Central Processing Unit
MPC561/MPC563 Reference Manual, Rev. 1.2
3-18
Freescale Semiconductor
3.7.5
Integer Exception Register (XER)
The integer exception register (XER), SPR 1, is a user-level, 32-bit register.
The bit descriptions for XER, shown in
, are based on the operation of an instruction considered
as a whole, not on intermediate results. For example, the result of the subtract from carrying (subfcx)
instruction is specified as the sum of three values. This instruction sets bits in the XER based on the entire
operation, not on an intermediate sum.
In most cases, reserved fields in registers are ignored when written to and return zero when read. However,
XER[16:23] are set to the value written to them and return that value when read.
Table 3-9. CR
n
Field Bit Settings for Compare Instructions
CR
n
Bit
1
1
Here, the bit indicates the bit number in any one of the four-bit subfields, CR0–CR7
Description
0
Less than, floating-point less than (LT, FL).
For integer compare instructions, (rA) < SIMM, UIMM, or (rB) (algebraic comparison) or (rA) SIMM, UIMM,
or (rB) (logical comparison). For floating-point compare instructions, (frA) < (frB).
1
Greater than, floating-point greater than (GT, FG).
For integer compare instructions, (rA) > SIMM, UIMM, or (rB) (algebraic comparison) or (rA) SIMM, UIMM,
or (rB) (logical comparison). For floating-point compare instructions, (frA) > (frB).
2
Equal, floating-point equal (EQ, FE).
For integer compare instructions, (rA) = SIMM, UIMM, or (rB).
For floating-point compare instructions, (frA) = (frB).
3
Summary overflow, floating-point unordered (SO, FU).
For integer compare instructions, this is a copy of the final state of XER[SO] at the completion of the
instruction. For floating-point compare instructions, one or both of (frA) and (frB) is not a number (NaN).
MSB
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
LSB
31
Field SO OV CA
—
BYTES
Reset
Unchanged
00_0000_0000_0000_0000_0
Unchanged
Addr
SPR 1
Figure 3-8. Integer Exception Register (XER)
Table 3-10. Integer Exception Register Bit Descriptions
Bits
Name
Description
0
SO
Summary Overflow (SO). The summary overflow bit is set whenever an instruction sets the
overflow bit (OV) to indicate overflow and remains set until software clears it. It is not altered by
compare instructions or other instructions that cannot overflow.
1
OV
Overflow (OV). The overflow bit is set to indicate that an overflow has occurred during execution
of an instruction. Integer and subtract instructions having OE=1 set OV if the carry out of bit 0 is
not equal to the carry out of bit 1, and clear it otherwise. The OV bit is not altered by compare
instructions or other instructions that cannot overflow.
Summary of Contents for MPC561
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