360 Assembly/Basic FAQ
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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.
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