I've got a board that has 512MB of RAM and using a PowerPC processor. From what I've been reading the PowerPC uses a relative jump scheme which is based on the PC which means it can only jump +/- 32MB. In the config.c and Makefiles it says to specify the RAM_LOW_ADRS and RAM_HIGH_ADRS which I've done as follows.
LOCAL_MEM_LOCAL_ADRS = 0x0 = 0
RAM_LOW_ADRS = 0x00010000 = 65k
RAM_HIGH_ADRS = 0x02000000 = 32MB
When the system powers up it will run out of ROM and uncompress the remaining section of the Bootrom at the RAM_HIGH_ADRS. The bootrom does it thing and initialize the hardware and then places a vxWorks Image (text, data & bss) at the RAM_LOW_ADRS.
The remainder of the memory 480MB (i.e. 512- 32mb) give or take is available as the system memory pool.