+---------+ <---- Up to 124K words (18-bit addressing)
        |         |
        |         |       Space available for use as VM
        |      <--------- volume if base address is set
        |         |       at 60K-word boundary
        |         |
        +---------+ <---- 60K-word boundary
        |         |
        |      <--------- Space available for use by XM programs
        |         |
        +---------+ <---- 28K-word boundary
        | RMON,   |
        | low     |
        | memory  |
        |         |
        +---------+