[ixp1200] [In ixp2400/2800 programming] chapter 5. nextblock?

[paragon]

Hello.
Dear ixp1200.

Present,  not trasmitted from RX to TX in enp-2611(ixp2400).

To see in chapter 5, 
    373    immed32(dl_next_block, SPI4_RX_NEXT_BLOCK)

What's Nextblock?

#define SPI4_RX_NEXT_BLOCK  0x24

what's difference in ixp2400?

In chapter 5, what's difference between ixp2400 and ixp2800?
How to modify in chapter.5 ?

thank you~~^^.