The LV-ROM player accesses data stored on the disc by reference to the picture number. It has to convert between picture numbers and disc sector numbers.

The service manual tells us that a sector of data is read in 1/75th of a second. As the disc is displaying pictures at a rate of 1/25th of a second, each picture stored on the disc holds three sectors of data.

Multiplying the picture number by 3 gives us the disc sector number.

Here, the disc has just been mounted and is at picture number 14. 14*3 = 42 which doesn't look right for the settle position of a newly mounted disc.

We have to make a further calculation to take into account VFS which uses, as with all Acorn 8-bit disc filing systems, sectors of 256 bytes. The VP415 service manual refers to 2048 byte sectors, so 8 VFS sectors will fit in an LV-ROM sector.

Dividing by 8 gives us 5-and-a-bit, which is probably about right after a mount operation, which reads the first 7 sectors of the disc, being 2 sectors of free space map followed by 5 sectors of root directory.

Click here to go return to LV-ROM Frames