Hello Moayuan,

Now I am trying to port ppcboot to a simple custom board, with ppc405cr, 32M RAM, 4M EEPROM, JTAG, UART.

Welcome!

I can not understand some Marcos in the header file include/configs/xxx.h.

#define CFG_INIT_DCACHE_CS 4 /* use cs # 4 for data cache memory */

there is no OCM in 405CR, so ppcboot needs data cache for initial stacks and data. but what does the "chip select" mean?

As I know, chip select should be used on some peripheral devices, like Flash, NVRAM, why is it needed for data cache which is inside the CPU core?

Is the CS only a concept inside ppcboot? or something according to the hardware?

You are right, the PPC405CR doesn't have the OCM, so the data cache has to be used for the initial stack. The Chip-Select is required to specify a memory region, so the PPC doesn't generate "bus-errors" upon accessing this cached memory region. Please use an unconnected CS (if possible) for this setup.

Best regards, Stefan.