四条指令的 PC 轨迹

运行下面这段程序，填写它打印出来的结果。

PROG = [0b00001001, 0b01000111, 0b10000011, 0b11000000]

def decode(w):
    return (w >> 6) & 3, (w >> 5) & 1, w & 31

def run(prog):
    R = [0, 0]
    pc = 0
    seen = []
    while pc < len(prog):
        op, rd, imm = decode(prog[pc])
        if op == 3:
            break
        if op == 0:
            R[rd] = imm
        elif op == 1:
            R[rd] = (R[rd] + imm) & 255
        else:
            R[rd] = (R[rd] - imm) & 255
        pc = pc + 1
        seen.append(R[0])
    return seen

pc = 0
seen = ""
while pc < len(PROG):
    op, rd, imm = decode(PROG[pc])
    seen = seen + str(pc)
    if op == 3:
        break
    pc = pc + 1
print(seen + "/" + str(pc))