Motorola MPC823e Reference Manual page 1300

MPC823e Instruction Set—stwcx.
B-142
If a reservation exists, but the memory address specified by the
stwcx. instruction is not the same as that specified by the load
and reserve instruction that established the reservation, the
reservation is cleared, and it is undefined whether the contents
of rS are stored into the word in memory addressed by EA.
If no reservation exists, the instruction completes without altering
memory.
The CR0 field is set to reflect whether the store operation was
performed as follows.
CR0[LT GT EQ S0] = 0b00 || store_performed ||
XER[SO]
EA must be a multiple of four. If it is not, either the system
alignment exception handler is invoked or the results are
boundedly undefined.
The granularity with which reservations are managed is
implementation-dependent . Therefore, the memory to be
accessed by the load and reserve and store conditional
instructions should be allocated by a system library program.
Other registers altered:
Condition Register (CR0 field):
Affected: LT, GT, EQ, SO
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