80-Bus News

attractive. In order to achieve this and still be compatible with previous CP/M software, (and also to allow programs to maintain economical 128-byte buffers rather than larger ones), some software is interposed between CP/M and the disk drivers. This software maintains a physical sector buffer in memory (512 bytes in size in our case) through which all the CP/M data transfers are passed.”

Associated with the buffer are some flags, and a record of which sector the buffer contains. (Drive number, track number, physical sector number). Let us start by considering what happens when the BDOS wants to read a logical sector (128 bytes of data). We will totally ignore writing for the moment. The BDOS starts by issuing drive, track and sector requests, followed by a Call to the BIOS Read routine. The Read routine starts by converting the CP/M logical sector number to a physical sector number. (It divides it by 4 as 512/128=4). Next it checks a flag to see if there is anything in the buffer. If there isn’t, it jumps on to do the actual read. If there is, it checks to see if it is the same drive/​track/​sector as the current request. If it is the same, then the read can be skipped as there is no point in overwriting the buffer with identical data! Once the buffer is full of data, the flag is set to indicate valid data is present, and the buffer pointers are updated to the correct drive/​track/​sector combination. Finally the transfer of 128 bytes of data to the BDOS follows. To locate which 128 bytes, the logical sector sector number is reloaded, and the lower two bits are isolated, (the remainder when divided by 4). These are then used as an index into the appropriate quarter of the buffer, which is then copied to the requested destination.

Writing follows a similar pattern, but has various extra quirks. The main one is that before a logical sector can be written into the buffer, the buffer must contain the full physical sector. This is because the BDOS Write is only modifying one quarter of the sector, and the other three quarters must be maintained intact. Thus a Write might actually require a pre-read to load the physical sector into the buffer. However the efficiency of the system can be increased by defering the physical write, (following the transfer of the 128 bytes to the buffer), because the odds are that the BDOS is performing a sequential Write, and so will be writing to another quarter of the same physical sector on the next Call. If the assumption is wrong, then nothing will have been lost, but if it is right, then the time taken to do a physical Write will have been saved. However a ‘Must Write’ flag has to be set to say that the buffer contains unwritten data in case the next request is a read or a write of another physical sector. In fact the code of both the Read and Write commands does check this flag, and if necessary Writes the buffer to disc before reusing it.

The BDOS also passes some additional information in register C to the BIOS on every Write Call. If register C is zero, then it is a normal write. (i.e. the BIOS handles it in the manner described above). If register C is set to 1, then the write is to a sector of the directory. In this case the buffer should be immediately re-written to disc, the write must not be defered. (Note This is in keeping with the ‘rugged’ approach of CP/M, in making it difficult for you to accidently destroy directories by removing or changing discs at the wrong time). If register C is set to 2, then the write is to the first sector of a newly allocated block of sectors on the disc. This last one is another ‘tweak’ to improve system performance. When the BDOS is writing a new file out to disc, (or extending an existing one), it tells the BIOS (by setting C=2) everytime it starts on a new block of sectors. As the BDOS is writing to an