The tape the register channel hands over is the state the run reads

A program that lays its own file arrives at its evaluation with a background built by the sweep; a program that is handed its file has that background from time zero. This file says the two agree: the marks the register channel writes are, slot for slot, what DescriptiveComplexity.Draw.Data.ixBack reads off the entry state at the handed file.

Only two slots need an argument, and both are about the file's ends:

• regFirst – at this channel the first register is not the least element of the universe but the greatest carrying no argument block, which is why the mark is DescriptiveComplexity.Draw.regSlotMark and not DescriptiveComplexity.Draw.slotMark (isTopNonArg_iff_least_marked);
• regLast – the last register is the last element, the argument tags being the greatest (isGreatest_iff_greatest_marked).

Everything else is a tag decision the layout repeats (the block one-hots, the name slots, the padding flag) or a track that starts clear.

The two ends of the handed file

The tape at time zero is the entry state's background

What the marking gives the run