My project is using a 9S12XEQ384, which the data sheet teels me has 24K of RAM. The data sheet does not specifically state how much of that RAM is available to the XGATE. Nor did any of the app notes I found (searched for XGATE)
In section 3.4.2.4, example 3-3:
The MCU FLASHSIZE is 64 Kbytes (0x10000) and MCU RAMSIZE is 32 Kbytes (0x8000).The XGATE RAMSIZE is 16 Kbytes (0x4000).
This implies that not all of the 24K is available to the XGATE. It suggests that maybe only 12K is available.
How much is available?
XGATE can access up to 32kB of onchip RAM. Unless you have more RAM than that (64k on S12XEP100), all RAM is available to XGATE.