The text doesn't actually tell us. We know that Iago wounds Cassio in the leg from behind, when Roderigo fails to kill him:

ROD:
I know his gait; 'tis he. Villain, thou diest!

CASSIO:
That thrust had been mine enemy indeed,
But that my coat is better than thou know'st;
I will make proof of thine.

ROD:
O, I am slain! 
[Iago from behind wounds Cassio in the leg, and exit.]
 
CASSIO:
I am maim'd forever. Help, ho! Murder! Murder! 

Roderigo is wrong, though. He isn't slain. He lies on the floor crying out for help, but no-one dares come in case he's a faker trying to set a trap so he can hurt them:

ROD:
O wretched villain!

LODOVICO:
Two or three groan; it is a heavy night:
These may be counterfeits; let's think't unsafe
To come in to the cry without more help.

ROD:
Nobody come? Then shall I bleed to death.

Roderigo lies there in the dark. Until Iago steps in with a light to see what is going on - and finishes the job:

IAGO:
O treacherous villains!
What are you there? Come in and give some help.

ROD:
O, help me here!

CASSIO:
That's one of them.

IAGO:
O murderous slave! O villain!

ROD:
O damn'd Iago! O inhuman dog! 

And that, stabbed in the dark, is the end of Roderigo.