Re: What is the function of each ROM chip (A2+)

[mjmahon@aol.com (Michael J. Mahon)]

Ernest asked:

>What is the function of each of the main ROM chips on an Apple II+ mainboard
>(F08, E08, etc.) I found a II+ with a dual ROM device plugged into ROM F08,
>and it made me curious about the purpose for each of the main ROMs.

The chips are simply five 2KB "chunks" of the upper address space of the
Apple ][+.  As a result, they are only approximately divided by "function".

Having said that, the upper ROM (the "F8" ROM, addresses from $F800
to $FFFF) is known as the "monitor" ROM, and contains all the standard
Apple ][+ monitor functions.  The piggybacked ROM you found is almost
certainly a copy de-protection aid, and is probably switch-controlled to
take the place of the standard ROM.

Frequently, such ROMs would replace some little-used function (like
the cassette routines) with routines that would copy RAM data from
places that are clobbered by a standard Reset into places where it
would be saved for examination and copying to disk.  Of course, the
standard Reset routines would be intercepted or patched to use the
new code.

Some "alternative" ROMs were quite extensive in their deprotection
functionality.  The most capable that I know of was the "Senior PROM".
The nicest configuration used a bank-switched 4KB EPROM and
contained both the standard ROM image and the alternate, so only
one chip was needed, with a switch controlling the high address line.

-michael

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