hello bmsp,
the bootloader allocates a dedicated carveout, TZ-DRAM, to run a secure OS.
please refer to security session, Trusty, a Trusted Execution Environment for reference.
BTW,
here’re discussion threads you should also check as see-also.
for example,
The difference between TZDRAM and TZRAM - #7 by JerryChang
Where is TZDRAM? - #3 by JerryChang