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Right before a int 21 call I saw XOR AX, 21CD, and was just curious what it does. I couldn’t find this type of parameter call on the wiki here http://en.wikipedia.org/wiki/MS-DOS_API
You’d have to know what was in the register before the XOR instruction, as this just flips the bits at the 1’s:
If the register originally contained zero, then this invokes function 21, which is random read. The AL register would be set to CD by the same instruction. This is a puzzle, because the AL register is not used for anything by this call in the documentation I can find.