I viewed, that the DOS version of Fasm uses only conventional memory for additional_memory, which may be enough for most purposes, but not to compile the Fasm Win32 IDE, which needs more than ~80000h bytes of additional_memory.

I fixed my version of Fasm (DOS) in a way which is similar to the LINUX/Windows version:
It now uses 3/8 of available xms for additional_memory (which is much more than conv. memory can provide) and the remaining 5/8 for the other memory.

It falls back to the current allocation strategy if less than 4Mb were detected.

A shot in the dark: Try using CWDPMI extender.

This is already implemented since 1.51 - DOS version checks whether it could gain more memory for the additional block by dividing the extended memory and in such case does it.

Sorry, got my head up in the sky again...