bios int 15h/ah=89h \ VOGONS

bios int 15h/ah=89h

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First post, by Goplat

Posted on 2011-10-01, 06:11

Goplat Offline

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Rank: Newbie
Posts: 1
Joined: 2009-05-20, 19:26

The dosbox implementation of the int 15h/ah=89h function in the BIOS is a bit off: it doesn't disable IRQs on the PIC, and (more importantly) it jumps to CX when it should just get the return address off the stack. Patch:

1--- src\ints\bios.bak	Mon May 10 10:43:54 2010
2+++ src\ints\bios.cpp	Fri Sep 30 21:28:19 2011
3@@ -700,8 +700,8 @@
4 		break;
5 	case 0x89:	/* SYSTEM - SWITCH TO PROTECTED MODE */
6 		{
7-			IO_Write(0x20,0x10);IO_Write(0x21,reg_bh);IO_Write(0x21,0);
8-			IO_Write(0xA0,0x10);IO_Write(0xA1,reg_bl);IO_Write(0xA1,0);
9+			IO_Write(0x20,0x10);IO_Write(0x21,reg_bh);IO_Write(0x21,0);IO_Write(0x21,0xFF);
10+			IO_Write(0xA0,0x10);IO_Write(0xA1,reg_bl);IO_Write(0xA1,0);IO_Write(0xA1,0xFF);
11 			MEM_A20_Enable(true);
12 			PhysPt table=SegPhys(es)+reg_si;
13 			CPU_LGDT(mem_readw(table+0x8),mem_readd(table+0x8+0x2) & 0xFFFFFF);
14@@ -710,10 +710,11 @@
15 			CPU_SetSegGeneral(ds,0x18);
16 			CPU_SetSegGeneral(es,0x20);
17 			CPU_SetSegGeneral(ss,0x28);
18+			Bitu ret = mem_readw(SegPhys(ss)+reg_sp);
19 			reg_sp+=6;			//Clear stack of interrupt frame
20 			CPU_SetFlags(0,FMASK_ALL);
21 			reg_ax=0;
22-			CPU_JMP(false,0x30,reg_cx,0);
23+			CPU_JMP(false,0x30,ret,0);
24 		}
25 		break;
26 	case 0x90:	/* OS HOOK - DEVICE BUSY */

Reply 1 of 1, by Qbix

Posted on 2011-10-01, 07:20

Qbix Offline

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Joined: 2002-11-27, 14:50
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do you have an example game that uses/fixes this ?
This code was added for one game, although I have to look in the archives if I still have it somewhere.

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