
MOTOROLA
2-10
REGISTERS
RCPU
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REFERENCE MANUAL
2.2.4.3 Condition Register CR
n
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 2-6
. A condition register field can also be
accessed by the
mfcr
,
mcrf
, and
mtcrf
instructions.
2.2.5 Integer Exception Register (XER)
The integer exception register (XER) is a user-level, 32-bit register.
The SPR number for the XER is one. The bit definitions for XER, shown in
Table
2-7
, are based on the operation of an instruction considered as a whole, not on in-
termediate results. For example, the result of the subtract from carrying (
subfc
x
)
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 and return zero
when read. However, XER[16:23] are set to the value written to them and return
that value when read.
Table 2-6 CR
n
Field Bit Settings for Compare Instructions
CR
n
Bit
1
NOTES:
1. Here, the bit indicates the bit number in any one of the four-bit subfields, CR[0:7]
Description
0
Less than, floating-point less than (LT, FL).
For integer compare instructions, (
r
A) < SIMM, UIMM, or (
r
B) (algebraic comparison) or (
r
A) SIMM,
UIMM, or (
r
B) (logical comparison).
For floating-point compare instructions, (
fr
A) < (
fr
B).
1
Greater than, floating-point greater than (GT, FG).
For integer compare instructions, (
r
A) > SIMM, UIMM, or (
r
B) (algebraic comparison) or (
r
A) SIMM,
UIMM, or (
r
B) (logical comparison).
For floating-point compare instructions, (
fr
A) > (
fr
B).
2
Equal, floating-point equal (EQ, FE).
For integer compare instructions, (
r
A) = SIMM, UIMM, or (
r
B).
For floating-point compare instructions, (
fr
A) = (
fr
B).
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 (
fr
A) and (
fr
B) is not a number (NaN).
XER
— Integer Exception Register
SPR 1
0
1
2
3
24 25
31
SO
OV
CA
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
BYTES
RESET: UNCHANGED
F
Freescale Semiconductor, Inc.
n
.