Scor­pio News

When a drive is logged in by calling the SELDSK entry in the BIOS jump table, the BIOS uses the drive code supplied by the BDOS as an index into the drive table and it then extracts the address of the XDPH for the required drive. If this address is zero then no physical drive exists and an appropriate error code is returned to the BDOS. If the drive does exist, the drive is logged in and the drive code is stored away for reference by the disk read and write routines (in variable @cdrv).

Similarly, when the READ or WRITE entry pointe in the BIOS jump table are called, these routines use the stored code for the current drive to index into the drive table and fetch the address of the appropriate XDPH. Bear in mind that any errors due to there being no physical drive present should have already been found by the SELDSK routine.

Once armed with the address of the correct XDPH, the SELDSK, READ and WRITE routines use it to extract any other information they require. A detailed description of each field of the XDPH is given below.

The INIT field provides the address of an initialisation routine for use with the drive addressed by the XDPH. The drives are initialised by the BIOS during cold boot. Other than this, the INIT routine will not be used except, perhaps, during error recovery.

The READ and WRITE fields of the XDPH are fairly self explanatory. These fields provide the addresses of the sector read and write routines for the selected drive and are extracted from the XDPH by the BIOS when called at the appropriate place in its jump table. Before either of these routines is called, other calls to the BIOS will have performed any necessary sector translation, logged in the appropriate drive, set the track and sector numbers, set the DMA address and indicated which memory bank is to be used in the coming disk transfer. All that is required of the BIOS in its simplest form is to select the required memory bank and transfer data to or from one physical disk sector.

A facility exists within CP/M 3 for the BDOS to instruct the BIOS to transfer multiple sequential sectors to a contiguous area of RAM, starting at the DMA address. The BDOS tells the BIOS how many sectors to read or write and the BIOS will, if this facility has been implemented, continue transferring sectors to or from disk until the required number have been copied. In case this facility has not been implemented by the writer of the BIOS, the BDOS will continue to set track and sector numbers and DMA address and will call the BIOS the appropriate number of times. It is up to the BIOS to ignore the surplus calls if it has already transferred the required amount of data.

The LOGIN field is used by SELDSK to find the address of the routine appropriate to the disk drive being selected.

The UNIT field is a single byte that holds the FDC relative drive code, i.e. the actual value that is output to the disk controller to select the required drive for use. The BIOS extracts this code when any of the SELDSK, READ or WRITE routines is called.

The TYPE entry of the XDPH is a scratchpad location for use by the BIOS. It may be used for any purpose but Digital Research recommend that it is used to hold drive relevant information such as current density in systems that support single and double density.

The CSV entry under CP/M 3 is identical to that of CP/M 2.2 but the checksums contained in the vector are calculated in a slightly different manner. Instead of adding together all 128 bytes in a directory record, CP/M 3 produces sub-totals for each of the four 32 byte directory entries in the record. These four sub-totals are then exclusive ORed together to produce the checksum byte. This XORing together reduces the number of bits lost due to overflow (due to adding as for CP/M 2.2) and therefore each checksum byte contains more information about the directory record. For this reason, the likelihood of identical checksums being returned for directory records on different disks is reduced and hence the possibility of a disk change being missed is also reduced.