Hello again...

XV5080 uses a 32 bit CPU actually addressing 1 GB or memory space. Due to hardware MMU limitation and operating system resources the real space is 512 MB shared between RAM and ROM. As you know most of that memory space is ROM-only.
64 MB for onboard samples
320 MB for SRX & SRJV samples
128 MB for SIMM addon RAM

There is some 8 MB onboard RAM that share its address with the last add on card's rom when it's inserted. Actually this space of 8 MB is useless for anything but SCSI sample read buffering. So the 128 MB simm addon memory is a must if you really want to use s700,s1000,AIFF,WAV samples.

The rest of onboard memory (mostly ROM) is used by OS and FX/DSP software algorithms.