*scratch*.el

Type BLInst
  Field N:UInt =  1024 * 1024 * 5 ' number of Lisp objects (doubles) to store
  Field hp:ULong = 0 ' heap pointer
  Field sp:ULong = N ' stack pointer    
  Field cell:Double[N]      

  Field nil_val:Double      
  Field quit_val:Double     
  Field tru_val:Double      
  Field err_val:Double      
  Field env_val:Double      
End Type

Function makeBLInst() : BLInst
  ...
End Function

Then add the BInst reference to all functions so they know for which
BLInst object they are working for on their behalf.