In the description of the Hack machine language in chapter 4, it is stated that in well-written programs a C-instruction that may cause a jump should not contain a reference to M, and vice versa. Discuss why this should be avoided.

The last instruction (0; JMP) effects an unconditional jump. Since the C-instruction syntax requires that we always effect some computation, we instruct the ALU to compute 0 (an arbitrary choice), which is ignored.
The programmer can use the A register to select either a data memory location for a subsequent C-instruction involving M, or an instruction memory location for a subsequent C-instruction involving a jump. Thus, to prevent conflicting use of the A register, in well-written programs a C-instruction that may cause a jump (i. e., with some non-zero j bits) should not contain a reference to M, and vice versa.