Here is an attempt at writing an assembly code version of the coin toss program. The loop control is a little different, but we are accomplishing the same task. Copy this program over to the CS Linux Server into your lab$\_5/$prog$\_7$B directory. Using the debug switch, $-$g$,$ assemble and link the code into an executable program. Run the program and check results.

Oh no$!$ We got the dreaded Segmentation fault! Where did it come from and what can we do about it$?$ There is something horribly wrong about this program. Do you see the error in the code? Probably not.

Of course, I am going to ask you to start the debugger. Set your first breakpoint at $\_$start.

Hint, there is exactly one line of code missing. All other lines that are given are without error.

Debug the assembly language code, add the missing line of code, and rebuild a working executable file.

Source Code

coin.s

$//$ coin toss program, written in assembly

$//$ different coding approach than the coin.c program

$.$text

$.$global $\_$start

$\_$start:

$//$ begin for$-$type loop control: loop $5-$times

mov x$19,$ #$5//$ preload count$-$down counter with $5$

loop$\_$top:

$//$ call coin$\_$flip function

bl coin$\_$flip $//$ branch w$/$ return address saved in link register

$//$ loop control

sub x$19,$ x$19,$ #$1//$ decrement the loop counter by one

cbnz x$19,$ loop$\_$top $//$ compare counter and jump back to top if not zero

exit:

$//$ return to the operating system

mov x$8,$ #$93//$ system call number for exit

mov x$0,$ #$0//$ set return code $=0$

svc #$0//$ call the kernel

coin$\_$flip:

$//$ coin flip function

$//$ generate random number and store results in 'buffer'

mov x$8,$ #$278//$ system call number for get random number

ldr x$0,=$buffer $//$ set address to the start of buffer memory

mov x$1,$ #$4//$ set buffer size to $4$ bytes

mov x$2,$ #$0//$ set flags to $0$

svc #$0//$ call the kernel

$//$ set the address for the output string

ldr x$1,=$str$\_$heads $//$ preload string address for 'heads'

ldrb w$2,[$x$0]//$ read first byte $($random number$)$ from buffer

and w$2,$ w$2,$ #$1//$ mask off all bits other than bit $0$

cbnz x$2,$ write $//$ compare bit and branch if $1$ to write

ldr x$1,=$str$\_$tails $//$ replace string address with 'tails'

write:

$//$ print string to the terminal

mov x$8,$ #$64//$ system call number for write

mov x$0,$ #$1//$ set to file #$1,$ standard output

mov x$2,$ #$6//$ write six characters

svc #$0//$ call the kernel

$\_$coil$\_$flip$\_$return:

$//$ return to calling function

br lr $//$ branch using the address in link register

$.$data

str$\_$heads:

$.$asciz "heads

$"$

str$\_$tails:

$.$asciz "tails

$"$

buffer:

$.$octa $0$x$0,0$x$0,0$x$0,0$x$0$