It is designed to run in real mode. The NT bootblock probably even loads it as if it was .COM, but it is too large for DOS's .COM loader. The actual binary is c

It is designed to run in real mode. The NT bootblock probably even loads it as if it was .COM, but it is too large for DOS's .COM loader. The actual binary is concatenation of two files, there is realmode part (on XP SP3 it is 0x4cd0 bytes long, most of which are either zeros or text strings, including the aforementioned error message) and rest of the file is 32 bit PE binary, complete with "This program cannot be run in DOS mode" stub. The PE binary part is obviously somewhat special because it has to be able to switch to real or vm86 mode in order to call BIOS and run NTDETECT.COM, which is valid DOS .COM (from quick look at the code it even looks like it takes command line arguments) except the obvious fact that it does not do any DOS calls. Edit: The message in the XP's NTLDR says that it needs 512kB of conventional memory (I think that I've seen the 639kB number on NT4 or 2000) and there is also message along the lines "Windows NT needs 7MB of expanded memory", which is also somewhat interesting given that BugCheck 0x0000007D INSTALL_MORE_MEMORY is defined as "more than 5MB" and the fact that when you boot NT normally on computer with 4MB RAM you get the 0x0000007D BSOD, not the message from NTLDR.