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CH375USB 0.5.1

CH375USB 0.5.1 is an open-source USB host driver stack with mass storage, keyboard, mouse support for 386-class DOS / Windows 95 systems using an ISA-connected CH375 controller at the common parallel-I/O mapping 0260h / 0261h. CH375USB has been tested on the Pocket386.

The Pocket 386 is a compact, retro-style mini laptop built for running legacy MS-DOS and early Windows software using a Ali M6117 (386SX-compatible, 40 MHz) processor and 8MB of RAM, and one USB A port controlled by the CH375.

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Version 0.5.1 keeps the established DOS mass-storage, HID keyboard, hotplug and Windows-aware paths, and makes the mouse stack self-contained: CH375USB.SYS now owns the DOS INT 33h mouse API directly, merges the BIOS PS/2 mouse with CH375 USB HID mouse input, provides the text-mode cursor requested by DOS applications, and feeds the independently written Win16 companion CH375MOU.DRV under Windows 95.

CuteMouse / CTMOUSE is no longer a runtime dependency. Do not load another DOS mouse driver on top of CH375USB 0.5.1.

Prebuilt 0.5.1 binaries are published in binary/: CH375USB.SYS, CH375MOU.DRV and the MTEST.COM mouse diagnostic. The supplied build scripts can also rebuild them from source.

Author: Davide "gat"
GitHub: https://github.com/davidegat
License: GNU GPL v3 or later (GPL-3.0-or-later)

CH375USB is an independent, unofficial project. It is not affiliated with or endorsed by Nanjing Qinheng Microelectronics Co., Ltd. (WCH). See documentation/NOTICE.md.

Table of contents

Compatibility

Device DOS support DOS hotplug Windows 95 support Windows 95 hotplug
PS/2 mouse Yes, through CH375USB internal INT 33h Do not hot-plug PS/2 mice on legacy hardware unless the system explicitly supports it. Yes through CH375MOU.DRV N/A
PS/2 keyboard No No No No
USB mouse Yes Yes Yes through CH375MOU.DRV on the tested Pocket386 setup First hotplug works; unplug then replug in the same Windows session is a known limitation
USB mass storage Yes Yes, when idle/closed Yes through the DOS-backed compatibility path when detected before Windows starts No native safe removal; do not unplug while Windows is running
USB keyboard Yes Yes No native Windows keyboard driver No
External USB hub Experimental Experimental No No

USB and PS/2 mouse

  • CH375USB.SYS provides its own resident DOS INT 33h mouse driver.
  • PS/2 mouse is received through the real BIOS INT 15h/C207 callback.
  • CH375 USB HID mouse reports feed the internal mouse core directly.
  • PS/2 and USB mouse input can coexist and control the same logical three-button DOS mouse.
  • DOS programs receive the normal INT 33h API without requiring CTMOUSE.EXE or MOUSE.COM.
  • Windows 95 GUI mouse support is provided by CH375MOU.DRV, which consumes the same resident INT 33h callback stream.

DOS cursor behavior

CH375USB 0.5.1 implements the text-mode software cursor requested by DOS programs through INT 33h/01h. The underlying text cell is restored when the cursor moves or is hidden, and the text-cursor masks configured through INT 33h/0Ah are honored.

The driver does not draw a permanent cursor at the DOS prompt merely because a mouse is connected. A DOS application must request the cursor through the normal mouse API. Programs and games that render their own graphical pointer continue to do so normally.

Programmable INT 33h/09h 16x16 graphics-cursor masks are not currently rendered by CH375USB; the hotspot/mask metadata is retained for API compatibility. This does not prevent software that draws its own graphical cursor from working.

USB flash drive / mass storage

  • Read/write support in DOS.
  • Hotplug works in DOS.
  • One DOS drive letter is reserved by CH375USB even when no USB storage is attached.
  • A storage device already detected by DOS can remain available in Windows 95 through the real-mode compatibility path.
  • Windows-side insertion/removal is not a native USB mass-storage Plug-and-Play implementation.

USB keyboard

  • Works in DOS.
  • Can be connected and disconnected while DOS is running.
  • Software typematic/repeat is implemented.
  • There is currently no Windows 95 protected-mode keyboard companion.

USB hub

  • One external hub with up to four downstream ports is partially implemented.
  • Hub support remains experimental and has not received the same validation as direct-root devices.
  • Low-speed downstream behavior should not be considered generally supported yet.

Mouse architecture and runtime dependencies

The 0.5.1 mouse stack is self-contained:


PS/2 mouse -> INT 15h/C207 callback ---+
                                       +---> CH375USB internal INT 33h ---> DOS
USB mouse -> HID report path ----------+              |
                                                      +---> CH375MOU.DRV ---> Windows

No external DOS mouse driver (eg. CTMOUSE.EXE or MOUSE.COM) is needed.

CuteMouse remains relevant as historical/open-source reference material documented in documentation/NOTICE.md; its source or binary is not incorporated or redistributed by CH375USB.

The PS/2 backend uses the real BIOS callback rather than CH375USB proxying all BIOS mouse services. The USB backend bypasses BIOS C207 entirely and feeds the common mouse core directly.

Hardware-tested status

The current 0.5.1 mouse architecture has been exercised on real Pocket386 hardware with diagnostic tests and with a range of real DOS programs and games.

DOS

Test Result
PS/2 mouse movement/buttons through MTEST Working
USB mouse movement/buttons through MTEST Working
USB mouse hotplug through MTEST Working
PS/2 + USB mouse simultaneously Working

Windows 95

Test Result
PS/2 mouse Working
USB mouse present at startup Working
First USB mouse hotplug after startup Working
PS/2 + USB simultaneously Working
USB unplug then replug in the same session Known limitation

DOS programs and games

The following DOS software has been tested on real Pocket386 hardware with both the PS/2 mouse and a CH375 USB mouse.

Program / game Type PS/2 USB mouse
MS-DOS Editor (EDIT) Program Working Working
DOS Shell (DOSSHELL) Program Working Working
QBasic 4.50 Program Working Working
Norton 4.55 Program Working Working
FastTracker II Program Working Working
The Secret of Monkey Island Game Working
Monkey Island 2 Game Working Working
The Games: Winter Challenge Game Working Working
Wolfenstein 3D Game Working Working
Ken's Labyrinth Game Working Working
Hexxagon Game Working Working
Cannon Fodder Game Working Working
Battle Chess Game Working Working
Tyrian 2000 Game Working Working
XQuest 2 Game Working Working
Warcraft: Orcs & Humans Game Working Working
Lemmings Game Working Working
SimEarth: The Living Planet Game Working Working
Sid Meier's Civilization 474.03 Game Working Working
Arkanoid Game Working Working
Strip Poker Professional Game Working Working

These tests establish the current Pocket386 baseline; they are not a blanket guarantee for every BIOS, DOS program, game or Windows 95 machine.

Tested setup

  • Pocket386 PC.
  • MS-DOS 7.x / Windows 95 DOS environment.
  • CH375 ISA USB controller at I/O base 0260h (data=0260h, command/status=0261h).
  • USB storage using the validated 1 GiB FAT16 layout documented below.
  • PS/2 mouse: DOS input through the internal INT 33h driver and Windows 95 input through CH375MOU.DRV.
  • USB HID mouse: DOS input/hotplug through the direct HID -> internal INT 33h path and Windows 95 input through CH375MOU.DRV.

The Pocket386 used during development has also been run with these performance-oriented BIOS settings:

  • AT Bus Clock: PCLK2/8
  • I/O Recovery: Disabled
  • 16Bit ISA Insert Wait: Disabled
  • Slow Refresh: 120 µs

These values are not CH375USB requirements. Standard or conservative BIOS settings may be more stable on another machine. If something behaves unexpectedly, test with BIOS defaults before changing low-level timing.

USB storage formatting

For cross-platform use between Linux, DOS and Windows 95, the currently validated conservative layout is:

  • MBR / msdos partition table;
  • one primary FAT16 partition;
  • partition size: 1 GiB (from 1 MiB to 1025 MiB in the Linux example below);
  • 512-byte logical sectors;
  • 64 sectors per cluster = 32 KiB clusters;
  • two FAT copies;
  • 512 root-directory entries;
  • boot flag enabled for broad DOS-era compatibility.

This replaces the earlier recommendation to use a near-2 GiB FAT16 partition with mkfs.fat choosing the geometry automatically. On the tested Pocket386 setup, that near-limit layout became unreliable once the filesystem contained many files and directories: the volume could be reported as corrupted when moved between Linux, DOS and Windows. The 1 GiB layout above was subsequently exercised with roughly 300 files and multiple DOS software trees without reproducing that corruption.

This is a real-hardware compatibility result, not a claim that every theoretically valid FAT16 geometry above 1 GiB is inherently broken.

Linux: create the validated layout

Replace /dev/sdX with the whole USB device, for example /dev/sdb. These commands erase the selected device completely.

sudo umount /dev/sdX?* 2>/dev/null
sudo wipefs -a /dev/sdX
sudo parted -s /dev/sdX mklabel msdos mkpart primary fat16 1MiB 1025MiB set 1 boot on
sudo partprobe /dev/sdX
sleep 1
sudo mkfs.fat -F 16 -S 512 -s 64 -f 2 -r 512 -n DOSUSB /dev/sdX1

After copying files from Linux, unmount the filesystem before removing the drive:

sudo umount /dev/sdX1

A normal successful unmount flushes Linux filesystem buffers.

DOS: format an already prepared 1 GiB partition

CH375USB exposes storage as a DOS block-device drive letter, not as a BIOS INT 13h hard disk. For that reason, the recommended way to create the MBR and 1 GiB partition itself is the Linux procedure above (or another partitioning environment that can access the USB disk directly). Do not rely on DOS FDISK to partition the CH375USB drive through this driver.

Once the 1 GiB partition already exists and CH375USB has mounted it as a DOS drive letter, it can be reformatted from MS-DOS using the normal FORMAT command. Replace E: with the drive letter printed by CH375USB at boot:

FORMAT E: /U /V:DOSUSB

This reformats the existing logical partition; If a particular DOS FORMAT build refuses to format the installable block-device volume, prepare both the partition and FAT16 filesystem with the validated Linux command instead. The important compatibility target is the resulting 1 GiB FAT16 geometry above.

Pocket386 BIOS / CMOS warning

The Pocket386 has an unusual and important hardware limitation: it does not have a separate CMOS backup battery dedicated to retaining BIOS/RTC settings. BIOS/CMOS retention depends on the same rechargeable battery that powers the computer itself.

As a result, when the main battery is allowed to discharge completely, BIOS settings can be lost or reset to defaults. This is not a CH375USB failure.

Two defaults are especially relevant to this project:

  • mouse support may revert to Disabled, which prevents the BIOS PS/2 mouse path from operating normally;
  • the floppy/FDC configuration may revert to Enabled, which can produce the familiar floppy-controller/FDC failure message or boot beeps on a machine without a usable floppy drive/controller configuration.

This also explains apparently spontaneous BIOS resets reported by Pocket386 users: the machine is using its main battery for retention instead of a separate coin-cell-style CMOS battery.

Saving/restoring BIOS settings with DOS utilities

Generic DOS utilities exist that can save CMOS/BIOS configuration data and restore it later. One possible workaround is to keep a known-good CMOS backup and invoke the corresponding restore utility from AUTOEXEC.BAT, before starting Windows with WIN.

CH375USB does not bundle, recommend, test, document commands for, or provide support for any CMOS backup/restore utility. Use such tools only if you already understand the specific utility and firmware involved.

Community-modified BIOS

A community-modified Pocket386 BIOS has been discussed online with the defaults changed so that:

  • mouse support is Enabled by default;
  • the floppy/FDC setting is Disabled by default.

Community thread supplied as a reference:

https://forum.vcfed.org/index.php?threads/pocket-386.1247640/page-6

A closely related Pocket386 discussion on VOGONS describes the same type of modification, made with AMIBCP 6.24, specifically to avoid the default floppy error/beeps and default-disabled mouse setting:

https://www.vogons.org/viewtopic.php?start=100&t=99751

CH375USB has not tested or validated this modified BIOS. The community report itself notes uncertainty about normal flashing tools and describes programming the firmware chip directly. Firmware flashing can render the machine unbootable; use any third-party BIOS image entirely at your own risk.

Files

Prebuilt binaries

Source and build files

  • CH375USB.ASM — unified resident driver source and DOS block-device interface.
  • internal_mouse.inc — resident INT 33h core, PS/2 backend, direct USB mouse feed and DOS text-cursor support.
  • CH375MOU.ASM — independently written Win16 mouse-driver bridge source.
  • CH375MOU.LNK — shared Open Watcom linker definition for the Win16 driver; used by both build scripts.
  • MTEST.ASM — tiny DOS INT 33h coordinate/button diagnostic source.
  • bios_mouse.inc — BIOS PS/2 compatibility / legacy virtual-device support.
  • win_hotplug.inc — bounded Windows root HID-mouse hotplug state machine.
  • ch375_defs.inc — CH375/USB constants.
  • ch375_hw.inc — low-level CH375 I/O and transaction helpers.
  • ch375_native.inc — CH375 native mass-storage path and storage abstraction.
  • usb_core.inc — USB enumeration and control transfers.
  • usb_msc.inc — USB Mass Storage BOT/SCSI support.
  • usb_hid.inc — USB HID keyboard/mouse support and typematic handling.
  • usb_hub.inc — experimental external-hub support.
  • usb_maint.inc — hotplug and deferred maintenance.
  • BUILD.BAT — native-DOS build of CH375USB.SYS, CH375MOU.DRV and MTEST.COM using NASM and Open Watcom from fixed DOS paths.
  • build.sh — Linux/Unix build of the same outputs; bootstraps Open Watcom and validates the Win16 NE image.
  • CHANGELOG.md — release history.
  • documentation/KNOWN_ISSUES.md — current open issues, hardware caveats and deferred work.
  • documentation/KNOWLEDGE.md — engineering notes and architecture.
  • documentation/NOTICE.md — project independence, provenance and external references.
  • LICENSE — GNU GPL v3 license text.

The published binaries are project build outputs from this source tree. The repository does not contain the vendor CH375DOS.SYS, FreddyV's CH375286.SYS binary, or CuteMouse binaries/source.

Basic DOS installation

Copy binary/CH375USB.SYS from the repository to the DOS boot drive as CH375USB.SYS and add it to CONFIG.SYS:

DEVICE=C:\CH375USB.SYS

Do not load CH375USB.SYS and CH375286.SYS at the same time. Both need exclusive access to the CH375 controller.

Do not load CuteMouse/CTMOUSE, MOUSE.COM, or another DOS mouse driver on top of CH375USB 0.5.1. CH375USB itself owns INT 33h.

For a machine used mainly in DOS, this single driver provides storage, keyboard, mouse and DOS hotplug support.

DOS / Windows 95 dual boot

Loading CH375USB normally from the Windows profile in CONFIG.SYS can make Windows 95 startup slower because the real-mode storage driver remains resident throughout the boot path.

A practical workaround tested during development is to use separate DOS and Windows profiles:

  1. Load CH375USB normally from CONFIG.SYS for the DOS profile.
  2. Leave CH375USB out of the Windows profile in CONFIG.SYS.
  3. In the Windows profile, load CH375USB from AUTOEXEC.BAT with FreeDOS DEVLOAD.COM immediately before Windows starts.
  4. Do not load a separate DOS mouse driver; CH375USB 0.5.1 provides INT 33h itself.
  5. For storage compatibility, insert the USB drive and make sure DOS sees it before starting Windows.
  6. Do not unplug that USB storage device while Windows is running; exit Windows or shut down first.
  7. Optionally, users who already maintain a CMOS backup with a third-party DOS utility may restore it in this pre-Windows part of AUTOEXEC.BAT; this is outside CH375USB support.

DEVLOAD is a FreeDOS utility that loads DOS device drivers from the command line and emulates a DEVICE= entry. Only DEVLOAD.COM is required at runtime.

Example CONFIG.SYS

[MENU]
MENUITEM=DOS,DOS with CH375USB
MENUITEM=WINDOWS,Windows 95
MENUDEFAULT=WINDOWS,5

[COMMON]
DOS=HIGH,UMB
FILES=40
BUFFERS=20
LASTDRIVE=Z

[DOS]
DEVICE=C:\CH375USB.SYS

[WINDOWS]
REM CH375USB is intentionally not loaded here.

Example AUTOEXEC.BAT

@ECHO OFF
GOTO %CONFIG%

:DOS
REM CH375USB.SYS is already resident from CONFIG.SYS.
REM No CTMOUSE/MOUSE.COM is needed.
GOTO END

:WINDOWS
REM OPTIONAL and unsupported by CH375USB:
REM If you use your own third-party CMOS restore utility, it may be called here.

REM Load CH375USB only now, immediately before Windows.
C:\DEVLOAD\DEVLOAD.COM C:\CH375USB.SYS

REM CH375USB itself supplies the DOS INT 33h mouse driver.
REM For USB storage, the drive must already be inserted and visible in DOS here.
WIN
GOTO END

:END

With this setup:

  • the DOS profile provides USB storage, keyboard, mouse and DOS hotplug;
  • the Windows 95 profile loads CH375USB only immediately before Windows starts;
  • a USB storage device already detected by DOS can remain available in Windows 95;
  • the PS/2 and CH375 USB mouse can coexist on the same Windows pointer;
  • Windows storage remains a DOS compatibility path.

Windows 95 mouse companion

Use the prebuilt binary/CH375MOU.DRV, or build CH375MOU.DRV locally with build.sh or BUILD.BAT.

Typical setup:

  1. Make sure CH375USB.SYS is resident before WIN.
  2. Do not load CuteMouse/CTMOUSE or another DOS mouse driver.
  3. Copy CH375MOU.DRV to:
C:\WINDOWS\SYSTEM\CH375MOU.DRV
  1. In C:\WINDOWS\SYSTEM.INI, set under [boot]:
mouse.drv=CH375MOU.DRV
  1. Restart Windows.

CH375MOU.DRV is a Win16 mouse-driver bridge, not a native USB HID stack. It registers a normal INT 33h callback with the resident CH375USB mouse core. When Windows enables the bridge, CH375USB re-arms the BIOS PS/2 callback so the mouse remains active after Windows enters enhanced mode.

The bridge sets a wide hidden INT 33h coordinate range and centers that internal counter before seeding its relative-motion baseline. This prevents the first Windows session from exhausting coordinate headroom in one direction before the visible pointer reaches the screen edge.

The Windows-side root USB mouse enumerator performs bounded work over multiple timer ticks rather than performing a long blocking enumeration inside one timer callback.

On the tested Pocket386 baseline:

  • a USB mouse already connected before Windows starts works;
  • a USB mouse can be hotplugged for the first time after Windows has started;
  • PS/2 and USB mice can work simultaneously;
  • unplugging the USB mouse and then plugging it back in again during the same Windows 95 session does not currently restore USB input. The PS/2 mouse continues to work. This is a known minor issue retained for later work.

MTEST mouse diagnostic

MTEST.COM is built from MTEST.ASM and continuously displays the current DOS INT 33h X/Y coordinates and button state without relying on application cursor drawing.

Run:

MTEST.COM

Move either the PS/2 or CH375 USB mouse and press the buttons. Press Esc to exit.

MTEST is useful for separating transport/input failures from application-specific cursor behavior: if coordinates/buttons change in MTEST but an application does not show a pointer, the mouse transport and INT 33h core are already functioning and the remaining problem is likely application/cursor-API compatibility.

Building

Linux / Unix

Requirements:

  • NASM;
  • unzip;
  • Python 3;
  • curl or wget if Open Watcom has not already been bootstrapped locally.

Run:

./build.sh

The script builds into the working-tree root:

  • CH375USB.SYS with NASM;
  • CH375MOU.DRV with Open Watcom WASM/WLINK;
  • CH375MOU.MAP as a linker map;
  • MTEST.COM with NASM.

The repository's published prebuilt copies live in binary/; local builds are intentionally produced in the project root by the current build script.

Open Watcom is downloaded into the project-local .toolchains/ directory when required. The build checks that CH375MOU.DRV is actually a Win16 NE DLL/driver before reporting success.

Native DOS / Windows 9x DOS mode

BUILD.BAT builds all 0.5.1 release outputs, using the same essential assembler/linker options as build.sh:

CH375USB.ASM -- NASM -f bin --------------------------> CH375USB.SYS
CH375MOU.ASM -- WASM -q -2 -fo=CH375MOU.OBJ --------> CH375MOU.OBJ
CH375MOU.OBJ -- WLINK @CH375MOU.LNK -----------------> CH375MOU.DRV
                                                       CH375MOU.MAP
MTEST.ASM ---- NASM -f bin --------------------------> MTEST.COM

The DOS build deliberately uses fixed, simple 8.3-safe tool locations. Before running BUILD.BAT, install/extract the tools exactly as follows:

C:\NASM\NASM.EXE
C:\OPENWAT\BINW\WASM.EXE
C:\OPENWAT\BINW\WLINK.EXE

Open Watcom V2 for DOS / Win16

Use the Open Watcom V2 C/C++ installer for DOS/16-bit Windows and install it with the root directory set to C:\OPENWAT.

For the native DOS host, Open Watcom's tools are under BINW; BUILD.BAT expects WASM.EXE, WLINK.EXE and the linker system-definition files there. It sets:

SET WATCOM=C:\OPENWAT
SET EDPATH=C:\OPENWAT\EDDAT
SET INCLUDE=C:\OPENWAT\H
SET PATH=C:\NASM;C:\OPENWAT\BINW;%PATH%

It also points WLINK_LNK at C:\OPENWAT\BINW\WLINK.LNK when that file is present. This is important because CH375MOU.DRV must be linked as a 16-bit Windows NE DLL/driver, not as an ordinary Windows executable.

NASM for DOS

Use a native DOS NASM executable, not the Win32/Win64 package. The documented DOS package for this build is NASM 3.01 from the FreeDOS repository:

The FreeDOS package page provides the DOS nasm.zip. Extract/copy NASM.EXE so that the final path is:

C:\NASM\NASM.EXE

NASM's flat bin output is used for the DOS .SYS and MTEST.COM; Open Watcom WASM emits the OMF object used by WLINK for the Win16 .DRV.

Run the DOS build

From the CH375USB source directory:

BUILD.BAT

A successful build produces in the source directory:

CH375USB.SYS
CH375MOU.DRV
CH375MOU.MAP
MTEST.COM

CH375MOU.OBJ is temporary and is removed after a successful link.

The batch file also refuses a CH375MOU.DRV build when WLINK reports Warning!, because a misconfigured Open Watcom linker can otherwise emit an apparently usable file with the wrong Windows image configuration.

The Linux build additionally performs a Python-level inspection of the final NE header. The native DOS batch does not require Python; it relies on the explicit CH375MOU.LNK Win16 DLL/driver definition, WLINK's exit status, warning check and final-file existence check.

Troubleshooting

DOS BUILD.BAT says NASM is missing

The batch file does not search arbitrary locations. It requires:

C:\NASM\NASM.EXE

Install/extract the native DOS NASM package there, then run BUILD.BAT again.

DOS BUILD.BAT says Open Watcom is missing

It requires the DOS-host Open Watcom tools at:

C:\OPENWAT\BINW\WASM.EXE
C:\OPENWAT\BINW\WLINK.EXE

If Open Watcom was installed somewhere else, reinstall/move it to C:\OPENWAT or edit the two root variables at the top of BUILD.BAT deliberately.

WLINK warns or CH375MOU.DRV is not produced

Do not ignore the warning. Make sure the DOS/Win16 Open Watcom distribution is installed, C:\OPENWAT\BINW is intact, and WLINK.LNK is present there. The named Windows linker definitions must be available when CH375MOU.LNK requests a Windows DLL/driver image.

PS/2 mouse worked before, but is now completely dead

Check the Pocket386 BIOS first. Because there is no separate CMOS backup battery, a fully discharged main battery can reset BIOS settings. Mouse support may revert to Disabled.

Re-enter BIOS and enable mouse support before assuming that CH375USB has regressed.

FDC failure / floppy error suddenly appears at boot

This can have the same cause: a BIOS reset can restore the default floppy/FDC setting to Enabled. On Pocket386 configurations without a usable floppy setup, this can produce an FDC failure message and/or boot beeps.

Check the BIOS and disable the unused floppy/FDC option again.

Mouse input works in MTEST but no cursor is visible in a DOS program

CH375USB draws a text-mode cursor only after an application requests it through INT 33h/01h. The driver does not display a permanent DOS-prompt cursor.

If MTEST shows movement/buttons but a specific program still has no pointer, verify whether that program expects a text cursor, supplies its own graphics cursor, or depends on an unimplemented graphics-cursor rendering detail. The software matrix above is the current real-hardware validation baseline.

Windows 95 does not see the USB drive

CH375USB does not provide native Windows USB mass-storage hotplug. Connect the USB flash drive before starting Windows and make sure DOS sees it before WIN.

If the drive was not detected by CH375USB first, do not expect Windows 95 to discover it later as a native USB device.

Can I unplug the USB drive?

  • DOS: yes, but only after closing files/programs using the drive and waiting for disk activity to stop. If SMARTDRV write-behind caching is enabled, run SMARTDRV /C first.
  • Windows 95: no. Do not unplug USB storage while Windows is running. Exit Windows or shut down first, then remove the drive.

The reason is that Windows sees the storage through CH375USB's DOS real-mode compatibility path, not through a native Windows USB mass-storage stack with a safe-removal protocol.

Windows 95 reports MS-DOS compatibility mode or starts very slowly

Do not load CH375USB from the Windows profile in CONFIG.SYS. Instead, use the dual-boot arrangement above and load it from AUTOEXEC.BAT with DEVLOAD immediately before starting Windows:

C:\DEVLOAD\DEVLOAD.COM C:\CH375USB.SYS
WIN

The USB storage device should already be connected and visible in DOS. This is a real-mode compatibility workaround, not native Windows USB support.

Windows 95 mouse does not work

Check all of the following:

  1. For the PS/2 mouse, Pocket386 BIOS mouse support is enabled.
  2. CH375USB.SYS is resident before Windows starts.
  3. The mouse works through CH375USB's INT 33h path in DOS before WIN (MTEST.COM is useful here).
  4. CH375MOU.DRV is present in C:\WINDOWS\SYSTEM.
  5. [boot] in SYSTEM.INI contains mouse.drv=CH375MOU.DRV.
  6. No competing DOS mouse driver such as CTMouse is loaded on top of CH375USB.

If DOS mouse support does not work before Windows starts, fix that first; CH375MOU.DRV depends on the resident CH375USB INT 33h path.

USB mouse works in Windows until it is unplugged and replugged

This is a known 0.5.1 limitation. A USB mouse can be present when Windows starts or can be hotplugged for the first time after Windows is running, but after an unplug followed by a replug in the same Windows 95 session the USB mouse currently does not resume input. The PS/2 mouse continues to work.

This is intentionally retained as a minor follow-up issue rather than changing the validated 0.5.1 architecture late in the release.

USB flash drive is not recognized or appears corrupted after being heavily populated

Use the validated 1 GiB FAT16 layout from USB storage formatting. Do not use the older near-2 GiB Linux recipe with geometry left entirely to mkfs.fat; that layout was observed to become unreliable after adding many files and directories on the tested Pocket386 workflow.

CH375USB currently supports 512-byte storage sectors and safely rejects unsupported sector sizes.

If one flash drive still fails with the validated geometry, try another, preferably an older/simple USB 2.0-era device. Flash-controller compatibility varies on retro USB host hardware.

USB hub support

Hub support remains experimental and has not yet received systematic real-hardware validation. For reliable use, connect storage, keyboard or mouse directly to the CH375 USB port.

Some DOS games do not see the USB keyboard

CH375USB translates boot-HID reports to PC-compatible scan codes, but some programs bypass conventional BIOS/DOS behavior or make assumptions about the keyboard controller. Such programs may still be incompatible.

Windows keyboard support

USB keyboard input remains DOS-only in 0.5.1. There is no Windows keyboard companion equivalent to CH375MOU.DRV.

Windows 3.1 / Windows for Workgroups 3.11

CH375USB 0.5.1 has not been validated as a Windows 3.x input solution. No Windows 3.1/WfW 3.11 compatibility claim is made.

Project documentation

License and source provenance

CH375USB is released under GPL-3.0-or-later.

The source code in this project is independently written. No proprietary WCH driver source code or vendor driver binary is included. No CuteMouse, Bret Johnson USBMOUSE, FreddyV CH375286 or other third-party source file is incorporated or redistributed as part of CH375USB.

Public hardware/protocol specifications, documented DOS/BIOS/Windows interfaces, historical implementations and community reports were used as interoperability and engineering references.

See documentation/NOTICE.md for the detailed provenance/reference record.

About

Open-source DOS / Windows 95 USB host driver stack for CH375 ISA controllers, with mass storage, keyboard, mouse support.

Topics

386386sx80386assemblych375ch375286dosfreedoshotplugisams-dosnasmpocket386retro-computingretrocomputingusb-devicesusb-driverwindows95x86

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