IBM AT 5170 Technical Reference page 441

The verifying program can look at the diskette controller's status
bytes in the ROM BIOS data area for additional information
about embedded errors. More information about copy protection
may be found on page 9-5 under "Copy Protection".
Any DOS program must be relocatable and insensitive to the size
~. of DOS or its own load addresses. A program's memory
requirement should be identified and contiguous with the load
module. A program should not assume that all of memory is
available to it.
There are several 80286 instructions that, when executed, lock
out external bus signals. DMA requests are not honored during
the execution of these instructions. Consecutive instructions of
this type prevent DMA activity from the start of the first
instruction to the end of the last instruction. To allow for
necessary DMA cycles, as required by the diskette controller in a
multitasking system, multiple lock-out instructions must be
seperated by JMP SHORT $+2.
Compatibility 9-15