main.S Notes $($ARM version: see main.S$)-$ PA$8$

This is the ARM$32$ version of Cmain.c$.$ Tasks are as follows:

You will need to finish the design of the stack frame and calculate the stack variables offsets as shown in the lecture. Use the local variables as defined in Cmain.c starter code to guide your design. You may add additional variables to the stack. If you use the registers carefully, additional stack space will not be needed.

In the starter code we have saved $4-r10$ on the stack for you. Be aware that this is an odd number of registers and you may need to adjust the size of the pad to make sure the entire size of the stack frame is evenly divisible by $8(8-$byte aligned$)$ depending on the number of variables you place on the stack. Also keep the two buffers $($inbuf and bookbuf$)8-$byte aligned. Here is a sample stack frame based on Cmain.c to get you started.

Figure out how you are going to use the preserved registers in your code by filling out the preserved register use table comment section. This documents your use and is helpful when writing your code.

Call setup$().$ setup$()$ is passed four arguments, using registers r$0-$r$3.$ Argc $($r$0)$ and $**$argv $(r1)$ are input parameters and $**$func$()($r$2)$ and $**$fpbook $($r$3)$ are both output parameters. It is important to note that the first two arguments to setup$()$ are argc $($in ro$)$ and arg $($in r$1).$ So all you have to do is pass a pointer to a stack variable $*$func$)$ in register $r2$ and a pointer to the stack variable $*$fpbook in register r$3.$ Setup will write to both these stack locations.

If setup$()$ failed $($returns RETRN$\_$FAIL$),$ you will need to exit the program with EXIT$\_$FAILURE. Make sure you branch to the exit code located at the bottom of the main$()$ function. In assembly language programming, all stack frame deallocation and function returns are done only at one place, and are located near the bottom of the function $($this differs from high level programming$).$ Do not forget that on setup$()$ failure, the bookfile is not open and the two stack variables $*$func$()$ and $*$fpbook will both contain NULL so do not call fclose$($fdbook$)$ in this case.

Prepare all the registers prior to the $\frac{1}{O}$ loop. You may want to store as many of the most referenced stack variables in preserved registers to avoid reading the stack all the time.

Main I$/$O loop is the same as in Cmain.c$.$ If there is an error, branch to the exit code at the bottom of the function. Be aware that when calling rdbuf$(),$ it has $5$ parameters, so the last one is on the stack.

There should be two branch targets near the end of main$()($below the $\frac{1}{O}$ loop$)$ for calling fprintf$()$ with either.Lrdmess or $.$Lwrmess. It is possible to have both targets share the same code section to make the call to fprintf$().$

Part of the code at the bottom of main$()($below the $\frac{\frac{?}{?}}{O}$ loop$)$ calls fclose$($fpbook$).$ Be aware that there should be a branch target for setup $()$ failure that should not call fclose $0$ as this may cause a segfault.

Make sure to return EXIT$\_$FAILURE or exit$\_$SUCCESS as appropriate when main$()$ returns.