Re: GEOS

["Michael J. Mahon" <mjmahon@aol.com>]

Linards Ticmanis wrote:

<snip>

> The Apple is more restricted regarding the memory location of the 
> visible hi-res screen than the C64. It can live only at $2000 to $3FFF 
> or at $4000 to $5FFF, and the memory is organized differently. So any 
> applications that write directly to screen memory will not be 
> compatible. Also, the resolution is different, it's 280x192 instead of 
> 320x200 (provided Apple GEOS runs at "Standard Hi-Res", not "Double 
> Hi-Res"; the latter would be 576x192). Any applications that depend on 
> 320x200 won't work on the Apple.

That shouldn't be a problem, since GEOS app windows are resizeable
and GEOS handles the screen for them.

> Beyond 64K and at double-hires resolution, things start to be very 
> different between the C128 and the 128K Apple IIe. The Apple does 
> 80-char text and double resolution graphics by using parallel sections 
> of memory from both banks. The C128 has a second, very different 
> graphics chip for this (the VDC), with its own dedicated VRAM. Programs 
> that depend on this sort of stuff are most likely going to be incompatible.

Text handling should be irrelevant, since GEOS apps are entirely
GUI-based.

> And of course, the floppy disk formats are different and Apple 5.25" 
> disks have only 140KB capacity; bootable disks need an OS, including the 
> low-level disk handling routines, on the disk, unlike the Commodores 
> which have disk handling routines in the drive ROM. Commodore drives are 
> a computer of their own with its own CPU, RAM and ROM; Apple drives are 
> not, all the hard work is done by the host CPU. This means also that 
> Apples can't really do much anything else (including reacting to the 
> mouse) while reading from or writing to the disk, since this has to 
> happen in very tight loops that leave no time for other things. No idea 
> if that would be any problem for GEOS.

It wouldn't be.  Minor mouse hiccups during disk access are acceptable.

The boot routine would, of course, load an RWTS, so the GEOS kernel
disk calls would work fine, without any file system dependence.

> Also the file structure on Apple 
> disks is usually different from commodore disks, there aren't any 
> pointers in the first two bytes of a sector, instead all files are 
> organized a bit like Commodore's "relative files", with side sectors. 
> Programs that depend on the low-level file organization are therefore 
> probably not portable, unless Apple GEOS copied the Commodore's file 
> structure instead of using the usual Apple structure.

Again, well-structured GEOS apps have no low-level dependencies.

> Apples don't have sorta-kinda-protected RAM area, such as the Commodore 
> has at $C000. Therefore, it's probably not possible to exit to BASIC 
> from GEOS, run a BASIC program, and then be able to reenter GEOS with 
> the Kernel usually still intact.

Fair enough--but probably irrelevant, since BASIC is not a GEOS app
in any case.  Calls could certainly be made to the BASIC ROMs if there
were reason to do so.

-michael

Music synthesis for 8-bit Apple II's!
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