Section 2.4

The Main Control Desk


The main control desk (sometimes called the console) has a level surface, behind which is a nearly vertical panel on which are the main controls and displays for the computer. On the level surface are, on the operator's left, the main TR5 paper-tape reader (*TRA) and, the operator's right, the main Flexowriter.


On the vertical panel of the main control desk are a number of displays for the use of the maintenance engineers. Most of the controls are also for the engineers' use and are in fact inoperative in the normal Programmers' Mode (see Sec. 2.3.2). The only keys which are normally operative are the Handkeys and the controls for the main tape-reader and Flexowriter (see Sections 2.4.8 and 2.4.9 below).


The Handkeys are a set of 48 single-acting switches arranged in two rows of 24 and spaced and labelled to correspond to the fields in a machine-instruction. The keys are also numbered 0 to 47 to show their bit-positions. We denote the keys thus: H0, H1, ..., H47. If the computer reads from pseudo-register 18 (see Section 2.6) a word is obtained which has 1-bits in those digital positions where the corresponding handkeys are down and 0-bits elsewhere. Pseudo-register 19 contains the inverse word (0-bits where the handkeys are down). It is strongly recommended that programs should not use the handkeys unless this is essential. These keys are primarily intended for engineers' use.


The displays take the form of lights, each corresponding to a bit in a register; the light is on if the bit is a one. These displays are useful only if the machine is stopped or on "slow"; they are therefore of no use in Programmers' Mode. The principal displays are the following:

  1. The Route Map. This occupies the left-hand end of the main control-panel; it is primarily of concern to engineers but includes a few lights of use to programmers, notably a red light which is on when the overflow-indicator is set, a row of 9 lights giving the datum-point and a row of lights showing the settings of the monitor indicators (to show, e.g. whether monitoring on signals is turned on, etc.)
  2. In the centre at the top of the control panel is a set of 48 lights, arranged in two rows of 24, which show the content of G or H (according to the position of a switch to the right of the display). The registers G and H are in the arithmetical unit.
  3. Below the G/H display are two rows of lights showing the contents of further registers in the arithmetic unit, viz. M and K on the left and J on the right (the upper half holding the "write address" and the lower half the control number).
  4. Below this are lights displaying the S, F, R and T bits in the current instruction. (Below these lights are the handkeys.)
  5. At the top towards the right of the control panel is a set of lights displaying the content of L, another register of the arithmetical unit.

The M and K registers are used to hold the mode and k bits, respectively, in the initiation of peripheral transfers (the k bits are the machine address of the peripheral device to be used). These registers are, however, also used for other purposes, in particular K is used to manipulate the shift-number in group 5 instructions and both M and K are used in floating-point instructions (for exponent arithmetic) and in the count instructions (80 to 83). The L register is used in multiplication and division and other instructions with double-length operands.


In Engineering Mode certain controls become operative which are important to those writing certain basic programs. They will therefore be described here. The chief controls are three double-acting keys. Each of these has three positions; up, central and down. The keys are the following:

  1. left-hand key:       Up ("insert") - for storing the handkeys,
                               Central ("automatic") - for obeying stored instructions.
                               Down ("manual") - for obeying the handkeys.
  2. centre key:           Up ("jump to 128") - sets control number to 128,
                               Central - normal (key is spring-loaded),
                               Down ("single step") - to obey one instruction.
  3. right-hand key:     Up ("slow") - to obey instructions at reduced rate
                               Central ("stop") - to stop the computer,
                               Down ("run") - to obey instructions normally.

These keys are mounted centrally near the bottom of the control panel, slightly towards its right-hand side.

Certain combinations of settings of these keys are prevented from having any effect by electronic interlocks (e.g. "single-step" and "run"). In general, one should not move the left or centre keys unless the computer is stopped (i.e. with the right-hand key in its central position). In the following description we suppose that all three keys are initially in their central positions - this will be called the Basic Position.

The control number is displayed in the lower half of J (see 2.4.4c above). Starting from the Basic Position, if the right-hand key is depressed (to "run") the computer will obey instructions from the working store, starting at the address displayed. Returning this key to its central position ("stop") at any time will cause the machine to stop as soon as it has completed the instruction (or compound instruction - see the Glossary, Section 0.3) it was obeying at that instant.

Pushing the centre key up ("jump to 128") while the computer is stopped will set both halves of J to 128, i.e. it will set both the control number and the write address to 128. This key springs back to its central position when released. If this key is pressed down ("single step") from the Basic Position, then the computer will obey a single instruction (or compound instruction) from the working store at the address equal to the control number; the control number will also be altered to the address of the next instruction (normally by having one added to it but it will be increased by more if a compound instruction is obeyed and will be set to a new value by a successful jump). In the case of most of the instructions the result written into the store will at this point be held in register H and can be displayed on the top-most set of lights (see Sec. 2.4.4 b above). The write address, displayed in the upper half of J, shows where this result was written. The S, F, R and T bits of the next instruction are also displayed at this moment (see Sec. 2.4.4 d) and the whole instruction is in register G, which can be displayed instead of H if desired.

Pushing the right-hand key up (to "slow") has the same effect as a series of single step operations following one another at a rate which can be altered by an adjacent knob. A program can be obeyed slowly by using this feature (but 157-instructions will not then cause a stop). It is very important not to try to go too fast when on "slow" as a long instruction (e.g. 142 pair) might then go wrong.

If, starting from the Basic Position, the left-hand key is pushed down (to "manual") then the computer is ready to obey machine instructions set up on the handkeys - these are called manual instructions. Such instructions are normally obeyed one at a time by using single step operations but they can, exceptionally, be obeyed on "slow" or "run". The control number will not be changed by obeying manual instructions (other than successful jumps) so that it is possible to stop a program, obey a manual instruction and then continue with the program. It is not possible to obey compound instructions manually.

Manual instructions may be used to display the content of any working store register. For example, to display the word in the register with machine-address 33 we stop the computer, set up the machine instruction

    04X   0   33

go to "manual" and do a single step operation. The word required is then in H and can be displayed on the upper set of lights.

If one starts from the Basic Position and pushes up the left hand-key (to "insert") then the computer is ready to insert (i.e. store) the word set up on the handkeys into the working store register whose machine address is in the upper half of J (the write address): it will actually insert the word when a "single step" is given; the write address is increased by one immediately afterwards so that another instruction can be set up and inserted into the next register. In this way one can easily insert short sequences of instructions into the working store; this is the foundation of the "bootstrap procedure" (see Sec. 2.4.6 below). It will be recalled that pushing up the centre key ("jump to 128") sets the write address to 128, as well as the control number. (The "insert" process also operates on "slow", which is always equivalent to a succession of single steps.)

To insert words from the handkeys into an address other than 128 it is necessary to be able to set the write address to an arbitrary value. This can be done by obeying a 2-address 74-instruction, which has the effect of setting the write-address equal to the address set up in X on the handkeys. If this is preceded by a manual 2-address 75-instruction to the same address then both halves of J will be set to this address. (Note that the 75 must precede the 74 since a successful jump interchanges the two halves of J.)


The 'bootstrap' procedure is used in Engineering Mode to get programs into the computer without using the built-in programs. This is necessary during commissioning and maintenance. The procedure is usually preceded by a manual 143-instruction to clear the working store.

A program to be read in by the bootstrap procedure must be punched in binary as machine instructions; the paper tape (7-track) or cards will therefore have been prepared earlier by computer. The following description assumes that the program is punched in 7-track paper tape. The basis of the procedure is that a single compound instruction of the form

    140.2    0    K
    142      W    N

can read the whole program into the working store - it will in fact read in N characters from peripheral device K and place them in the store starting at machine address W. The peripheral device is normally the main tape reader but the values to be used for W and N will depend on the program being read in. In order to avoid having to use a different procedure for each program a standard procedure is used which reads in a single 142-instruction (containing the variable data) which is punched at the beginning of the program.

This standard procedure is to insert into 128 and 129 the instructions

    140.2    0     K     (In 128)
    142      129   8     (In 129)

which are then obeyed. These read 8 characters (i.e. a word) from peripheral device K (the main tape-reader) into 129, i.e. over the 142-instruction above. These 8 characters represent the new 142 appropriate to the program being read in. We now have in 128 and 129 the compound instruction needed to read in the program, and this is obeyed.

The standard bootstrap procedure is therefore as follows:

  1. Go to Basic Position (all three keys central).
  2. Load tape in main tape reader with the first non-UC character in the reading position.
  3. Left and centre keys both up (i.e. "insert" and "jump to 128"); centre key springs back.
  4. Set up on the handkeys the instruction:
    140.2  0      K
  5. Centre key down ("single step").
  6. Set up on the handkeys the instruction:
    142    129    8
  7. Centre key down ("single step").
  8. Left key central ("automatic").
  9. Centre key up, then down ("jump to 128" and "single step").
  10. Repeat step 9.

Notes on this procedure (H12 means handkey no. 12):

Step   3: the write address is set to 128.

Step   4: the mode is in the m.s. five bits of X so H12 should be down.

Step   5: the key will spring back when it is released; this operation causes the 140-instruction to be inserted in 128.

Step   7: the 142 is inserted into 129.

Step   8: prepare to obey stored instructions.

Step   9: set jump address to 128 and obey the compound instruction to read new 142 from tape into register 129. Tape in reader moves.

Step 10: obey new form of compound instruction in 128 and 129 to read in the program.

Many of the programs which are read in by means of this procedure are arranged to start in 128 and can therefore be entered by "jump to 128" and "run" on the centre and right-hand keys, respectively. A program which has to be entered at some other machine address can be entered by obeying a manual 75-instruction.


If the computer encounters a 157-instruction in Engineering Mode it will stop, displaying its effective addresses. The computer can be caused to continue at the next instruction by moving the right-hand key to its central position ("stop") followed by either "run" (right-hand key down) or "single step" (centre key down).


Near the bottom of the main control panel, slightly to the left of centre is a group of controls and displays for the main tape-reader and Flexowriter. There is a row of small rectangular lights for each device; some of these lights serve also as push-buttons. As a general rule a button may be pressed if it is lit up in white. The upper row are the controls for the main Flexowriter and consists of four lights as follows (from left to right):-

  1. "Input Selected".  This light is normally white; it turns blue when the computer is reading from the Flexowriter keyboard; the white light on the Flexowriter is then also lit.  If pressed it calls in the Monitor Program to read from the Flexowriter.
  2. "Output Selected". This white light turns blue when the computer is typing (and possibly punching) on the Flexowriter.  If this light is pressed it causes run out (repeated punching of UC) provided the Flexowriter is disengaged.
  3. "Engage".  When white this may be pressed to engage the Flexowriter; it then turns green.
  4. "Disengage".  This is red if the Flexowriter is disengaged.  It is otherwise white, when pressing it will disengage the Flexowriter (e.g. to allow the stationery to be changed).

A "Select" control is also mounted on the Flexowriter itself.  This is marked "Accept".


The lower row of lights are the displays and controls for the main tape-reader. These consist of four lights as follows (from left to right):-

  1. 7-track/5-track. This control is divided into two halves. The upper half displays "7-track" in yellow when the "associated electronics" of the tape-reader are set for 7-track tape. When set for 5- track tape the lower half of the light displays "5-track" in blue. Pressing this light switches the associated electronics from one state to the other (the tape reader itself has to be mechanically adjusted as well).
  2. "Select". This white light is pressed to call in the Monitor Program to read the heading of a tape (see Sec. 5.7). This light turns blue when the reader is selected, i.e. is actually being used by the computer.
  3. "Engage". This button is green when the tape-reader is engaged. When white it may be pressed to engage the reader.
  4. "Disengage". This is red when the tape-reader is disengaged. When white it may be pressed to disengage the reader (for example to change tapes).

The "Disengage" button may be used if a knot of tape is approaching the reader; the "Engage" button can then be pressed to allow reading to continue.

The detail in this section refers only to Orion 1. The control panel for Orion 2 and the actual operation of the machine from the panel (in Engineers Mode) is different. The naming of the control registers for Orion 2 is not the same as Orion 1, so that Orion 2 J's register is not the same as Orion 1's.

The sections that do apply to Orion 2 are 2.4.1, 2.4.2 and, 2.4.8 except for the position of the lights.