Write a MIPS program that prompts the user for a hexadecimal digit and then displays the decimal equivalent multiplied by 2. For example, the program displays 10 for hex '5', 20 for 'A', etc. If the user enters an invalid value, the program should display an error message and asks the user to try again.

Write a MIPS program that prompts the user for a 4-bit binary number, reads it as a string, and outputs its decimal equivalent. When calculating the decimal value, use a loop to process the 4-character input string. Assume the user enters a valid binary number. Save Program and Modify your code as to display the hexadecimal representation of the binary number instead of the decimal value. Assume the user enters a valid binary number. 

Write a MIPS program that reads from the keyboard a string of no more than 20 characters and then outputs the string in a single line even if the input is written on multiple lines. The input string will be terminated by an character '0', which doesn't count as part of the string.