DLL load fails although instructions appear valid

[paml27]

I recently moved to 64-bit NASM from 32-bit MASM, and I have a simple addressing problem. 

I am writing a DLL in NASM.  The following two lines assemble and link with the NASM assembler / GoLink, but when I try to load the DLL, I get an OS error "invalid access to memory location."  RCX contains a pointer that is passed into the DLL: 

mov rax,254
mov [rcx+8],rax

When I try it this way, it also doesn't work: 
mov rax,254
mov [rcx+8],qword rax

The rest of the DLL is fine, just those two lines cause a load failure. 

Thanks for any ideas. 

[dreamCoder]

Hard to tell from this snippet alone. Need more information. It could be that RCX is by itself not a valid pointer. It could be something else, like stack alignment, register corruptions and stuff. 64-bit is a different beast.