When i suggested modulo for 7XNN i meant it would be simpler and in one line instead of an if line and essentially the same base code twice. By "You also are setting the Register of Nibble 3, not Nibble 2." I meant this: 8XY7: 8 = Nibble 1 X = Nibble 2 Y = Nibble 3 7 = Nibble 4 V[nib[3]] =...

I suggest using the modulo operator for your 7XNN opcode instead of manually removing 256 V[nib[2]] = (v2 + nn) % 256 In 8XY4, Even if it does carry or not, you never modulo or manually remove 256 from the result Again in 8XY5, If it does go negative, It isn't fixed. In 8XY6, the result must be math...

Have you tried using Tronix's test rom? It test a bunch of different opcodes and makes sure that registers are what they should be.

http://www.cyberforum.ru/post919567.html

Its attached on that post, sctest_12.zip