processor, reside in main memory. Information can be transferred from auxiliary memory to main memory when needed.

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Computer Organization and Architecture | Device Mapping | Translator Mapping | Space-to-space Mapping | Bridge Mapping

Memory Mapping Types


There are a few different kinds of mappings that can be specified in the map attribute. All use the format described in the previous section.


Device Mapping
The most common kind of mapping. It is used for devices, RAM and ROM objects. The target field is not set.
Translator Mapping
Sometimes the address has to be modified between memory-spaces, or the destination memory-space depends on the address or some other aspect of the access such as the initiating processor. In these cases a translator can be used. A translator mapping is specified with the translator in the object field, and the default target as target. The translator has to implement the TRANSLATE interface. When an access reaches a translator mapping, the translate function in the TRANSLATE interface is called. The translator can then modify the address if necessary, and specify what destination memory-space to use. If it doesn't specify any new memory-space, the default one from the configuration is used. The following fields can be changed by the translator: physical_address, ignore, block_STC, inverse_endian and user_ptr.
Translate to RAM/ROM Mapping
Used to map RAM and ROM objects with a translator first. The object field is set to the translator, and target is set to the RAM/ROM object.
Space-to-space Mapping
Map one memory-space in another. Both object and target should be set to the destination memory-space object.
Bridge Mapping
A bridge mapping is typically used for mappings that are setup by some kind of bridge device. The purpose of a bridge mapping is to handle accesses where nothing is mapped, in a way that corresponds to the bus architecture. For a bridge mapping, the object field is set to the bridge device, implementing the BRIDGE interface. The target field is set to the destination memory-space. If both a translator and bridge is needed, they must be implemented by the same object. If an access is made where nothing is mapped, the memory-space by default returns the Sim_PE_IO_Not_Taken pseudo exception. But if the access was made through a bridge mapping, the bridge device will be called to notify it about the unmapped access. It can then update any internal status registers, specify a new return exception, and set the data that should be returned in the case of a read access. Since the bridge is associated with the mapping and not the memory-space itself, several bridges can exist for one space, and devices doing accesses directly to the memory-space in question will not affect the bridge for non-mapped addresses. In the latter case, the device itself has to interpret the Sim_PE_IO_Not_Taken exception. The Sim_PE_IO_Error exception, indicating that a device returned an error is also sent to the bridge. Finally, bridges are called for accesses that generate Sim_PE_Inquiry_Outside_Memory, i.e. an inquiry access where nothing is mapped. In this case the bridge may have to set a default return value, such as −1.

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