Embedded Linux Primer

I bought "embedded Linux Primer" and read chapters 4 and 16. To build kernel source tree using command line as

make ARCH=ppc CROSS_COMPILE=ppc_85xx mpc8540_ads_defconfig

and compile a linux kernel for ppc.
make ARCH=ppc CROSS_COMPILE=ppc_82xx uImage

My question is how do I know I should use ppc_85xx and ppc_82xx?

----henry zhang