3-20
MPC565/MPC566 Reference Manual
MOTOROLA
User Instruction Set Architecture (UISA) Register Set
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.
3.7.7
Count Register (CTR)
The count register (CTR), SPR 9, is used to hold a loop count that can be decremented
during execution of branch instructions with an appropriately coded BO field. If the value
Table 3-10. Integer Exception Register Bit Descriptions
Bit(s)
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 to one 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.
MSB
0
1
2
3
456789
10
11
12
13 14 15
16
17
18
19
20
21 22 23 24
25
26
27
28
29 30
LSB
31
Branch Address
RESET: UNCHANGED
Figure 3-9. LR — Link Register