Section 3.6

Group 6

The instructions of this group are 'jump' or 'discrimination' instructions.  With function-numbers 60 to 65 a comparison is made between the signed 48-bit numbers y and z while with function-numbers 66 and 67 the comparison is between pY and z.  (In 2-address form z = 0 for all function-numbers).  Depending on the result of this comparison the next instruction obeyed is either that stored in register X or the one stored in the next-higher numbered register to that containing the 'jump' instruction itself.  This is achieved by arranging for the control-number either to be set to X (strictly, to the machine-address represented by X, which may be a Basic or a Symbolic Address) or to be increased by unity from its current value.

The replacement and modification facilities operate in the normal way; in particular the destination address X may be replaced and/or modified.  Such treatment of the X-address yields a 'switch', that is, a single instruction which causes the computer to follow one or another of several possible courses of action, depending on the result of a comparison between two numbers and on the value of a third number (a modifier).  Note that when a jump instruction does not cause a jump, the destination address is not checked for lock-outs or reservations, unless replaced.


Function number 60  [0110 000]

3-address: Jump to X if y = z            2-address: Jump to X if y=0.


Function number 61  [0110 001]

3-address: Jump to X if y ≠ z            2-address: Jump to X if y≠0.


Function number 62 [0110 010]

3-address:  Jump to X if y > z (i.e. z < y)
2-address:  Jump to X if y > 0 (i.e. y positive)


Function number 63  [0110 011]

3-address:  Jump to X if y ¬> z (i.e. z ≥ y)
2-address:  Jump to X if y ¬>0 (i.e. y negative or zero).


Function number 64  [0110 100]

3-address:  Jump to X if y < z (i.e. z ≥ y)
2-address:  Jump to X if y < 0 (i.e. y negative, ys=1)


Function number 65  [0110 101]

3-address:  Jump to X if y ¬< z ( i.e. z ≤ y)

2-address:  Jump to X if y ¬< 0 (i.e. y non-negative, ys = 0)


Function number 66  [0110 110]

3-address:  Jump to X if pY = z

2-address:  Jump to X if pY = 0

Although pY can be the content of any of the pseudo-registers, the most common use of this instruction is to test the state of the overflow indicator, by reference to P4, P5, P6 or P7.


Function number 67  [0110 111]

3-address:  Jump to X if pY ≠ z

2-address:  Jump to X if pY ≠ 0.

See comment under function-number 66.