The three registers that are visible to the assembler programmer are A, S and PC. The others (MAR, MBR and TMP) hide behind the scenes and cannot be changed explicitly by an assembler program, anyway. The PC register can never have a negative number in it, so its value is shown simply as an unsigned decimal number. Notice also the four condition bits: C, N, V and Z which show the values on these lines coming out of the ALU after the most recently executed instruction.