Memory system including guarded pointers

DETAILED DESCRIPTION The present invention relates to several aspects of a memory system which may be used independently or together.
The invention is particularly applicable in a virtual addressing, multiprocessor environment which requires sharing of data among multiple tasks across multiple nodes.
In accordance with one aspect of the invention, a data processing system comprises shared memory for storing instructions and data for plural programs, the shared memory being accessed in response to pointers.
Guarded pointers address memory locations to which access is restricted.
Each guarded pointer is a processor word which fully identifies a protected segment in memory and an address within the protected segment.
Processor hardware distinguishes guarded pointers from other words and is operable under program control to modify guarded pointers.
Modification of guarded pointers is restricted so that only addresses within the identified segment can be created.
Thus, access outside of a protected segment is prevented.
A permission field in the guarded pointer indicates permissible access to the identified memory segment such as read only or read/write.
By providing the full virtual address, segment information, and a permission field, segment checks and permission checks can be performed during a memory access without requiring additional machine cycles.
Preferably, each guarded pointer comprises a length field and an address field.
The value in the length field indicates a division of the address field into a segment subfield which identifies a segment location and an offset subfield which identifies an offset within an identified segment.
The value in the length field is preferably logarithmically encoded using a base 2 logarithm.
A tag field may be provided to identify the word as a guarded pointer, and the pointer must be so identified if it is to be used to access a memory location.
By limiting the ability to set the flag bit and to freely modify addresses in pointers to the operating system, the creation of forged pointers by application programs to gain access to protected segments is avoided