Section 2.3

Modes of Operation


Most of this Reference Manual and the published descriptions of Orion and its programming techniques are concerned with the normal or Programmers' Mode in which the system operates. When an interruption occurs the Monitor Staticisor (also known as HKFF) is turned on and the computer is thrown into the Monitoring Mode, in which part of the Monitor Program operates; the Monitor Program returns the computer to Programmers' Mode by obeying a 152-instruction. The Monitoring Mode can be thought of as a subsidiary mode of the Programmers' Mode.

The following events occur in Monitoring Mode and not in normal Programmers' Mode.

  1. Lockouts have no effect.
  2. There are no interruptions on the completion of peripheral transfers.
  3. The timer is out of action.
  4. The reservation-checking system is inoperative, and the datum-point register is cleared (to zero).
  5. Interruptions due to peripheral incidents are delayed, by being stored in the appropriate peripheral control unit, until return to Programmers' Mode.
  6. The instructions of group 15 (see Sec. 3.15) are valid.
  7. Program failures (e.g. illegal instructions or impermissible operands) cause the instruction to be repeated - they must therefore never be allowed to occur.
  8. The l40-instruction operates differently in that its Y-address contains the machine address (k-bits) of the peripheral device concerned and not the programmer's name.
  9. The 141-instruction similarly specifies the machine-address of the drum location referred to and not the programmer's address (see Sec. 3.14).


There is a key-operated switch on the main control panel of the computer which can be used to put the machine into Engineering Mode. In this mode the computer behaves exactly as in the Monitoring Mode described above but with the following exceptions:

  1. Lockouts remain effective, but a locked-out instruction does not cause an interruption - it is simply repeated, over and over again (without being completed), until the lockout is lifted.
  2. The 152-instruction does not return the computer to Programmers' Mode.
  3. The 157-instruction, which causes the computer to wait (see Sec. 3.15) must not be obeyed in a part of the store which is subject to a lockout (this causes the microprogram pulse to be lost). The same is true of the 153- and 156-instructions.
  4. A set of control keys becomes operative which allow the computer to be stopped, to obey instructions set up on the handkeys and to be operated in other ways necessary for its proper maintenance (see Sec. 2.4.5).

The key to operate the switch may be inserted and withdrawn only when the switch is turned to the Normal (Programmers' Mode) position.


The same key-operated switch referred to in Sec. 2.3.2, above, can also be used to put the computer into Engineers' Time-Sharing Mode.  The machine then behaves exactly as in normal Programmers' Mode (or Monitoring Mode, since interruptions are allowed) except that the engineers' control keys described above are operative.  This mode facilitates maintenance of the time-sharing features of the system.


The information in this Section refers in general to both Orion 1 and Orion 2. Described here are the differences to be noted when reading these sections for Orion 2.

Section 2.3.1.

Subsection (d)   On Orion 2 the datum point is ignored not cleared.

Section 2.3.2.

Subsection (c)   On Orion 2 no pulse is lost and so these instructions may be obeyed in these circumstances.