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A DiskOnChip ISA card gives a retro PC solid-state storage through a tiny slice of memory address space: the host maps an 8 KB window, while the chip’s controller handles access to its internal flash. Scott M. Baker’s design adds configurable address decoding and an optional real-time clock (RTC) on the same card. DiskOnChip is the name of a specific M-Systems product family, not a generic term for today’s disk-on-module products.
What a DiskOnChip is
M-Systems’ DiskOnChip integrated flash storage and controller logic in an IC package with a pinout compatible with common memory devices. Instead of presenting itself as a conventional spinning disk, it exposes flash through a memory-mapped window. Baker describes an 8 KB window: the host addresses that region, and the chip manages paging through its larger internal storage.
The small window is useful in an early x86 system with limited address space. It does not mean the storage itself is only 8 KB; it is the portion of processor address space used to communicate with the device. Baker’s project page describes the design and the relevant chip versions.
Capacities depend on which product range is meant
Published capacity ranges differ because they describe different scopes. Baker’s 2018 project account describes IC versions from 8 MB to 384 MB. Hackaday’s 2018 overview gives a broader DiskOnChip family range of 16 MB to 1 GB. These figures should not be collapsed into one specification: they do not establish that every capacity used the same package or fits this particular board.
#1 Best Overall
- Support MP3 format, automatically play when power on, and the red LED indicator flashes when playing.
- Onboard 2W mono power amplifier (5V power supply up to 3W)
- Connect directly to the speaker (4Ω 3W speaker is recommended), and the 3.5mm audio port is the audio output port, which can be connected to earphones or external speakers.
- If you use the U disk playback mode, 5V power supply is recommended, and some U disks do not support 3.7V power supply
- Interface: Type-C interface Power supply range: DC 3.7~5.5V
| Source and scope | Reported capacity |
|---|---|
| Scott M. Baker, 2018; IC versions relevant to the design | 8 MB to 384 MB |
| Hackaday, 2018; broader product-family range | 16 MB to 1 GB |
Hackaday’s 2018 overview also describes the device as having a standard 8K JEDEC-compatible appearance. That historical description is not a substitute for a datasheet when identifying or wiring a particular chip.
How Baker’s ISA card maps the chip
The card uses a 74HCT688 magnitude comparator to decode a selected ISA memory address. DIP switches set one comparator input; address lines set the other. When the values match, the comparator asserts chip select. Because the device uses an 8 KB window, address lines A13–A19 select where that window is placed; 0xE0000 is Baker’s usual example.
Rank #2
- The information below is per-pack only
- With high quality, onboard 2W single channel power amplifier.
- Convenient to wire and refit; 4 buttons on the board for setting operation.
- Support MP3 format, playing power on automatically, LED indocator light when module is working.
- Mp3 lossless decoders power amplifier Mp3 player module Mp3 decoder board support TF card and USB.
A switch can disable the DiskOnChip’s addressability. That provides a way to prevent the device from responding at its mapped address if its BIOS extension interferes with booting. The card is therefore configurable, but the described arrangement does not establish compatibility with every XT-class machine: firmware behavior and system-specific requirements are not documented for all targets.
The BIOS extension and booting
A DiskOnChip can provide a BIOS extension that loads during PC startup, which is part of what makes the card interesting as a bootable-storage experiment. Whether a specific computer can use it depends on that system’s firmware and configuration. The project account does not promise universal support, nor does it document a complete compatibility matrix or provide an established source for DOS software. Readers should not assume that a chip, card, or BIOS extension will work unchanged across machines.
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Rank #3
- Micro SD Card Module: The module includes 74HC125 and AMS1117 chips, enabling voltage level conversion between 3.3V and 5V systems, ensuring stable communication between the Micro SD card and host devices with different voltage levels.
- Interface level: 3.3V or 5V
- Supported Interface: SPI
- Supported Card Type: Micro SD Card (TF Card)
- Socket: Pop-up
The RTC is a separate option
The card can host an RTC circuit as well as the DiskOnChip, but Baker says the two sections are separable and have independent address decoders. The RTC is not required for the flash-storage interface; the DiskOnChip section needs the chip and its address-decoding components, including the comparator, pull-ups, and DIP switch. The author summarizes the board as one that can serve as a DiskOnChip board, an RTC, or both.
Historical hardware, not a current buying recommendation
Hackaday reported in 2018 that SanDisk acquired M-Systems and discontinued DiskOnChip devices in 2007. Baker’s 2018 example used a used chip. This is obsolete hardware, so the historical coverage does not establish present inventory, prices, or a dependable source for chips. Baker also noted that fabrication files had been available through OSH Park at the time; current file availability, fabrication service, and assembled-board availability are not established.
Rank #4
- [Non-Real-Time Data Exchange] Connect your industrial computer or embedded system to USB devices for seamless non-real-time data transfer. This ISA to USB card facilitates communication between hardware and modern peripherals, ensuring smooth operation during industrial processes.
- [Broad System Compatibility] Works with DOS versions 3.x through 7.x, and supports 3.X, 95/98/ME operating systems. This ISA to USB board integrates easily into older industrial control environments without requiring major system upgrades, preserving your existing setup.
- [Minimal Driver Footprint] Requires only one CH375DOS.SYS driver file under 5K, with just 4K memory usage after loading. This ISA to USB industrial card keeps system resources free for other tasks, making it perfect for memory-constrained industrial computers and embedded controllers.
- [Hot-Swappable USB Support] Plug in a USB flash drive only when needed thanks to hot-swap functionality, simplifying data retrieval and transfer. This ISA to USB adapter card lets you exchange data quickly without rebooting, saving valuable time during maintenance or production runs.
- [Stable and Reliable Performance] The driver activates only during read/write operations, leaving the original DOS system and industrial control computer unaffected when idle. With a durable printed circuit board construction and driver CD included, this ISA to USB interface card delivers dependable long-term service for aging equipment.
Questions about DOS software for the board and compatibility with MD2200-series chips are not resolved by the project page. Do not infer either from the card’s general design description. For a real build, verify the exact chip’s identity and documentation, the target machine’s firmware behavior, and whether the board files and components can actually be obtained.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to assess this approach for a build
DiskOnChip is most relevant if the goal is to explore a memory-mapped flash device in a retro PC and the target has usable address space for its window. Before committing to a build, check these points:
Best Value
- WIDE MICROCONTROLLER COMPATIBILITY: Designed to seamlessly integrate with a variety of development boards. Fully compatible with popular AVR microcontroller boards including the UNO R3, MEGA 2560, and Due. Also works excellently with ESP32 and RP2040-based platforms, making it a versatile choice for your data storage needs.
- FLEXIBLE POWER & SPI INTERFACE: Features a standard 6-pin SPI interface (CS, SCK, MOSI, MISO, VCC, GND) for straightforward connection. An onboard voltage regulator and logic level shifter allow the module to operate safely with both 3.3V and 5V systems, eliminating the need for external level conversion components.
- RELIABLE EXTERNAL DATA STORAGE: Easily add high-capacity, removable storage to your projects. Ideal for applications like data logging from sensors, storing configuration files, saving user settings, or playing audio and image files, preserving your microcontroller's limited internal flash memory.
- COMPACT AND READY TO USE: This lightweight and compact module is designed to fit easily into any project enclosure. Each board comes with a pre-soldered 6-pin header, allowing for immediate connection to your microcontroller or breadboard without any soldering required.
- ONBOARD LEVEL SHIFTER FOR ROBUST PERFORMANCE: The integrated chip level conversion ensures stable and reliable communication between the 3.3V logic level of the SD card and the host microcontroller, whether it operates at 3.3V or 5V. We provide comprehensive after-sales support: complete digital documentation including user guides and technical references is available through our store customer service, and our support team is ready to assist with installation, programming, and troubleshooting to help you get started quickly.
- Address mapping: confirm that an 8 KB region can be assigned at a suitable address and does not conflict with the machine’s other hardware.
- Startup behavior: establish whether the target can execute the chip’s BIOS extension and whether that configuration supports the intended boot process.
- Exact chip version: verify that the available IC matches the board design; the historical capacity ranges alone do not prove package or chip-family compatibility.
- Practical availability: confirm that the chip, decoding parts, and usable board files are obtainable before treating the project as a buildable plan.
The cited sources do not provide enough evidence to rank this design against CompactFlash, XT-IDE, floppy emulators, or other retrocomputer storage options. Those comparisons require machine-specific compatibility and availability information.
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