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:
  • 0 ; Service Interrupt Vector not installed
  • 1 ; Service Interrupt Vector installed

MartyPC Service Interrupt Functions

Interrupt functions are specified via AH, aligned with typical convention.

00h Service Control

Input:

• BX == DEADh
• CX == BEEFh
• AL:
  • 00h ; disable MartyPC services
  • 01h ; enable MartyPC services
  • 02h ; query service state

Output:

• AL:
  • 00h ; services disabled
  • 01h ; services enabled