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Marjolein Breysens
Marjolein Breysens

Reputation: 83

IEEE 754 floating point rounding mode hardware/software

There are several rounding modes in the IEEE 754 floating point standard, my question is: is the rounding mode selected based on the guard/round/sticky bits or does a programmer specify what mode to use?

Upvotes: 2

Views: 537

Answers (1)

Pascal Cuoq
Pascal Cuoq

Reputation: 80276

The rounding mode is chosen by the programmer. Either the rounding mode is stored in a “control register” inside the FPU, and is changed through a specific (expensive) instruction, or each floating-point instruction has a few bits reserved to indicate what rounding mode it is intended to be executed in.

Either way, the programmer controls the rounding mode that applies each time a floating-point instruction is executed, and the rounding mode influences how the guard/round/sticky bits are interpreted to round the result, not the other way round.

Upvotes: 2

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