A set of tools and programs to manage the overall working of a computer using a defined set of hardware components is called an operating system.
Example:
1. DOS
2. Windows.

1. Purchase of goods for sale
2. Administrative, selling and distribution expenses
3. Manufacturing expenses

Purposes of subsidiary books:
(i) Purchases Book: It records only credit purchases of goods by the trader.
(ii) Sales Book: It records only credit sale of goods by the trader.
(iii) Purchases Returns Book: It records the goods returned by the trader to the suppliers.
(iv) Sales Return Book: It records the goods returned by the customers.
(v) Bills Receivable Book: It records the receipt of bills from customers.
(vi) Bills Payable Book: It records the bills accepted by the trader.
(vii) Cash Book: It records all the cash transactions of the business. (i.e.,) the receipts and payments of cash.
(viii) Journal Proper: It is the journal which records entries which cannot be entered in any of the subsidiary books.

Need for preparing trial balance:
(i) Trial balance helps to check the arithmetical accuracy of entries made in the accounting records.
(ii) Trial balance serves as a lubricant for the smooth movement and completion of the accounting cycle.
(iii) Trial balance helps in discovering errors which may have been committed in the accounting records.

He will be maintaining the following other books to be maintained:
1. Triple column each book.
2. Petty cash book (Analytical).
3. Purchase book for credit purchase
4. Sales book for credit sales.
5. Purchase returns book.
6. Sales returns book.
7. Business book account (i.e. current account to the maintained).
8. Proper journal for other assets maintaining:
(i) All cash transactions book recorded is cash book.
(ii) All petty expenses are to the recorded is analytical petty cash book.
(iii) All credit transactions to the recorded is special purpose books (i.e. purchase book, sales book, purchase return book and sales return book and proper journal.