Unnecessary SORT on Order By DESC

select * from RESR where REG between 'A' and 'Z' order by REG; select * from RESR where REG between 'A' and 'Z' order by REG DESC;

RESR has INDEX REGINDEX on REG

I don't understand way ORACLE need to sort the values in the 2nd SELECT (which is very slow on a big database)
ORACLE has a Index on that field all he need to do is to go backword! Any idea how to improve the proformance of the 2nd query?

My ideas is
1. Create another index by REG DESC.
2. Limit to number of record included in the Select to say to 100 if all I need is just a limited number of records, How you do samething like that?

TIA Muly.

The show plan of the commands:

select * from RESR where REG between 'A' and 'Z' order by REG;

OPERATION                   OPTIONS                       OBJECT_NAME
------------------------------ ------------------------------ ------------
SELECT STATEMENT
TABLE ACCESS             BY ROWID                      RESR
INDEX                               RANGE SCAN                REGINDEX

select * from RESR where REG between 'A' and 'Z' order by REG DESC;

OPERATION                 OPTIONS                        OBJECT_NAME
------------------------------ ------------------------------ -----------
SELECT STATEMENT
SORT                              ORDER BY
TABLE ACCESS           BY ROWID                       RESR
INDEX                             RANGE SCAN                REGINDEX