User Guide

268 x87 Floating-Point Programming
AMD64 Technology 24592—Rev. 3.15—November 2009
• FDIVR—Floating-Point Divide Reverse
• FDIVRP—Floating-Point Divide Reverse and Pop
• FIDIVR—Floating-Point Integer Divide Reverse
The FDIV instruction syntax has forms that include one or two source explicit operands that may be
single-precision or double-precision floating-point values or 16-bit or 32-bit integer values. In the one-
operand form, the instruction reads a value from memory, divides ST(0) by the memory operand, and
writes the result to ST(0). In the two-operand form, both source operands are located in stack registers.
The instruction divides the first operand by the second operand and writes the result to the first
operand.
The FDIVP instruction syntax has forms that include zero or two explicit source operands. In the zero-
operand form, the instruction divides ST(1) by ST(0), writes the result to ST(1), and pops the stack. In
the two-operand form, both source operands are located in stack registers. The instruction divides the
first operand by the second operand, writes the result to the first operand, and pops the stack.
The FIDIV instruction reads a 16-bit or 32-bit integer value from memory, converts it to the double-
extended-precision format, divides ST(0) by the memory operand, and writes the result to ST(0).
The FDIVR and FDIVRP instructions perform the same operations as FDIV and FDIVP, respectively,
except that the source operands are reversed. Instead of dividing the first operand by the second
operand, FDIVR and FDIVRP divide the second operand by the first operand.
Change Sign
• FABS—Floating-Point Absolute Value
• FCHS—Floating-Point Change Sign
The FABS instruction changes the top-of-stack value, ST(0), to its absolute value by clearing its sign
bit to 0. The top-of-stack value is always positive following execution of the FABS instruction. The
FCHS instruction complements the sign bit of ST(0). For example, if ST(0) was +0.0 before the
execution of FCHS, it is changed to -0.0.
Round
• FRNDINT—Floating-Point Round to Integer
The FRNDINT instruction rounds the top-of-stack value, ST(0), to an integer value, although the value
remains in double-extended-precision floating-point format. Rounding takes place according to the
setting of the rounding control (RC) field in the x87 control word.
Partial Remainder
• FPREM—Floating-Point Partial Remainder
• FPREM1—Floating-Point Partial Remainder
The FPREM instruction returns the remainder obtained by dividing ST(0) by ST(1) and stores it in
ST(0). If the exponent difference between ST(0) and ST(1) is less than 64, all integer bits of the