Holds one of the following:

• address of the next character in the message being written on a blackboard or spoken
• coordinates of the next location to move to in pursuit of ERIC
• number of movements remaining to navigate a staircase ($AC)
• walkabout or intermediate destination x-coordinate ($AC)
• walkabout or intermediate destination pace counter ($AD)
• number of movements remaining to wipe the blackboard ($AC)
• x-coordinate of the next column of the blackboard to wipe ($AD)

Used by the routines at $09E9, $0A58, $0C3F, $0E35, $108B, $1129, $12B5, $136F, $13F3, $16E9, $1A04, $1EE3, $1FA0, $2241, $246B, $2500, $2757, $277D, $28BC, $2E13 and $BE09.