Interrupts used by Intel GEM
GEM consists of two modules which provide services:
- The VDI loads first. It handles the loading of drivers and provides device independence. Immediately it is loaded, it runs a program (usually GEM.EXE, the AES) and its functions remain available only as long as the program it has loaded is running.
- The AES can only be loaded if the VDI is present, and is normally loaded directly by the VDI. It handles higher "windowing" functions.
GEM/1 and GEM/2 hook three interrupts. GEM/3 and ViewMAX hook two...
- INT 24h
- The critical error handler. This is hooked by the AES, which offers the choices "Retry" or "Cancel". "Retry" returns as normal, while "Cancel" pops the stack and returns to the application which made the INT 21h call with the Carry flag set and AX=40h+critical error code (see the Interrupt List, table 1798). GEM/2 only supports the DOS 2.x critical error numbers.
- INT E0h
- Calls with CL=73h are intercepted by the GEM/1 and GEM/2 VDIs,
emulating the GSX-86 graphics system.
If CH is not 4, the call returns immediately. Otherwise, the call goes to the VDI with DS:DX pointing to the parameter block.
The ViewMAX and GEM/3 VDIs do not hook this interrupt.
- INT EFh
For calls to the AES: DX=0; ES:BX -> AES parameter block; CX=00C8h or 00C9h. For calls to the VDI: DS:DX -> VDI parameter block; CX=0473h (ignored by the VDI, but used to get the call past the AES).If the AES is not loaded, all calls go to the VDI, which ignores CX. The installation check below should be used to find whether the AES or VDI are present.
Formats of the parameter blocks
AES parameter block
DD control ;Control array. 5 words. DD global ;Global variables. 15 words. DD int_in ;Integer parameters DD int_out ;Integer results DD addr_in ;Address (DWORD) parameters DD addr_out;Address (DWORD) results
control is five words in size:
DW function ;AES function number, 10-132 DW no. of words in int_in DW no. of words available in int_out DW no. of words in addr_in DW no. of words available in addr_out
VDI parameter block
DD contrl ;Control array. 11 words. DD intin ;Integer parameters DD ptsin ;(x,y) pair parameters DD intout ;Integer results DD ptsout ;(x,y) pair results
contrl is 11 words:
DW function ;VDI function number, 1-132 DW [in] no. of words in ptsin DW [out] no. of words in ptsout DW [in] no. of words in intin DW [out] no. of words in intout DW subfunction DW graphics handle DD [in] pointer parameter DD [out] pointer return
Two bytes after the INT EFh entry point, one of the following ASCII sequences should appear:
- The GEM VDI (only) is present.
- The GEM AES and VDI, or the ViewMAX AES and VDI, are present.
- The ViewMAX VDI (only) is present.
Either sequence is followed by a zero-terminated ASCII number. Known combinations are:
- GEM/1 AES
- GEM/2, GEM/3, ViewMAX AES; GROWBOX.ACC
- GEM/4 AES
- GEM/1 VDI
- GEM/2 VDI
- GEM/3 VDI
- "GEMVDI32 31-Mar-89"
- GEM/4 VDI
- ViewMAX/1 and ViewMAX/2 VDIs
John Elliott 8-2-1998