https://homeostasis.scs.carleton.ca/wiki/index.php?action=history&feed=atom&title=Operating_Systems_2019F_Lecture_9 2026-05-18T21:41:56Z Revision history for this page on the wiki MediaWiki 1.42.1 https://homeostasis.scs.carleton.ca/wiki/index.php?title=Operating_Systems_2019F_Lecture_9&diff=22485&oldid=prev 2019-10-03T17:29:15Z

<p>Created page with &quot;==Video== Video from the lecture given on October 2, 2019 [https://homeostasis.scs.carleton.ca/~soma/os-2019f/lectures/comp3000-2019f-lec09-20191002.m4v is now available]. =...&quot;</p> <p><b>New page</b></p><div>==Video==<br /> <br /> Video from the lecture given on October 2, 2019 [https://homeostasis.scs.carleton.ca/~soma/os-2019f/lectures/comp3000-2019f-lec09-20191002.m4v is now available].<br /> <br /> ==Notes==<br /> <br /> &lt;pre&gt;<br /> Lecture 9<br /> ---------<br /> <br /> Trying to explain assembly code!<br /> <br /> Key concepts<br /> * saving and loading of registers from stack<br /> * call/ret<br /> * passing arguments<br /> * using base pointer register for local variables<br /> <br /> <br /> When you enter a function, you save any registers that you will clobber<br /> - they are saved by pushing them on the stack<br /> - they must be popped when the function exits<br /> - register save/restore (like any instruction) can be optimized away<br /> <br /> Registers are marked by a %<br /> Constant values are marked by a $<br /> <br /> The stack pointer contains the current &quot;top&quot; of the stack (note that the stack grows down in memory).<br /> <br /> The base pointer contains the value of the stack pointer as it was<br /> at the start of the function.<br /> <br /> We use the base pointer to access parameters that are passed to a function and to access local variables.<br /> <br /> To allocate local variables, we just decrement the stack pointer (to make space). We can get rid of them by restoring the stack pointer from the base pointer (on function exit).<br /> <br /> <br /> Key steps to make a function call<br /> * push arguments onto stack or store in registers<br /> * call function (save current instruction on stack, jump to function)<br /> <br /> Key steps on entry to a function<br /> * save base pointer<br /> * copy stack pointer to base pointer register<br /> * decrement stack pointer to make room for local variables<br /> <br /> Key steps on exiting a function<br /> * copy base pointer register to stack pointer register<br /> * do ret (pop instruction pointer from stack, jump to it)<br /> <br /> After function call finishes<br /> * take arguments off stack, get return value<br /> <br /> What happens when a signal is delivered to a process?<br /> * the currently running instruction is saved to the stack<br /> * kernel replaces instruction pointer value with signal handler location<br /> (after putting proper arguments on stack)<br /> &lt;/pre&gt;<br /> <br /> ==Code==<br /> <br /> &lt;source lang=&quot;c&quot; line&gt;<br /> #include &lt;stdio.h&gt;<br /> <br /> int something(char *s)<br /> {<br /> printf(&quot;You said: %s\n&quot;, s);<br /> <br /> return 0;<br /> }<br /> <br /> <br /> int main(int argc, char *argv[], char *envp[])<br /> {<br /> printf(&quot;Entering main.\n&quot;);<br /> if (argc &gt; 1) {<br /> something(argv[1]);<br /> } else {<br /> printf(&quot;No argument given.\n&quot;);<br /> } <br /> printf(&quot;Exiting main.\n&quot;);<br /> <br /> return 0;<br /> }<br /> &lt;/source&gt;</div>

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