[TUHS] Hello World compiled in v1/v2

Noel Chiappa jnc at mercury.lcs.mit.edu
Mon Jul 14 09:21:20 AEST 2014

    > From: Dave Horsfall <dave at horsfall.org>

    > crt0.s -> C Run Time (support). It jiggers the stack pointer in some
    > obscure manner

It's the initial startup; it sets up the arguments into the canonical C form,
and then calls main(). (It does not do the initial stack frame, a canonical
call to CSV from inside main() will do that.) Here are the exact details:

On an exec(), once the exec() returns, the arguments are available at the
very top of memory: the arguments themselves are at the top, as a sequence of
zero-terminated byte strings. Below them is an array of word pointers to the
arguments, with a -1 in the last entry. (I.e. if there are N arguments, the
array of pointers has N+1 entries, with the last being -1.) Below that is a
word containing the size of that array (i.e. N+1).

The Stack Pointer register points to that count word; all other registers
(including the PC) are cleared.

All CRT0.s does is move that argument count word down one location on the
stack, adjust the SP to point to it, and put a pointer to the argument
pointer table in the now-free word (between the argument count, and the first
element of the argument pointer table). Hence the canonical C main() argument
list of:

	int	argc;
	int	**argv;

If/when main() returns, it takes the return value (passed in r0) and calls
exit() with it. (If using the stdio library, that exit() flushes the buffers
and closes all open files.) Should _that_ return, it does a 'sys exit'.

There are two variant forms: fcrt0.s arranges for the floating point
emulation to be loaded, and hooked up; mcrt0.s (much more complicated)
arranges for process monitoring to be done.


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