nomemory means that operations with ebx and eax, edx registers doesn’t touch memory, only register-to-register copies. Really this code is bad - it doing write to memory with ebx as index.
parm - means that argument is ebx.
modify means that eax and ebx may be or not may be modified and compiler anyway need to save content of that registers.
exact means that eax and edx registers will be exactly modified. Used to help compiler with analyzing.
For example with Watcom 11 you can do the following without memory access:
unsigned __int64 rdtsc( void);
#pragma aux rdtsc = “db 0x0F 0x31” value [edx eax] parm nomemory modify exact [edx eax] nomemory;
and use it just as: