NASM 2.05 based x86 Instruction Reference[ch127]

A.5.107 FTST: Test ST0 Against Zero FTST ; D9 E4 [8086,FPU] FTST compares ST0 with zero and sets the FPU flags accordingly. ST0 is treated as the left-hand side of the comparison, so that a "less-than" result is generated if ST0 is negative.