User`s guide

BDOS FUNCTION 7: AUXILIARY INPUT STATUS
Entry Parameters:
Register C: 07H
Returned Value:
Register A: Auxiliary Input Status
The Auxiliary Input Status function returns the value OFFH in register A if a
character is ready for input from the logical auxiliary input device, AUXIN:. If no
character is ready for input, the value OOH is returned.
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BDOS Function Calls CP/M 3 Programmers Guide