This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
The ROM 3 Apple IIgs offers many minor enhancements vs. the ROM-01 which make the machine more functional and pleasant to use: - You get a machine that is about 5-10% faster for GUI, floppy disk loading and RAM Disk operations (due to updated smartport firmware and System 5 tools being in ROM). - A machine that is more flexible when working with AppleTalk and slots (you don't have to give up an extra slot and you can stick a card in slot 4 and still use the mouse in GS/OS). - A nicer text Control Panel that lets you resize RAM Disk with a warm-boot and a cleaner way to size it too (no min/max size junk). Also a 'Mouse' menu and other existing things cleaned up and made better. - A just over 1 megabyte of RAM built-in to start off with; so, you can have 5 MB of DMA compatible memory in total. - The MB0 signal provided in slots 1 through 6, so you can stick in a Video Overlay Card and Second Sight in together, and not worry about having to reserve slot 3. - Hardware shadowing of text page 2 with Alternative Display Mode (no slowing down your system to a crawl when you see a screen full of 2's running 8-bit software). - A newer ADB keyboard microcontroller with built-in sticky keys, keyboard mouse and compatibility with the indicator LEDs on extended keyboards. - A removable Lithum battery (in a snap case. Just pop the old one out when your clock and Control Panel settings stop working). - Less power consumption and electrical noise from the motherboard, - A set of pins (location 'S1') on the motherboard to make the text Control Panel disappear, making your GS settings tamper proof where young children are around There are even a couple of more goodies, like the Step/Trace commands in Monitor or the improved disassembler. Even if you are not a programmer, they're handy for peeking at SHR graphics still in memory (from Monitor hit 'S' and then the return key). Things like sticky keys and mouse keyboard come in handy too-- like if you are eating or drinking with one hand, you can still reset the computer using the other. The down side is that some old GS games and demos won't run. However many of the more worthwhile ones have been patched. By: Mitchell Spector