Changes

strb w0,[r1,0] // like str but writes one byte only - note the use of w0 instead of r0 for the source register name

stur r0,[r1,0] // store register r0 to the address pointed to by (r1 + 0) - the mnemonic means "store ''unscaled'' register"

msub r0,r1,r2,r3 // load r0 with r3-(r1*r2) (useful for calculating remainders)

madd r0,r1,r2,r3 // load r0 with r3+(r1*r2)