The first question you have to ask is, where do I copy them to? Well the only address you have that is valid in that userspace is the stack pointer.
What you are going to want to do is use the stack to store the arguments and then give the user program a modified stack pointer.
You are going to need to count the total number of strings you bring in as well as the total number of character (including the null terminators).
Take the total size of what you need to store, (num_strings * size of a char pointer) + (num_of_chars) and subtract this from the stack pointer (stack grows down) . Now that you have reserved the size you need you can write to it. Remember that pointer access to an array grows upwards so write the first pointer of your list to stackptr - total size.
Interestingly you want to give the user program the new stack pointer so it doesn't over write your data and you also need to give it a pointer (in userspace) to the argument list. These are the same thing!
Here is a picture of the stack that should make all of this more clear.
Remember the stack pointer must be aligned to 4 bytes so round up to the next highest multiple of 4.
Wrote this one quick. If you have any questions leave a comment. There may be other ways of doing this but beware DumbVM does some fairly dumb things so make sure you fully understand what is going on because if you are reusing physical memory unintentionally your values may work now but not when you implement paging.
- FlounderingZ
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