360 Assembly/Basic FAQ

This attempts to define terms related to the IBM 360 and similar mainframes.

• Base-displacement. Addresses on the 360 series mainframe which are given in an instruction are given in base-displacement form, in which the address is referenced by a 4-bit number representing one of the 16 registers, numbered 0 to 15, which is used as a base, and a 12-bit index which is added to the contents of that register (unless that base register is register 0), as a displacement. The value of the displacement, plus the contents of the specific register, create the base-displacement address.
• Op code. The first byte of an instruction determines what instruction it is. This byte is called the op code.
• Program check. An error in a program that causes hardware to stop its execution as an exception. A program will be terminated unless the program has issued a request to the operating system to trap the exception. Program checks include any of the following:
  • Data exception. Program has attempted to perform packed arithmetic upon invalid packed data.
  • Divide exception. Program has attempted to divide by zero.
  • Execute exception. Program has attempted to use an EXECUTE instruction as the target of an EXECUTE instruction
  • Operation exception. Program has attempted to execute an invalid instruction.
  • Privileged operation exception. Program has attempted to execute a privileged or I/O instruction.
  • Protection exception. Program has attempted to read or write memory it is not allowed to access.