On 23 July 2017 at 15:47, Andrea Carpi via Ql-Users <
ql-users@lists.q-v-d.com> wrote:

>   Hello everybody
> Trying to transfer large files between Windows 10,
> QPC2, and QL-Aurora-SGC-Qubide I have noticed that there are big
> limitations in the maximum length of the files.
> I mean:
> - On QXL.WIN
> hard drive in QPC2 I'm not able to generate files longer than 50Mb (End
> Of file error)
> - On the Qubide hard drive the limit is 19Mb (ROM 2.01),
> but perhaps also depends on the partition creation choices
> - On Ram
> Disk (in QPC2 maximum RAM 128 Mb) I did not find any limits unless the
> size of the RAM
> - I did not find limits on DOS devices from QPC2 except
> those of the file system in use on Windows (NTFS)
> Specifically for
> QXL.WIN and QUBIDE do anyone know the exact length limits and
> why?

The QDOS file system stores the file position in the channel definition
block as two 16-bit words - one for the block number and one for the byte
position within one block. So, when using 512-byte blocks, the maximum file
length will be 65535*512 bytes or just under 32MB (or 16MB when using
signed arithmetic). When using 2K byte blocks, the limit will be 128 or
64MB respectively.

I know mdv and flp use 512-byte block size and ED flp have 2K byte sectors
but I'm not sure if the latter also uses 2K blocks. The same goes for
(virtual) win drives - sectors are usually grouped to keep the map within
limits but I don't know off-hand if that also affects the block size (it
might as well be 512 bytes, depending on the driver).

This use of word-sized block numbers within QDOS is an unfortunate design
flaw - as is the standard FS.MDINF trap which returns word-sized sector
counts - but could TT back in 1984 foresee that within five years there
would be a storage medium for the QL that surpassed the 32MB limit? In the
PC world there are multiple examples of this - remember the 32MB partition
size limit in DOS 3.3, then the 528MB limit on CHS-addressed hard disks,
and the initial 128GB limit on LBA which was supposed to 'fix' CHS.

Storage capacity has grown so much that any 'X MB ought to be enough for
everybody' design decision has proven wrong eventually...

*Jan Bredenbeek* | Hilversum, NL | j...@bredenbeek.net
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