Chop, chop, chop. I hope you weren't planning on doing anything with that arm, 'cause you're most likely gonna lose it.

When wounds appeared to be too bad to treat, to the point that they endangered the life of the wounded, the bog-standard procedure was amputation. The battlefield medics of the Middle Ages weren't university-trained physicians, they were barber-surgeons: experts in bloodletting, plague (really), cauterization, and amputation.

Cauterization--to stop the bleeding of a wound by burning the artery or vein shut--would have been an option for smaller wounds. Andalusi physician and 'father of surgery' Al-Zahrawi, whose text was translated into Latin around 1170, describes a process of using one's fingers to physically block the problem blood vessels, then heating several metal implements over a fire, "blowing on them to make them very hot." Then the fingers are removed and the medic very quickly places the cauteries against the blood vessels, holding them there just until the bleeding has stopped.

But for larger wounds, and indeed in the rush of battle even for smaller wounds, it seems that Western European barber-surgeons favored quick resort to amputation. Crusade-era Muslim chroniclers rant about the barbarism of Frankish battlefield medicine; even expert Nuremberg barber-surgeon Hans Folz found himself sued by a patient claiming he had amputated unnecessarily.

For your amputation, good sir knight, I hope you have a pretty good pain tolerance, because we don't carry anesthesia around with us. (But if you're nice, we might give you a blindfold so you don't have to try not to watch). The medic would tie a tourniquet above and possibly below the wound--texts prescribe the use of both, but artistic depictions sometimes omit the lower tourniquet. Then the wounded soft tissue would be sliced away. Further cuts would be made into the healthy skin further up the arm, and this skin would be peeled back and protected with linen (this will be pulled down at the end to cover the bone stump).

Now it's time to saw. Yes, with a saw. Oh, there's no saw available this time? Don't worry. The barber-surgeon might just place a sharp knife against the bone and pound it down with a heavy mallet until the bone splits. (I'm not aware of this being used as a battlefield technique in the Middle Ages, but it has been identified in remains of people who suffered amputation in other situations, so perhaps. Here's a later illustration of the practice. )

And you thought by suffering the saw you could avoid the horrific pain of burning? Nope! The stump would then be cauterized to stop the bleeding. Finally, if you were still alive what with all the blood loss and any other trauma you might have suffered, the medic would peel down the skin to cover the stump, wrap a linen bandage around the stump and tie it off. I do not know whether he would have stitched the skin flaps together or simply let them try to heal up on their own.

It's impossible to calculate mortality rates for amputations in the Middle Ages. In 1830s Paris, which is still pre-antibiotics, the survival rate for amputations was around 40%, but how does that compare to a world discovering gunpowder? The best we can say is that a lot of people died, but some people did live--for example, the unfortunate amputee who sued Folz, or the Holy Roman Emperor himself, Frederick III. Probably your chances of survival would vary based especially on how quickly the barber-surgeon reached you after the wound, i.e. how much blood you had lost. If you survived the amputation, it was probably up to infection.

As for the aftermath, if you did survive? In the Middle Ages, you would not have had a functional prosthesis. I'm not personally aware of any contemporary descriptions of phantom limb pain today, but there is a possible medieval interpretation. An amputee, say the texts, frequently feels cold on the side of the body with a missing appendage.

I do not know if there is a historical antecedent for the possibility of Jaime Lannister becoming a badass left-handed knight in the last seasons (...or books...) of Game of Thrones.