Modify Index and Skip
Organizational graphic tag to help group instructions



Short and Long format of instruction

The purpose of this instruction is:

  1. To modify (increment or decrement) the contents of an index register (1, 2, or 3), the contents of the instruction-address register, or the contents of a word in core storage, and
  2. To skip a word or to branch when the location being modified (except the instruction-address register contents) either changes sign or is zero after the modification.

Note: In no case are the contents of the accumulator, its extension, the overflow indicator, or the carry indicator modified or changed.

A skip causes the program to skip over the next word in storage and go to the second word in sequence. This means that when this instruction could cause a skip it should be followed by a short-format instruction. If a long-format instruction were to follow, a skip would send the program to the second word in the instruction and a programming error would result. A skip does not occur if the contents of the modified location do not change sign or do not go to zero as a result of the modification.

Short-Format Description

The displacement is expanded to 16 bits by duplication of the sign bit eight positions to the left of the high-order position. The expanded displacement is added to the register specified by the tag bits of the instruction as follows:

Tag Bits   Operation
00   Displacement added to instruction-address register
01   Displacement added to index-register 1
10   Displacement added to index-register 2
11   Displacement added to index-register 3

When the tag bits are 00, this instruction performs a no-operation or modifies the instruction-address register, depending on the value of the displacement. Because the instruction-address register contains the address of the next instruction, a displacement of zero accesses the next instruction. Any displacement results in a branch to the modified address. The displacement can be either positive or negative.

A typical operation is:

  1. Assume:
    1. Tag bits = 01, specifying index-register 1.
    2. Index-register 1 contains FFFF (a minus one).
    3. The displacement is 04.
  2. The expanded displacement is added to the contents of index-register 1:
      FFFF    From XR1
     +0004    Expanded displacement
      0003    Result in XRl
  3. A skip of one word occurs because the sign of the contents of XR1 changes from negative to positive.

Long-Format Description

Modification is accomplished according to the tag and modifier bits or indirect-address fields of the instruction. If the tag bits are 00, the expanded displacement (bits 8 through 15 of the first word of the instruction) is added to the contents of the storage location specified by the address field in the instruction. The displacement is expanded to 16 bits by duplicating the sign bit eight positions to the left of the high-order position. If the tag bits are not 00, the contents of the address field of the instruction are added to the contents of the index register that is specified by the tag bits.

Tag Bits   Index Register
01   1
10   2
11   3

Indicators: The carry and overflow indicators are not affected by execution of a modify-index-and-skip instruction.


Modify Index and Skip

Assembler Language Coding Hexadecimal Value Description of Instruction
Label   Operation   F T    
21 25
27 30
  32 33   35..40..
    MDX         DISP 70XX Add expanded DISP to I (no skip can occur)
    MDX     1   DISP 71XX Add expanded DISP to XR1
    MDX     2   DISP 72XX Add expanded DISP to XR2
    MDX     3   DISP 73XX Add expanded DISP to XR3
    MDX   L     ADDR,DISP 74XXXXXX Add expanded positive or negative DISP to CSL at Addr (Add to memory)
    MDX   L 1   ADDR 7500XXXX Add Addr to XR1
    MDX   L 2   ADDR 7600XXXX Add Addr to XR2
    MDX   L 3   ADDR 7700XXXX Add Addr to XR3
    MDX   I 1   ADDR 7580XXXX Add V in CSL at Addr to XR1
    MDX   I 2   ADDR 7680XXXX Add V in CSL at Addr to XR2
    MDX   I 3   ADDR 7780XXXX Add V in CSL at Addr to XR3

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