How to insert arm instructions into a function in a native library?

Question

I am new and am still learning assembly languuage. In a native android app library that has been disassembled i found this function which had 1 instruction.

addres    hex     arm instruction                            function
2cc3ad   71704708 stmdaeq r7, {r0, r4, r5, r6, ip, sp, lr} ^ function0(unsigned char)

I have read in articles that arguments used to call a function are are stored on r0,r1 and r2 respectively. I wanted to add 200 into register r0 so that the instruction can store the value into the the memory referenced by those registers. So i inserted the the hex value of a a mov instruction at the address 2cc3ad so that in a hex editor it appeared like this. mov ro, #200 is C800A0E3 in hex.

address     Hex              Instruction    
2cc3ad      C800A0E3         mov ro, #200   
2cc3b1      71704708         stmdaeq r7, {r0, r4, r5, r6, ip, sp, lr} ^

After editing and adding those bytes i saved to the file. Before using the edited library i tried to redisassemble it but the disassembler gave an error as well as the app which used the library. In my understanding by adding that byte to the library i corrupted the whole file. Is there a way or an instruction i can use to to assign a certain value to r0 or to store the value to the memory referenced by those registers in that function without modifying the whole library?

Answer 1

Actually what I needed to do was to branch to an empty code cave and insert my code there. Also, I was using a disassembler which was not correctly analysing the function, for example this 71704708hex value was decoded as a thumb instruction set on another disassembler while on the disassembler I first used it was an ARM instruction set. The starting address of the function was also incorrect.