Memory system including guarded pointers

• US 5,845,331 A
• Filed: 09/28/1994
• Issued: 12/01/1998
• Est. Priority Date: 09/28/1994
• Status: Expired due to Fees

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Dally, William J., Keckler, Stephen W., Carter, Nicholas P.
• Primary Examiner(s): Chan, Eddie P.
• Assistant Examiner(s): Verbrugge, Kevin
• Application Number:
  US08/314,013
• Time in Patent Office: 1,525 Days
• Field of Search: 395/474, 395/477, 395/479, 395/490, 395/416, 395/412, 395/413, 395/417, 395/419, 395/200.07, 395/200.13, 395/200.16, 711/152, 711/163, 711/164
• US Class Current: 711/163
• CPC Class Codes: G06F 12/0292 using tables or multilevel address translation means G06F12/023 takes precedence; address translation in virtual memory systems G06F12/10
• CPC Class Codes: G06F 12/1036 for multiple virtual address spaces, e.g. segmentation G06F12/1045 takes precedence
• CPC Class Codes: G06F 12/1045 associated with a data cache
• CPC Class Codes: G06F 12/1072 Decentralised address translation, e.g. in distributed shared memory systems
• CPC Class Codes: G06F 12/145 the protection being virtual, e.g. for virtual blocks or segments before a translation mechanism

Abstract

A multiprocessor system having shared memory uses guarded pointers to identify protected segments of memory and permitted access to a location specified by the guarded pointer. Modification of pointers is restricted by the hardware system to limit access to memory segments and to limit operations which can be performed within the memory segments. Global address translation is based on grouping of pages which may be stored across multiple nodes. The page groups are identified in the global translation of each node and, with the virtual address, identify a node in which data is stored. Pages are subdivided into blocks and block status flags are stored for each page. The block status flags indicate whether a memory location may be read or written into at a particular node and indicate to a home node whether a remote node has written new data into a location.

Claims

1. A data processing system for processing plural programs, the system comprising:

shared memory for storing instructions and data for the plural programs, the shared memory being accessed in response to pointers;

guarded pointers for addressing memory locations with restricted access, each guarded pointer comprising a processor word which fully identifies without table lookup a protected segment of memory and a virtual address within the protected segment;

and

processor hardware which distinguishes guarded pointers from other words and which is operable under program control to modify guarded pointers, modification of guarded pointers being restricted to prevent access outside of a protected segment.

2. A system as claimed in claim 1 wherein each guarded pointer comprises a length field and an address field, a value in the length field indicating 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.

3. A system as claimed in claim 2 wherein the value in the length field is compressed by encoding.

4. A system as claimed in claim 3 wherein the length value is logarithmically encoded.

5. A system as claimed in claim 4 wherein the length value is encoded using a base 2 logarithm.

6. A system as claimed in claim 3 wherein the guarded pointer further comprises a permission field which indicates permissible access to a memory segment and a tag field which identifies the word as a guarded pointer.

7. A system as claimed in claim 2 wherein the processor hardware is operable to generate a second guarded pointer from a first guarded pointer, the second guarded pointer identifying a subsegment within the segment identified by the first guarded pointer, by changing a value in the length field to decrease the length of the offset field for decreased offset range and to increase the length of the segment field for finer segment location resolution.

8. A system as claimed in claim 2 wherein the processor hardware is operable to generate a second guarded pointer from a first guarded pointer by performing an arithmetic operation on the offset, the processor hardware checking the second guarded pointer for over or under flow by detecting a change in value in the segment subfield.

9. A system as claimed in claim 1 further comprising a permission field which fully indicates permissible access to a memory segment.

10. A system as claimed in claim 9 wherein the processor hardware is operable to modify the permission field of a guarded pointer to generate a pointer having only a more restricted access.

11. A system as claimed in claim 9 further comprising a tag field for identifying a word as a guarded pointer.

12. A system as claimed in claim 1 further comprising a tag field for identifying a word as a guarded pointer.

13. A system as claimed in claim 1 wherein the guarded pointers include ENTER pointers which may be processed by the processor hardware only to jump to the identified address within the protected segment and to execute.

14. A system as claimed in claim 1 wherein operations by the processor hardware on guarded pointers are restricted such that the guarded pointers include read only pointers, read/write pointers, execute pointers, enter pointers which may be used only to jump to a location and execute, and key pointers which may not be modified.

15. In a data processing system, a method for processing plural programs, the method comprising:

storing instructions and data for the plural programs in shared memory;

addressing a memory location through an address carried in a guarded pointer, the guarded pointer comprising a processor word which fully identifies without table lookup a protected segment of memory, a virtual address within the protected segment and a level of authorization for access to the addressed location, access to the memory location being restricted by that level of authorization; and

restricting modification of guarded pointers.

16. A method as claimed in claim 15 further comprising indicating a division of an address field in a guarded pointer by means of a value in a length field, the division dividing the address field into a segment subfield which identifies a segment location and an offset subfield which identifies an offset within an identified segment.

17. A method as claimed in claim 16 further comprising identifying a word as a guarded pointer by means of a tag field.

18. A method as claimed in claim 16 further comprising generating a second guarded pointer from a first guarded pointer, the second guarded pointer identifying a subsegment within the segment identified by the first guarded pointer, the second guarded pointer being generated by changing a value in the length field to decrease the length of the offset field and to increase the length of the segment field.

19. A method as claims in claim 16 further comprising generating a second guarded pointer from a first guarded pointer by performing an arithmetic operation on the offset of the first guarded pointer and checking the second guarded pointer for over or underflow by detecting a change in value in the segment subfield.

20. A method as claimed in claim 15 further comprising generating a second guarded pointer from a first guarded pointer, the second guarded pointer identifying a subsegment within the segment identified by the first guarded pointer.

21. A method as claimed in claim 15 further comprising modifying the level of authority indicated in the guarded pointer to generate a guarded pointer having only a more restricted authority to access.

22. A method as claimed in claim 15 further comprising identifying a word as a guarded pointer by means of a tag field.

23. A method as claimed in claim 15 further comprising jumping to an address identified in an enter pointer, the level of authority identified in the enter pointer preventing modification of the pointer address prior to jumping to the identified address.