Study plan varies from person to person but if i am in ur place I would have tried to complete dcld first , then COA and then OS and would have give 5 to 6 days for each subject.
DCLD is easy 1 day for KMAP, 2nd day for combinational circuit, 3rd day for sequential and 4th day for no. system and IEEE floating point conversion. 5th day revision.
COA is little tough 1st day do disk scheduling, 2nd day cache memory , 3rd day pipelining ,4th day Addressing modes and 5th day ALU and data paths.
OS :- 1st day paging, 2nd day scheduling, 3rd day process synchronization ,4th day memory management , 5th day file system.
Just do previous yr questions along with theory.
By doing this ....ur 3 subjects would be roughly completed till 1st week of jan.
Then u should only focus on revision and full lenght test.
CN is a lengthy subject ...do it if u get time otherwise u should leave it.It took me almost 1 month to learn it and still i think my concepts are weak in this subject.
note:- these are just my opinions.