一份带前后值的 trace

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]
    seen = []
    for w in prog:
        op, rd, imm = decode(w)
        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
        seen.append(R[0])
    return seen

R = [0, 0]
rows = []
for w in PROG:
    op, rd, imm = decode(w)
    if op == 3:
        break
    before = R[0]
    if op == 0:
        R[rd] = imm
    elif op == 1:
        R[rd] = (R[rd] + imm) & 255
    else:
        R[rd] = (R[rd] - imm) & 255
    rows.append(str(before) + ">" + str(R[0]))
print("|".join(rows))