Info Catalog
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as: Alpha Syntax
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9.2.3.2 Register Names
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The 32 integer registers are referred to as '$N' or '$rN'. In addition,
registers 15, 28, 29, and 30 may be referred to by the symbols '$fp',
'$at', '$gp', and '$sp' respectively.
The 32 floating-point registers are referred to as '$fN'.