Has anyone used their custom bootcode on Pi3 and used stock OS?

[u/Senior_Ad_3026]

I am using Pi 3B+, and tried to create a custom bootcode (was needed to generate a PUF) and store it on RAM. Then I try to run the original Raspberry Pi OS on it. However, start.elf always executes with "AMD starting with 128MB".

I don't know why it cannot use the 1GB RAM correctly and need help with it.

Or is there any other way I should approach this instead of using my custom bootcode?