MOTOROLA
Chapter 3. Central Processing Unit
3-19
User Instruction Set Architecture (UISA) Register Set
3.7.4.3
Condition Register CRn Field — Compare Instruction
When a specified CR field is set by a compare instruction, the bits of the specified field are
interpreted as shown in
Table 3-9. A condition register field can also be accessed by the
mfcr, mcrf, and mtcrf instructions.
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
Table 3-10, 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.
2
Floating-point invalid exception (VX).This is a copy of the final state of FPSCR[VX] at the completion of the
instruction.
3
Floating-point overflow exception (OX).This is a copy of the final state of FPSCR[OX] at the completion of
the instruction.
Table 3-9. CRn Field Bit Settings for Compare Instructions
CRn 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
123456789
10
11
12
13
14
15
16
17 18 19
20
21
22
23
24
25 26 27 28
29
30
LSB
31
SO
OV
CA
Reserved
BYTES
RESET:
U
UUUUU 0000
0000
00000000
000
U U U U U U
U
Figure 3-8. XER — Integer Exception Register
Table 3-8. Bit Settings for CR1 Field of CR (continued)
CR1 Bit
Description