With MacsBug installed, boot your system with the control key down and it will drop you into MacsBug right after putting "Debugger Installed" under "Welcome to Macintosh." To exit MacsBug, you can enter the command "g" to go to the next instruction (which in this case will continue the boot process). To reboot the machine, you can use the command "rb".
The following commands will find the information that we need to have in order to clear video interrupts on your system. It might be useful to make a log file of your session with MacsBug. You may do this with the "log filename" command where filename will be the log file's name--found in the "System Folder".
The first step is to list the drivers in the system. This and the subsequent examples are from my Quadra 700 with a Macintosh 2-page display card and monitor installed and nothing attached to the internal video port. To list the drivers, use the 'drvr' command. In the examples below, what you type is bracketed with >>>> and <<<< I tried to make it bold, but at least Netscape renders "bold" preformatted text just like regular preformatted text.
>>>> drvr <<<< Displaying Driver Control Entries dRef dNum Driver Flg Ver qHead Storage Dely Drvr at DCE at FFFE 0001 .Sony bPO #2 00000000 00000000 0000 4086C3E0 00006220 FFFC 0003 .Sound bPO #0 00000000 00006630 0000 408E59F0 000065F0 FFFB 0004 .Sony bPO #2 00000000 00000000 0000 4086C3E0 00006220 FFFA 0005 .AIn bPC #6 00000000 00000000 0000 4086ABCA 000067D0 FFF9 0006 .AOut bPC #6 00000000 00000000 0000 4086ABE2 00006810 FFF8 0007 .BIn bPC #6 00000000 00000000 0000 4086ABFA 00006850 FFF7 0008 .BOut bPC #6 00000000 00000000 0000 4086AC12 00006890 FFDE 0021 .PTAD!011.6F bPO #0 00000000 00002064 0001 00013468 000125C0 FFDA 0025 .PTAD!051.6F bPO #0 00000000 00002070 0001 0000A558 00008EB0 FFCF 0030 .Display_Video_Apple_WVC bHO #0 00000000 00002124 0000 00006FD0 00006670 FFCC 0033 .EDisk bPC #0 00000000 00000000 0000 408E7500 000065B0 #64 Unit Table entries, #11 in use, #53 free
Note that the italicized line is for my display card. Most graphics display drivers will start with ".Display". Of this information, we want the value in the next to the last column, "Drvr at"--in this case, 00006FD0. This is the block of memory at the head of the driver. We are looking for the drivers "open" routine as that is most likely where the driver installs it's video interrupt handling routine.
>>>> dm 6fd0 <<<< Displaying memory from 6fd0 00006FD0 4C00 0000 0000 0000 0134 0000 01DA 05F8 [text representation of bytes]The 8th and 9th bytes from this address make up the offset to the open routine. So we find an offset of 0134 from 00006FD0. We disassemble the code from that point, looking for the _SIntInstall trap. For this video card, that was quite a ways down. MacsBug will also let you search for this routine, but I chose to do it the hard way.
>>>> il 6fd0 + 134 <<<<
Disassembling from 6fd0 + 134
No procedure name
00007104 MOVEQ #$1A,D0 | 701A
[...]
0000757C RTS | 4E75
0000757E LEA *+$0356,A2 ; 000078D4 | 45FA 0354
00007582 MOVEQ #$10,D0 | 7010
00007584 _NewPtr ,Sys,Immed ; A71E | A71E
00007586 BNE.S *+$0026 ; 000075AC | 6624
00007588 MOVE.W #$0006,$0004(A0) | 317C 0006 0004
0000758E MOVE.L A2,$0008(A0) | 214A 0008
00007592 MOVEA.L $002A(A1),A2 | 2469 002A
00007596 MOVE.L A2,$000C(A0) | 214A 000C
0000759A MOVE.B $0028(A1),D0 | 1029 0028
0000759E _SIntInstall ; A075 | A075
000075A0 BNE.S *+$000C ; 000075AC | 660A
000075A2 ADDA.L #$000A0000,A2 | D5FC 000A 0000
Look for the value that is being placed in $0008(A0). This is a
pointer to the interrupt handling function for this card. In this
case, it's the contents of register A2, which is loaded with the
value 78d4. The value that is placed in $000C(A0) is what will be
in register A1 in the interrupt routine. In this case, it is being
pulled out of a structure--I happen to know that it's the slot's
base address. You may have to trace the driver's Open routine to
find this out in the general case, but that will vary from driver
to driver. If you can't figure it out, it's usually a reasonable
guess that the value is the slot's base address.
>>>> il 78d4 <<<<
Disassembling from 78d4
No procedure name
000078D4 MOVE.L A1,D0 | 2009
000078D6 ADDA.L #$000A0000,A1 | D3FC 000A 0000
000078DC CLR.B (A1) | 4211
000078DE ROL.L #$8,D0 | E198
000078E0 ANDI.W #$000F,D0 | 0240 000F
000078E4 MOVEA.L JVBLTask,A0 | 2078 0D28
000078E8 JSR (A0) | 4E90
000078EA MOVEQ #$01,D0 | 7001
000078EC RTS | 4E75
We can see here that the card takes the $000A00000 and adds it to the contents of A1 (which is the base address for this nubus slot. Then a byte is cleared at that address and the VBL task manager is called. The part that we are interested in is clear operation. To clear an interrupt on this card, all you have to do is write a zero to that address. Well, at least CLR.B that byte which will actually perform a read operation, too, if I recall correctly. Some cards will have a much more complex routine. If necessary, you can simply list the routine from your MacsBug log (RTS usually marks the end of a routine, as it does here) when you send in your report.
Remember, enter the 'g' command here, and your mac will continue booting to the Finder.
If you have an older kernel, you can get this info with the Slots program. This should also be available on Apple's ftp site and from puma. Make sure the the "Use Config ROM" option on the "Slots" menu is checked, and look at the slot information for your card. It should look something like the following:
The information that we need is referenced off the sResource directory entry '0x80' and then off the sResource directory entry '0x01'. For my video card, the information is:
0003 0001 0001 0006This tells us that this card is an Apple display card of type 6.