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The as recognizes a number of predefined symbols for the
various processor registers. A register specification in one of the
instruction formats is an unsigned integer between 0 and 15. The specific
instruction and the position of the register in the instruction format
denotes the type of the register. The register symbols are prefixed with
‘%’:
%rN | the 16 general purpose registers, 0 <= N <= 15 |
%fN | the 16 floating point registers, 0 <= N <= 15 |
%aN | the 16 access registers, 0 <= N <= 15 |
%cN | the 16 control registers, 0 <= N <= 15 |
%lit | an alias for the general purpose register %r13 |
%sp | an alias for the general purpose register %r15 |