8-24 Vol. 1

PROGRAMMING WITH THE X87 FPU

8.3.11 

x87 FPU Control Instructions

The following instructions control the state and modes of operation of the x87 FPU. They also allow the status of the 
x87 FPU to be examined:
FINIT/FNINIT

Initialize x87 FPU

FLDCW

Load x87 FPU control word

FSTCW/FNSTCW

Store x87 FPU control word

FSTSW/FNSTSW

Store x87 FPU status word

FCLEX/FNCLEX

Clear x87 FPU exception flags

FLDENV

Load x87 FPU environment

FSTENV/FNSTENV

Store x87 FPU environment

FRSTOR

Restore x87 FPU state

FSAVE/FNSAVE

Save x87 FPU state

FINCSTP

Increment x87 FPU register stack pointer

FDECSTP

Decrement x87 FPU register stack pointer

FFREE

Free x87 FPU register

FNOP

No operation

WAIT/FWAIT

Check for and handle pending unmasked x87 FPU exceptions

The FINIT/FNINIT instructions initialize the x87 FPU and its internal registers to default values.
The FLDCW instructions loads the x87 FPU control word register with a value from memory. The FSTCW/FNSTCW 
and FSTSW/FNSTSW instructions store the x87 FPU control and status words, respectively, in memory (or for an 
FSTSW/FNSTSW instruction in a general-purpose register).
The FSTENV/FNSTENV and FSAVE/FNSAVE instructions save the x87 FPU environment and state, respectively, in 
memory. The x87 FPU environment includes all the x87 FPU’s control and status registers; the x87 FPU state 
includes the x87 FPU environment and the data registers in the x87 FPU register stack. (The FSAVE/FNSAVE 
instruction also initializes the x87 FPU to default values, like the FINIT/FNINIT instruction, after it saves the original 
state of the x87 FPU.) 
The FLDENV and FRSTOR instructions load the x87 FPU environment and state, respectively, from memory into the 
x87 FPU. These instructions are commonly used when switching tasks or contexts.
The WAIT/FWAIT instructions are synchronization instructions. (They are actually mnemonics for the same 
opcode.) These instructions check the x87 FPU status word for pending unmasked x87 FPU exceptions. If any 
pending unmasked x87 FPU exceptions are found, they are handled before the processor resumes execution of the 
instructions (integer, floating-point, or system instruction) in the instruction stream. The WAIT/FWAIT instructions 
are provided to allow synchronization of instruction execution between the x87 FPU and the processor’s integer 
unit. See Section 8.6, “x87 FPU Exception Synchronization,” for more information on the use of the WAIT/FWAIT 
instructions.

8.3.12 

Waiting vs. Non-waiting Instructions

All of the x87 FPU instructions except a few special control instructions perform a wait operation (similar to the 
WAIT/FWAIT instructions), to check for and handle pending unmasked x87 FPU floating-point exceptions, before 
they perform their primary operation (such as adding two floating-point numbers). These instructions are called 
waiting instructions. Some of the x87 FPU control instructions, such as FSTSW/FNSTSW, have both a waiting and 
a non-waiting version. The waiting version (with the “F” prefix) executes a wait operation before it performs its 
primary operation; whereas, the non-waiting version (with the “FN” prefix) ignores pending unmasked exceptions. 
Non-waiting instructions allow software to save the current x87 FPU state without first handling pending exceptions 
or to reset or reinitialize the x87 FPU without regard for pending exceptions.