The “in memory undo” buffer is actually a buffer for the redo that describes the changes to the undo.
If you look at x$kcrfstrand you can see some information about redo threads (buffers), both public and private. In the example below I have 2 public threads and 10 private threads, but four of the private threads have been dynamically disabled so only 6 of the private threads are showing a buffer address.
select indx,
ptr_kcrf_pvt_strand,
from
x$kcrfstrand
;
INDX PTR_KCRF STRAND_SIZE_KCRFA
---------- -------- -----------------
0 00 3494400 public
1 00 3494400
2 20F27C80 66560 private
3 20F38080 66560
4 20F48480 66560
5 20F58880 66560
6 20F68C80 66560
7 20F79080 66560
8 00 66560 dynamically disabled
9 00 66560
10 00 66560
11 00 66560