7-24
FLOATING-POINT UNIT
The FLDENV and FRSTOR instructions allow FPU state information to be loaded from
memory into the FPU. Here, the FLDENV instruction loads only the status, control, tag, FPU
instruction pointer, FPU operand pointer, and opcode registers, and the FRSTOR instruction
loads all the FPU registers, including the FPU stack registers.
7.4.FLOATING-POINT DATA TYPES AND FORMATS
The IA FPU recognizes and operates on seven data types, divided into three groups: reals, inte-
gers, and packed BCD integers. Figure 7-17 shows the data formats for each of the FPU data
types. Table 7-8 gives the length, precision, and approximate normalized range that can be repre-
sented of each FPU data type. Denormal values are also supported in each of the real types, as
required by IEEE Standard 854.
With the exception of the 80-bit extended-real format, all of these data types exist in memory
only. When they are loaded into FPU data registers, they are converted into extended-real format
and operated on in that format.
Figure 7-16. Real Mode FPU State Image in Memory, 16-BitFormat
0
0
2
4
6
8
10
12
16-Bit Real-Address Mode and
Control Word
15
Status Word
Tag Word
Virtual-8086 Mode Format
0 0 0 0 0 0 0 0 0 0 0 0
Opcode 10...00
0
FPU Instruction Pointer 15...00
IP 19..16
OP 19..16
FPU Operand Pointer 15...00
