Implement number parsing

postfix.s

@@ -39,6 +39,25 @@ wordloop:
 	jmp wordloop
 
 doword:
+	mov $0, %rax
+	mov $0, %rcx
+numloop:
+	## Multiply current value by ten
+	imul $10, %rax
+
+	## Add numeric value of digit
+	movzx (%rsi, %rcx), %rbx
+	sub $0x30, %rbx
+	add %rbx, %rax
+
+	## Increment index and loop if not at end of word
+	inc %rcx
+	cmp %rcx, %rdx
+	jg numloop
+
+	push %rax
+
+endword:
 	## Go back to start if the terminating character (which is in R8)
 	## of the word was a line feed
 	cmp $0x0a, %r8