2.14.3 Automatic Teller Machines and Bank accounts

• An ATM or cashmachine allows transfer of funds between accounts.
• Accounts are held at various machines belonging to different banks
• Accounts offer the following operations

  deposit

  Place an amount of money in an account

  withdraw

  Take an amount of money from an account

  balance

  Get the current value in an account

• Operations implemented as read() and write() of values, so withdraw x from A and deposit x in B implemented as
  1. A.write(A.read() - x)
  2. B.write(B.read() + x)