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FISUB

Subtracts an internal floating-point value from another and stores the result in the destination operand.

The following table covers what the source and destinations can be:

DO NOT support LOCK

FISUB is available in 64-bit mode, but it is primarily used for compatibility with x87 floating-point code. It operates on the x87 floating-point stack and does not affect the general-purpose registers or EFLAGS.

The destination operand ST(i) MUST NOT be ST(0) when using the memory-source form, as the subtraction is performed between ST(0) and the source, and the result is stored in ST(i). If ST(0) is the destination, the operation is performed in-place.

Precision and rounding control are determined by the x87 control word. Operations may trigger #P, #U, or #O depending on the result. If the operands are not valid floating-point numbers, the instruction SHALL signal #I.