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https://www.os2museum.com/wp/hang-with-early-dos-boot-sector/

While installing various versions of DOS for the DOS history series of articles, I was faced with a mysterious problem: Some versions of DOS would hang right away when booting from fixed disk, but not from floppy. I already knew that DOS 4.x is very sensitive to BIOS stack usage; if a BIOS needs more than 100 bytes or so of stack to process a disk read request, it will fail to boot DOS 4.x from fixed disk, even though the same DOS 4.x can access the same disk just fine when booted from floppy.

In DOS 4.0, IBMBIO.COM/IO.SYS grew beyond 30KB and could no longer be loaded in one go at all, even if it were contiguous (that would in essence always cause the hang problem described above). There just wasn’t enough space between 70h:0 and 0:7c00h anymore. The BIOS component was therefore loaded in two stages, which avoided the problem. However, the staged loading caused the previously mentioned issue with tight stack space—but that’s a different story.