.1. X86
Bit Abbreviation (meaning) Description
0 IM (Invalid operation Mask)
1 DM (Denormalized operand Mask)
2 ZM (Zero divide Mask)
3 OM (Overflow Mask)
4 UM (Underflow Mask)
5 PM (Precision Mask)
7 IEM (Interrupt Enable Mask) Exceptions enabling, 1 by default (disabled)
8, 9 PC (Precision Control)
00 — 24 bits (REAL4)
10 — 53 bits (REAL8)
11 — 64 bits (REAL10)
10, 11 RC (Rounding Control)
00 — (by default) round to nearest
01 — round toward−∞
10 — round toward+∞
11 — round toward 0
12 IC (Infinity Control) 0 — (by default) treat+∞and−∞as unsigned
1 — respect both+∞and−∞
The PM, UM, OM, ZM, DM, IM flags define if to generate exception in the case of a corresponding error.
Status Word
Read-only register.
Bit Abbreviation (meaning) Description
15 B (Busy) Is FPU do something (1) or results are ready (0)
14 C3
13, 12, 11 TOP points to the currently zeroth register
10 C2
9 C1
8 C0
7 IR (Interrupt Request)
6 SF (Stack Fault)
5 P (Precision)
4 U (Underflow)
3 O (Overflow)
2 Z (Zero)
1 D (Denormalized)
0 I (Invalid operation)The SF, P, U, O, Z, D, I bits signal about exceptions.
About the C3, C2, C1, C0 you can read more here: (1.19.7 on page 234).
N.B.: WhenST(x)isused, theFPUaddsxtoTOP(bymodulo8)andthatishowitgetstheinternalregister’s
number.
Tag Word
The register has current information about the usage of numbers registers.
Bit Abbreviation (meaning)
15, 14 Tag(7)
13, 12 Tag(6)
11, 10 Tag(5)
9, 8 Tag(4)
7, 6 Tag(3)
5, 4 Tag(2)
3, 2 Tag(1)
1, 0 Tag(0)Each tag contains information about a physical FPU register (R(x)), not logical (ST(x)).
For each tag:
- 00 — The register contains a non-zero value