Answer:

use temp 

use zero 

lim zero 0 

label loop 

lim jump_target done 

jeq jump_target rest zero	// stop if at end of list 

load temp rest			// load next list value 

add sum sum temp		// add value to sum 

aim rest +1			// get address part of cell 

load rest rest			// load pointer to next cell 

lim jump_target loop		// go back for more 

junc jump_target 

label done