
MOTOROLA
Chapter 3. Central Processing Unit
3-15
User Instruction Set Architecture (UISA) Register Set
FEX and VX are the logical ORs of other FPSCR bits. Therefore these two bits are not
listed among the FPSCR bits directly affected by the various instructions.
A listing of FPSCR bit settings is shown in
Table 3-5.
Table 3-4. FPSCR Bit Categories
Bits
Type
[0], [3:12], [21:23]
Status, sticky
[1:2], [13:20]
Status, not sticky
[24:31]
Control
MSB
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Field FX FEX VX
OX
UX
ZX
XX
VXSN
AN
VXISI VXIDI VXZDZ VXIMZ VXVC FR
FI
FPRF0
Reset
Unchanged
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
LSB
31
Field
FPRF[1:4]
0
VX
SOFT
VX
SQRT
VXCVI
VE
OE
UE
ZE
XE
NI
RN
Reset
Unchanged
Figure 3-6. Floating-Point Status and Control Register (FPSCR)
Table 3-5. FPSCR Bit Descriptions
Bits
Name
Description
0
FX
Floating-point exception summary. Every floating-point instruction implicitly
sets FPSCR[FX] if that instruction causes any of the floating-point exception
bits in the FPSCR to change from 0 to 1. The mcrfs instruction implicitly clears
FPSCR[FX] if the FPSCR field containing FPSCR[FX] has been copied. The
mtfsf, mtfsfi, mtfsb0, and mtfsb1 instructions can set or clear FPSCR[FX]
explicitly.
Sticky bit
1
FEX
Floating-point enabled exception summary. This bit signals the occurrence of
any of the enabled exception conditions. It is the logical OR of all the
floating-point exception bits masked with their respective enable bits. The
mcrfs instruction implicitly clears FPSCR[FEX] if the result of the logical OR
described above becomes zero. The mtfsf, mtfsfi, mtfsb0, and mtfsb1
instructions cannot set or clear FPSCR[FEX] explicitly.
Not sticky
2
VX
Floating-point invalid operation exception summary. This bit signals the
occurrence of any invalid operation exception. It is the logical OR of all of the
invalid operation exceptions. The mcrfs instruction implicitly clears
FPSCR[VX] if the result of the logical OR described above becomes zero. The
mtfsf, mtfsfi, mtfsb0, and mtfsb1 instructions cannot set or clear FPSCR[VX]
explicitly.
Not sticky
3
OX
Floating-point overflow exception.
Sticky bit
4
UX
Floating-point underflow exception.
Sticky bit