How did MacLisp's garbage collector "run in the register set"? Olin Shivers, ‘History of T’:


  Maclisp on the [PDP]-10 had used a mark&amp;sweep GC (one version of which famously "ran in the register set," though that is another story)


This implies, in my interpretation, that the garbage collector used no storage other than the registers of the machine. Is my understanding of this correct, and if so, how did it achieve this efficiently? Was a paper ever published on its implementation?
The 16 high-speed registers of the PDP-10 ISA were addressible as locations 0-15 of physical memory, so that the code literally could run in the registers if it was short enough and you could devote a block of registers to it.  Not only could it use registers for temporary storage, it could use them as a code cache as well.