Motorola MPC823e Reference Manual page 417

15.5.4.2.5 Loop Control. The LOOP bit in the RAM word allows you to run a repetitive
program fragment a specific number of times. The first time the LOOP bit is asserted in a
RAM word, the memory controller recognizes it as a Loop Start Word. At this time, the
memory loop counter is loaded with the corresponding contents of the Loop field shown in
Table 15-6. See the appropriate machine mode register (MxMR) for more information. The
next RAM word encountered with the LOOP bit set is recognized as a Loop End Word. At
this time, the memory loop counter is decremented by one.
Continued loop execution depends on the memory loop counter. If the loop counter is not
zero, the next RAM word executed is the Loop Start Word. Otherwise, the next RAM word
executed is the word following the Loop End Word. Loops can be sequentially executed, but
not nested.
REQUEST SERVICED
Read Single Beat Cycle
Read Burst Cycle
Write Single Beat Cycle
Write Burst Cycle
Periodic Timer Expired
15.5.4.2.6 Exception Handling. When an access to a memory device is initiated by the
MPC823e under UPM control, the external device may assert the TEA, SRESET, or
HRESET signal. An exception occurs when one of these signals is asserted by an external
device and the MPC823e begins closing the memory cycle transfer. When an exception is
recognized and the EXEN bit is set in the RAM word, the next RAM word will branch to the
special exception start address (EXS). See Table 15-4 for more information. You must
provide an exception handler to handle output signals controlled by the UPM. For DRAM
control, you would provide a handler that would negate RAS and CASx to prevent data
corruption. The EXEN bit is similar to an exception mask in that if it is 0, then it defers the
exception and continues executing. If a RAM word is encountered with the EXEN bit set, the
UPM performs a branch to the exception start address. When the branch to the exception
start address is performed, the UPM continues reading until the LAST bit is set in the RAM
word.
MOTOROLA
Table 15-6. MxMR Loop Field Usage
MPC823e REFERENCE MANUAL
Memory Controller
MxMR Loop Field
RLFx
RLFx
WLFx
WLFx
TLFx
15-59