Determining register availability for register renaming

A method of data processing with an apparatus for data processing, said method comprising the steps of:

• mapping from register specifiers of an architectural set of register specifiers to registers of a physical set of registers, said architectural set of register specifiers representing registers as specified by instructions within an instruction set and said physical set of registers are physical registers for processing instructions of said instruction set;
• storing in an instruction memory register mapping data indicative of register mappings used by instructions that are issued so as to identify physical registers storing values still required for at least some outcomes of said issued instructions, said register mapping data for at least one issued instruction is kept in said memory if said issued instruction is an unresolved speculative instruction and is kept in said memory if said issued instruction is an instruction which is vet to read one or more registers;
• and
• in response to a current state of said apparatus, identifying which physical registers of said physical set of registers are available to be mapped to a register specifier of an instruction to be processed.

Apparatus for processing data, said apparatus comprising:

• a physical set of registers;
• register renaming means for mapping from register specifiers of an architectural set of register specifiers to registers of said physical set of registers, said architectural set of register specifiers representing registers as specified by instructions within an instruction set and said physical set of registers being physical registers for processing instructions of said instruction set;
• and
• available-register identifying means, responsive to a current state of said apparatus, for identifying which physical registers of said physical set of registers are available to be mapped by said register renaming logic to a register specifier of an instruction to be processed;
• wherein
• said available-register identifying means includes an instruction memory means for storing register mapping data indicative of register mappings used by speculative instructions that are issued so as to identify physical registers storing values still required for at least some outcomes of said issued instructions, said register mapping data for one of said issued instructions is kept in said memory if said one of said issued instructions is an unresolved speculative instruction and is kept in said memory if said one of said issued instructions is an instruction which is yet to read one or more registers.