The only effect it has is that you can only address the ROM on word boundaries. Since the ROM is full of code anyway, you wouldn't want to address it any other way. If you plugged some other card in there, presumably it would be a custom card, so if you need to be able to address off word boundaries for your application, then you can hook that function up to A0 and A1 which are available in the ROM SIMM slot as listed. They're just not used on a ROM SIMM. But they're there in the slot.
I don't know whether a VM memory expansion scheme would need off-word-boundary addressing or not.
The Portable also has a bunch of space available for "Custom ROMs" and a slot to install such a thing in. All kinds of potential for the portable.I've been looking at the Portable ROM slot's 8 MB of burst mode addressable memory space as a great spot to stick some VM hacked SRAM on a card, There's also a nice additional chunk of memory space "reserved for Apple" . . .. . . that I'm fairly certain they've no longer got any serious plans to use! }
By the way, have you also got a copy of DCaDftMFH? :?:
I had to special order my copy back in the day!
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Yes, I have DCaDftMFH. I think I got it through Amazon a few years ago. Reading it taught me two things. 1) I need a greater familiarity with "Inside Macintosh". 2) I need to learn (at least rudimentary) Pascal to understand the (presumably helpful) examples.
The Portable has never interested me in and of itself. The thing I find interesting about it is that it has 8 MB of address space allocated for RAM. I have several Outbound Model 125 Laptops and amongst other modifications I'd like to make to them, I'd like to change the memory map so that they can use 8 MB of memory. They have eight SIMM sockets inside, but four of them are dedicated to a RAM disk. Happily, the RAM disk sockets recognize 4 MB SIMMs so one can have a 16 MB RAM Disk which is huge on a Mac Plus equivalent (but 2X clock) machine.