Hi Eric, > not new... Floppy supported 128 to 1024 bytes, for example... Yes, > FreeDOS only supports 512 bytes at the moment. For unknown reason, > this became even more hardcoded than it was before short ago. As > far as I remember, the biggest that MS DOS supported was 4096 for > WORM, but only with driver.sys? > Another source mentions 2048 bytes used for MO drives... And wasn't > there some far-east floppy format with 1024 byte sectors, too?
Technically, MS-DOS should support 8192 too. Bret mentioned this in some notes. It would also fit the byte count used as directory entry index into a single sector (256 * 32 = 8192). > Note that supporting bigger sectors will need bigger buffers at > some point Yes, any buffer needs to have a (f.e.) 8192 byte data field then. Even for drives which have a smaller sector size. With the MS-DOS compatible buffers that FreeDOS currently uses (?) the useless space at the end of these large buffers can't be re-used when buffering drives with smaller sector size. > and some way of detecting actual sector size. On DOS's side, simply call the "Get BPB" device function (and insure the BPB is valid, i.e. no crucial values zero or -1). The bytes per sector field must be set correctly then. The default block device has to obtain the sector's size from the BIOS. There doesn't seem to be a standard Int13 call for that. LBA BIOSes may provide Int13.48 (Get drive parameters), the returned table always contains a "bytes per sector" field. [I tested this on a 2002 standard PC BIOS, and it supports this call and returns valid data.] Otherwise, we either have to assume 512 bytes per sector, or read a single sector into an initialized 8192-byte buffer and determine how many bytes the read actually changed. > I do not > know whether MS DOS ever could BOOT from drives with big sectors. I don't know either, but with the single-file boot architecture used by FreeDOS (which I'll adapt too) this depends solely on the boot loader in the boot sector. > Note that we should also check whether kernel, FORMAT, FDISK, DOSFSCK > and maybe other tools have "sign overflow" problems for 512 byte per > sector drives above 1024 GB. Suggestion: Just _never_ use signed data types for anything were a negative value doesn't make sense, such as file sizes and sector numbers ;-) > Of course as long > as FDISK is happy (or you simply use Linux FDISK) you can limit your > self to drive letters of at most 1024 GB each, avoiding any problems. Of course as long as you are happy with it you could restrict yourself to 8 GiB drives which can be addressed by CHS. With this reasoning we can avoid any more effort for developers! > Talking about GPT and UEFI: Tom often mentioned that we should add > GPT handling to our partition table parser (initdisk.c). Note that > this might make the parser significantly more complex - not sure. I'm pretty sure the GPT stuff allows partitioning with 64-bit sector addressing. This raises the question how this can be supported by us. Do EFI compatibility BIOSes support LBA with actual 64-bit addressing? (The LBA packet's sector number field is a 64-bit word, but actual implementations [that I've seen] used only 28 or 48 bit of these; known as LBA28 and LBA48. On the other hand, 48-bit addressing already enables very large drives.) DOS file systems currently use 32-bit addressing for sectors. This can't be worked around by the OS, a new file system is required for that. However, such a 32-bit file system could reside at positions on the (GPT) disk were it needs to be addressed by LBA48 or "LBA64". This means: the DOS file system driver doesn't need to know about 64-bit sector addressing; the block device installed for DOS when booting does have to. > On the other hand, not THAT many places would have > to be changed to make FreeDOS aware of other sector sizes, at least if > only block devices loaded AFTER boot would have to support other sizes. How to access installed hard disks with larger sector sizes then? (Or anything which looks like a hard disk or floppy disk, i.e. can be accessed by DOS when booting.) Bret mentions in some places that a few USB pen drives do have larger sector sizes, such as 2048 bytes. DOS couldn't boot from these then, too. Regards, Christian ------------------------------------------------------------------------------ Let Crystal Reports handle the reporting - Free Crystal Reports 2008 30-Day trial. Simplify your report design, integration and deployment - and focus on what you do best, core application coding. Discover what's new with Crystal Reports now. http://p.sf.net/sfu/bobj-july _______________________________________________ Freedos-kernel mailing list Freedos-kernel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freedos-kernel
