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Name: USB Activity Hook

Hook Pointer Block Address: 9BD4

Hook Enable BCALL: 528A

Hook Disable BCALL: 528D

Hook Call BCALL: (none known)

Hook Active Flag: 0, (iy+3Ah)

This hook is called from the interrupt routine when USB activity is detected.

Using the Hook

  • The hook is called whenever any of bits 4-0 of port 55h are low.
    • C holds the complement of these bits.
  • B always contains the constant 2Ch.
  • Return with B=0 to abort linking, similar to the silent link hook.


This hook was introduced in OS 2.30. Apps should check the OS version before attempting to use this hook.

Credits and Contributions

  • 84plusfreak (Sernin van de Krol) discovered and analyzed this hook.