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

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~~^^.