Large Drive CBM DOS Mapping Proposals

In the beginning, there were 170kB drives, with 1 head and 8 bits were used to hold track and sector values.

Then, came 500kB disks (8050) and the rules still held.

The 8250 and SFD1001 did not break the rules.

Later, CMD HD came along and played by the same rules. 255 tracks of 256 sectors, each with 256 bytes of data. The 16MB native partition was born. CMD HD units also used unit number to represent what we now call a HD partition, permitting 255 partitions, raising the maximum referenceable disk space to 4GB or so.

But, times have changed and drives now have 200GB or more on them.

I'm reading up on IDE64 DOS commands for block level access, and have been looking at the proposed CMD HD DOS+ solution to creating sector addressable partitions, but I'm interested in ideas from the community. Some questions:

IDE64 appears to forego emulating the older DOS addressing commands in favor of new ones (B=*), while HD-DOS+ appears to add a new byte to the existing command syntax. One offers a clean break, while the other offers more compatibility. Does anyone care? If so, which is preferred?

Commodore drives contain 256 byte sectors, but newer drives all standardized on 512 byte sectors. One favors compatibility, the other favors ease of firmware development. Should a drive represent a disk in 256 byte sectors, or 512 byte ones?

Newer ATA-based drives can access 48 bits of sectors, and each sector is 512 bytes in length. Assuming 512 byte sectors, and assuming such a disk is split into 255 partitions, that's 40 bits or 5 bytes of data needed to reference a location on disk. Currently, DOS commands allow T&S&buffer pointer. Do we add 1 or 2 more bytes?

Are there other areas to consider? Someone suggested using u0>h1 and u0>h0 to offer other ways to increase reference space.

Do we really need to worry about it? Are 16MB partitions large enough? If not, what is large enough? 4GB?

Jim
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Relevant Pages

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