Int 21 Fn 2506  P - Phar Lap 386/dos-extender - Set Int To Always Gain Cn  [E]

   AX = 2506h
   CL = interrupt number
   DS:EDX = CS:EIP of protected-mode interrupt handler

Return: CF clear

Notes: this function modifies both the real-mode low-memory interrupt
     vector table and the protected-mode Interrupt Descriptor Table (IDT)
   interrupts occurring in real mode are resignaled in protected mode
   this function is also supported by FlashTek X-32VM