Mark,
The distinction between a stack frame as against no stack frame is the one you are looking for here. A leaf procedure that only uses mnemonics can usually be written without a stack frame whereas a procedure that calls other procedures or locals usually has a stack frame. The choice here is to either use a stack frame of not use one. There are a couple of macros that control this which are simple enough to use.
NOSTACKFRAME ; turn the stack frame off
STACKFRAME ; turn it back on again.
You put these before and after a procedure that does not have a stack frame.
Now with the integer registers, you can modify rax rcx rdx r8 r9 r10 r11, the rest need to be protected so that they are the same on procedure exit and with MASM, avoid using push/pop as the preferred method is to use local variables to save and restore registers.
64 bit MASM does not use prototypes which is a blessing in that you don't have to write them but it means you must get the argument count right.
Now with calling procedures, you can manually call simple procedures with up to 4 arguments by writing the arguments in the order of rcx rdx r8 & r9 and using the CALL mnemonic but with a higher argument count, you are better off to use the "invoke" technique as arguments 5 and up are written to the stack and can be messy and complicated to do manually. This is due to the structure of the Microsoft FASTCALL calling convention.
LATER : Something I should have added, a good starter debugger/disassembler is called ArkDasm
http://www.arkdasm.com/ and it was very useful to me when I first started to work on 64 bit MASM. There are more complex and more powerful ones around but this one is simple and its dis-assembler will show you much of what you are looking for.
