Re: IIgs RAM card Question

[Roy and/or Janet Miller <millers@inetnebr.com>]

Supertimer wrote:

> Mainly the fact that the IIe treats the memory as an 8-bit
> computer would and the IIGS treats its memory as a
> 16-bit computer with a 24-bit address would.  The IIe can
> only see 64k of memory at once,

Actually, the 6502 and 65816, like all CPUs with an 8 bit data bus, only see
one byte at at time.

> so larger memory cards
> divide the memory into banks and the IIe switches
> between these to access memory beyond the first 64k
> bank.  This limits the things it can do with the memory
> somewhat although for skillfully programmed applications
> like AppleWorks, the technique is pretty transparent from
> the user's point of view.
>
> The IIGS is different.  Its CPU can see 16MB of memory
> at once.  Not surprisingly, the IIGS is much more flexible

> from a programmer's point of view and this is one reason
> why you see more interesting software for the IIGS than
> for the other Apple IIs.
>
> 1MB on a IIe is really 16 banks of 64k.  1MB on a IIGS
> is 1MB.

1 MB on the 65816 is 16 banks of 64K, just like on an 8088. The 6502 can
only address a single bank of 64K. To address more, different banks (or
pages from different banks) have to be switched in and out of that address
space, just like EMS on an IBM would do. The 65816 can address 256 banks of
64K. The 8086/8 can address 16 banks of 64K. The 65816 and 8086/8 have a
segmented memory, segments of 64K that is. To be even more technical, the
6502 addresses 256 pages of 256 bytes, while the 65816 sees 256 blocks of
256 pages of 256 bytes.

Roy