Hi There!
There have been source codes for HIMEM.SYS!
But they are a bit old. (Well himem.sys is also a bit)
If you want to learn from them, then i suggest you to read the sources
of PMODE 3.07 by Thomas Pytel which is a DOS-Extender in ASM and so
also handles XMS and Raw/Real Mode Protected Mode Handling and is released
with full sources and for free.
Ok, if you don't know where to get this or the himem.sys-docs, than you  
can send me a mail.
Mirko Hannemann
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