OS-9 has one of the most modular I/O systems there is . Basically there are (from emmoey) the following modules : Device managers : SCFMAN (Sequenatial character devices, terminal, printers, etc); RBFMAN (block-structured devices, disks), PIPEMAN (interprocess pipes) Device drivers : code to actually access a particular it of hardware Device descriptors : tables of parameters for a particular device The last 2 levels can best be explained by example. If you had 8 identical serial poets, you'd have one driver, but 8 descriptors. The latter would contain the addressies of the serial chips (to be read by the driver when accessing that port), various parameters like baud rate, word lenth, etc. Anyway, wityh a system that modular and thus configurale, I can't beleive _all_ OS-9 systems used the same hardware blcok size on a hard disk. -tony