Following on from the previous post, I started wondering where the different DOS-3.3-style boot sector types diverge so I constructed the following visualization.

This is just a stacked bar chart with the x-axis representing the number of bytes into the sector and the bars showing the proportion of disks with identical bytes up until that point.

The brightest bars (at the bottom of the stack) represent the normal DOS 3.3 boot sector and so sudden drops indicate a larger number of disks that diverge from that point. The biggest drops are labeled with the offset at which they occur.

In a normal DOS 3.3 boot sector, the code runs from 01 to 4C, bytes 4D to 5C contain a table of data (mapping logical sectors to physical sectors), bytes 5D to FC aren’t directly used by the boot process but can be used for DOS patches, and bytes FD to FF are some more data.

The fact a drop happens after 4A rather than after 4C suggests the final JMP has been changed. I’ll explore the variations in more detail in future posts.