Determining a tag value for use in a tag-guarded memory

A method for determining, for use in a tag-guarded memory, a selected tag value from a plurality of tag values, the method comprising:

• receiving an excluded tag vector comprising a plurality of fields, where each field is associated with a tag value in said plurality of tag values and identifies whether the associated tag value is excluded from use;
• employing ordered list generation circuitry to generate, from said excluded tag vector, an ordered list of non-excluded tag values;
• employing count determination circuitry to determine, using said excluded tag vector and an identified start tag value, a count value indicative of a number of non-excluded tag values occurring in a region of the excluded tag vector bounded by an initial field and a field corresponding to said start tag value;
• determining the selected tag value from said ordered list based on said count value and an identified offset, the identified offset indicating a required number of non-excluded tag values between said start tag value and said selected tag value;
• associating the selected tag value with an address identifying an addressed location in a memory system;
• and
• in response to a tag-guarded memory access operation based on a target address identifying a particular addressed location in the memory system:
• comparing an address tag associated with the target address with a guard tag associated with a block of one or more memory locations that includes the particular addressed location identified by the target address, and
• generating an indication of whether a match is detected between the guard tag and the address tag.

An apparatus for determining, for use in a tag-guarded memory, a selected tag value from a plurality of tag values, comprising:

• ordered list generation means for receiving an excluded tag vector comprising a plurality of fields, where each field is associated with a tag value in said plurality of tag values and identifies whether the associated tag value is excluded from use, the ordered list generation means for generating, from said excluded tag vector, an ordered list of non-excluded tag values;
• count determination means for determining, using said excluded tag vector and an identified start tag value, a count value indicative of a number of non-excluded tag values occurring in a region of the excluded tag vector bounded by an initial field and a field corresponding to said start tag value;
• and
• tag selection means for determining the selected tag value from said ordered list based on said count value and an identified offset, the identified offset indicating a required number of non-excluded tag values between said start tag value and said selected tag value;
• wherein the apparatus is arranged to associate the selected tag value with an address identifying an addressed location in a memory system;
• and
• the apparatus further comprises means for accessing memory responsive to a tag-guarded memory access operation based on a target address identifying a particular addressed location in the memory system to:
• compare an address tag associated with the target address with a guard tag associated with a block of one or more memory locations that includes the particular addressed location identified by the target address;
• and
• generate an indication of whether a match is detected between the guard tag and the address tag.