Emulator behavior differences

Most game console emulators are never intended to emulate the real hardware perfectly, as their goal is just to provide a "good enough" emulation to play games on. Even in cases where the emulator does want to replicate some certain feature, emulation accuracy bugs can be difficult to avoid due to the intricacies of the hardware. This page documents some known behavior differences between emulators and real consoles.

GB/GBC

Overview

The following table summarizes some features on GB/GBC where emulators commonly behave differently from the real hardware.

Platform	Invalid opcodes	Invalid banks	VRAM access	Echo RAM	Invalid stop	Non-executable memory sections
Game Boy Color (CGB-001)	Hang	PASS	PASS	PASS	PASS	None
Game Boy Advance SP (AGS-001)	Hang	PASS	PASS	PASS	PASS	None
Game Boy Player	Hang	PASS	PASS	PASS	PASS	None
3DS Virtual Console	Ignore	FAIL2	PASS	PASS	FAIL	VRAM, SRAM
Stadium 2 GB Tower	Message	PASS	FAIL3	FAIL4	FAIL	?
bgb 1.4.1	Debugger	PASS	PASS	PASS	PASS	None
VBA-RR v24 svn440	Message	PASS	PASS	PASS	FAIL	?
VBA 1.7.2	Message	FAIL1	FAIL	FAIL	FAIL	?
MyOldBoy v1.1.0	Ignore	PASS	PASS	PASS	FAIL	?
LameboyDS v0.12	Ignore	PASS	PASS	PASS	PASS5	?

1 - the result is 127 instead of 124 - this behavior needs to be more thoroughly investigated

2 - (probably) always returns 0

3 - (probably) always returns 255 - so VRAM access is enabled not during the V-Blank period, but during the V-Blank interrupt, which changes things dramatically

4 - some other data seems to be stored in the echo RAM area

5 - some corrupted stops work, some not - this is to be expected with undefined behavior

Invalid opcodes

Out of all 256 possible values for a byte, most are defined as the opcode for some GB ASM instruction (as a special case, 0xCB is defined as the first byte of a group of two-byte ASM instructions). However, there are 11 invalid opcodes, i.e. byte values that don't correspond to any assembly instruction, namely 0xD3, 0xDB, 0xDD, 0xE3, 0xE4, 0xEB, 0xEC, 0xED, 0xF4, 0xFC, and 0xFD.

• On real consoles, invalid opcodes hangs the game completely, hard-locking the CPU. The screen freezes as is, and the game audio becomes "stuck" on the note that was playing at the time of the freeze, if any.
• Some emulators display an error message upon encountering an invalid opcode, in addition to freezing the game. Since the real console would be unable to take input or generate meaningful output anyway, this is not considered an emulation inaccuracy.
  • Stadium 2 GB Tower displays the following message: "The saved data has been corrupted, so it is impossible to CONTINUE. Please reset the game and choose NEW GAME", even though the save data is not actually corrupted (unless, of course, it is a corrupt save that causes the invalid opcode to be encountered).
  • VBA displays the infamous message "Unknown opcode xx at yyyy".
  • BGB breaks into a debugger every time an undefined opcode is encountered.
• Some emulators silently ignore invalid opcodes, which is considered an emulation inaccuracy since this may cause glitches to exhibit behaviors that do not happen on real hardware, possibly destructive (e.g. corruption of the save file).
  • In particular, the 3DS Virtual Console ignores invalid opcodes, probably because the emulator hooks them to communicate with the hardware, essentially using them as an extension of the GB ASM instruction set.

References

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