3-20
MPC561/MPC563 Reference Manual
MOTOROLA
User Instruction Set Architecture (UISA) Register Set
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.
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.
3.7.6
Link Register (LR)
The link register (LR), SPR 8, supplies the branch target address for the branch conditional
to link register (bclrx) instruction, and can be used to hold the logical address of the
instruction that follows a branch and link instruction.
Note that although the two least-significant bits can accept any values written to them, they
are ignored when the LR is used as an address.
Both conditional and unconditional branch instructions include the option of placing the
effective address of the instruction after the branch instruction in the LR. This is done
regardless of whether the branch is taken.
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.
2
CA
Carry (CA). In general, the carry bit is set to indicate that a carry out of bit 0 occurred during
execution of an instruction. Add carrying, subtract from carrying, add extended, and subtract
from extended instructions set CA if there is a carry out of bit 0, and clear it otherwise. The CA
bit is not altered by compare instructions or other instructions that cannot carry, except that shift
right algebraic instructions set the CA bit to indicate whether any ‘1’ bits have been shifted out of
a negative quantity.
3:24
—
Reserved
25:31
BYTES
This field specifies the number of bytes to be transferred by a Load String Word Indexed (lswx)
or Store String Word Indexed (stswx) instruction.