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4.2 Registers and Predication

Defoe has a set of 64 programmer visible general purpose registers which are 64 bits wide. As in the MIPS architecture, register R0 always contains zero. Predicate registers are special 1-bit registers that specify a true or false value. There are 16 programmer visible predicate registers in the Defoe named PR0 to PR15. All operations in Defoe are predicated, i.e., each instruction contains a predicate register field (PR) that contains a predicate register number. At run-time, if control reaches the instruction, the instruction is always executed. However, at execution time, if the predicate is false, the results are not written to the register file and side effects such as TLB miss exceptions are suppressed. In practice, for reasons of efficiency, this may be implemented by computing a result, but writing back the old value of the target register. Predicate register 0 always contains the value 1 and cannot be altered. Specifying PR0 as the predicate performs unconditional operations. Comparison operations use a predicate register as their target register.


next up previous contents
Next: 4.3 Instruction Encoding Up: 4 Defoe: An Example Previous: 4.1 Function units   Contents
Binu K. Mathew