I am working on an GBA emulator and stuck at implementing CPU cycles. I just know the basic knowledge about it, each instruction of ARM and THUMB mode as each different set of cycles for each instructions. Currently I am simply saying every ARM instructions cost 4 cycles and THUMB instructions cost 2 cycles. But how do you implement it like the CPU documentation says? Does instruction cycles vary depending on which section of the memory it's currently accessing to? http://nocash.emubase.de/gbatek.htm#cpuinstructioncycletimes According to the above specification, it says different memory areas have different waitstates but I don't know what it exactly mean. Furthermore, what are Non-sequential cycle, Sequential cycle, Internal Cycle, Coprocessor Cycle for? I saw in some GBA source code that they are using PC to figure out how many cycles each instruction takes to complete, but how are they doing it?