
The PowerPC Core
MOTOROLA
MPC801 USER’S MANUAL
6-23
6
6.3.1.2.3 Fixed-Point Exception Cause Register.
The following table provides the bit
assignments for the fixed-point exception cause register (XER).
SO—Summary Overflow
This bit is set whenever an instruction (except
mtspr
) sets the OV bit. Once set, the SO bit
remains set until it is cleared by an
mtspr
or
mcrxr
instruction. It is not altered by compare
instructions or other instructions that cannot overflow. Executing an
mtspr
instruction to the
XER, supplying the values zero for SO and one for OV, causes SO to be cleared and OV to
be set.
OV—Overflow
This bit is set to indicate that an overflow has occurred during the execution of an instruction.
Add, subtract from, and negate instructions have OE = 1 set the OV bit if the carry out of the
MSB is not equal to the carry out of the MSB + 1, and clear it otherwise. Multiply low and
divide instructions have OE = 1 set the OV bit if the result cannot be represented in 64 bits
(
mulid
,
divd
,
divdu
) or in 32 bits (
mulw
,
divw
,
divwu
) and clear it otherwise. The OV bit is
not altered by compare instructions that cannot overflow (except
mtspr
to the XER and
mcrxr
).
CA—Carry
This bit is set during the execution of the following instructions:
Add carrying, subtract from carrying, add extended, and subtract from extended
instructions set CA if there is a carry out of the MSB and clear it otherwise.
Shift right algebraic instructions set CA if any 1 bits have been shifted out of a negative
operand and clear it otherwise.
The CA bit is not altered by compare instructions or by other instructions that cannot carry
(except shift right algebraic,
mtspr
to the XER, and
mcrxr
).
Bits 3–24—Reserved
These bits are reserved and should be set to 0.
BCNT—Byte Count for Load/Store String Operations
These bits specify the number of bytes to be transferred by a
lswx
or
stswx
instruction.
XER
BIT
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
FIELD
SO
OV
CA
RESERVED
BIT
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
FIELD
RESERVED
BCNT