You must have made a mistake during your comparison. The IRQ vector differs between AppleWin's and MESS's ROM, as Tom said. MESS has $FA86, which is correct for an old monitor ROM (non-autostart). AppleWin's has been hacked to $FF65. (This wrecks IRQ and BRK.) An autostart ROM would have an IRQ vector of $FA40. Cheers, Nick.