Why did we use the word ‘theoretically’? Because the 1541 DOS reserves some space for itself. By dividing 174848 by 1028 (1k) we get 171KB which is exactly the size of a D64 file as shown by the host file system. By multiplying 683*256 we get the value of 174848 bytes which is theoretically the total storage capacity of a diskette formatted with a 1541. Every sector is 256 bytes long and it contains, other than the stored data, a header to identify the sector itself. Every disk is divided in 4 areas with a different clock rate.ĭoing a quick sum of all the sectors of each area we get to know that the total number of sectors in a disk is 683. Graphical representation of a 1541 disk (image from the book ‘Inside Commodore DOS’) read from) the disk. Why is there a difference in the number of sectors per track? Does this mean the disk rotates with a different speed based on the header’s position? Not really, the disk always rotates at a constant speed of 300 rpm (round per minutes), the difference is provided by varying the clock rate at which data are written on (or 1. The formatting creates a total of 35 concentric tracks, starting from 1 (the outermost one) up to 35 (the innermost one), then each track is divided into a variable number of sectors. The sectors will be used to store information. During the formatting process, the 1541 DOS (Disk Operating System) organizes the disk surface in tracks and sectors. Additionally, since a D64 is the physical representation of a 1541’s single-sided disk in a file format, knowing its structure you can easily understand the functionality of a real Commodore 1541 Disk Drive. Nothing really fancy so far, we are all used to work with emulators… but if you are still reading this article, it means you want to know more about the structure of a D64 file and how it works.
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