MartyPC Service Interrupts

MartyPC Service Interrupts

MartyPC has a service interrupt vector, which is F5h by default as of 0.4.2.

I plan to use this vector to provide various MartyPC-specific DOS utilities that do otherwise "impossible" things, like crossing guest-host boundaries.

Identifying MartyPC

The actual service interrupt vector is configurable and should not be assumed to be a specific value. The exact vector can be queried via int 2Fh, the DOS mux-chain interrupt. This interrupt discovery mechanism will work even outside of DOS, since MartyPC hooks it at the CPU level.

Int 2Fh Probe

Input:

• AX = F500h
• BX = DEADh
• CX = BEEFh

Output if running in MartyPC:

• AL = FFh ; AL==FFh = installed
• BX = 4D50h ; "MP"
• CX = MartyPC version
  • MSB: Major Version
  • LSB: Minor Version
• DL = service interrupt vector
• DH = service flags
• SI = API version (currently 0100h)

Service Flags

• Bit 0: Service Interrupt Vector installed (0:no/1:yes)