请教问题

[fenghaoxuan99]

always @ (posedge clk) begin
{y_e, y_data} <= 0;
if (i_sof) begin
{y_bbuf, y_bcnt, y_eof} <= 0;
end else begin
bbuf = y_bbuf | ({x_bb,223'h0} >> y_bcnt);
bcnt = y_bcnt + {1'd0,x_bn};
if (bcnt >= 9'd64) begin
y_e <= 1'b1;
for (i=0; i<8; i=i+1) begin
y_data[i*8+:8] <= bbuf[407:400];
if (bbuf[407:400] == 8'hFF) begin
bbuf = {1'h0, bbuf[399:0], 7'h0};
bcnt = bcnt - 9'd7;
end else begin
bbuf = { bbuf[399:0], 8'h0};
bcnt = bcnt - 9'd8;
end
end
//y_data <= bbuf[407:344];
//bbuf = {bbuf[343:0], 64'h0};
//bcnt -= 9'd64;
end
y_bbuf <= bbuf;
y_bcnt <= bcnt;
y_eof <= x_eof;
end
end
码流拼接阶段中，bbuf[407:0]的大小使用了408位，请问为什么是这个大小呢？
我注意到每个时钟周期通过y_data输出64位数据，请问为什么刚好是这个大小呢？