├── cregfix.sys
├── LICENSE
├── README
└── cregfix.asm


/cregfix.sys:
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https://raw.githubusercontent.com/mintsuki/cregfix/HEAD/cregfix.sys


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/LICENSE:
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 1 | Copyright (C) 2023-2025 mintsuki and contributors.
 2 | 
 3 | Permission to use, copy, modify, and/or distribute this software for any
 4 | purpose with or without fee is hereby granted.
 5 | 
 6 | THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
 7 | REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
 8 | FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
 9 | INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
10 | LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
11 | OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
12 | PERFORMANCE OF THIS SOFTWARE.
13 | 


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/README:
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 1 | CREGFIX
 2 | =======
 3 | 
 4 | CREGFIX is a small DOS driver that works around an issue of many modern CSM
 5 | BIOS implementations where certain control register bits are left "dirty", thus
 6 | causing issues when trying to run legacy operating systems such as Microsoft
 7 | Windows 9x (see "While initializing device VCACHE: Windows protection error"
 8 | issue) or while trying to run EMM386.EXE.
 9 | 
10 | This program can be built using "nasm":
11 | 
12 |   nasm cregfix.asm -fbin -o cregfix.sys
13 | 
14 | but included in this repository is the prebuilt "cregfix.sys" file.
15 | 
16 | CREGFIX is a DOS-compatible .SYS driver which can be added to the beginning of
17 | the CONFIG.SYS file and it will sanitise the control register bits every time
18 | DOS boots up. For example:
19 | 
20 |   DEVICE=C:\CREGFIX.SYS
21 | 


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/cregfix.asm:
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 1 | bits 16
 2 | org 0
 3 | 
 4 | ; Minimal device driver header
 5 | dd -1
 6 | dw 0x8000
 7 | dw strategy
 8 | dw interrupt
 9 | db 'CREGFIX '
10 | 
11 | strategy:
12 |     ; Strategy is called right before interrupt; save request
13 |     ; header address for later
14 |     mov [cs:req], bx
15 |     mov [cs:req+2], es
16 |     retf
17 | 
18 | interrupt:
19 |     push eax
20 |     push ebx
21 |     push ecx
22 |     push edx
23 |     push es
24 |     pushf
25 | 
26 |     ; ES:BX = Request header
27 |     les bx, [cs:req]
28 | 
29 |     ; If the command DOS requests is not INIT (0), do nothing
30 |     cmp byte [es:bx+2], 0
31 |     jne .done
32 | 
33 |     cli
34 | 
35 |     ; Clear control registers
36 |     mov eax, 0x10
37 |     mov cr0, eax
38 | 
39 |     xor eax, eax
40 |     mov cr2, eax
41 |     mov cr3, eax
42 |     mov cr4, eax
43 | 
44 |     xor edx, edx
45 |     mov ecx, 0xc0000080
46 |     wrmsr
47 | 
48 |     ; Set driver size to 0 (don't stay resident)
49 |     mov word [es:bx+14], 0
50 |     mov word [es:bx+16], cs
51 | 
52 |   .done:
53 |     ; Status = done
54 |     mov word [es:bx+3], 0x0100
55 | 
56 |     popf
57 |     pop es
58 |     pop edx
59 |     pop ecx
60 |     pop ebx
61 |     pop eax
62 |     retf
63 | 
64 | req dd 0
65 | 


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