BBC Model B + MMFS: Command Guide

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BBC Model B + MMFS: Complete Command Guide for SD Card Systems

If you’ve upgraded your BBC Model B with the MMFS SD-card interface, you’ve opened up a whole new way to use your retro computer.

No more fragile floppy disks MMFS lets you browse, load, and run hundreds of disk images (.SSD or .MMB) straight from an SD card.
This guide collects every useful command you’ll need to explore, manage, and boot software quickly on your setup.

Quick Start — Create & Use Your Own MMFS Disk

Here’s a simple first project to prove everything’s working.

Goal: Create a blank disk, format it, write a BASIC program that prints “Hello, World!”, save it, and run it again.

You can watch my YouTube video below and see the guides below for more commands.

Create or Select a Blank Disk Slot

Insert your SD card and power on.

To check what’s available:

*DCAT

Then choose an empty slot (for example, disk 100):

*DIN 0 100

This mounts disk 100 into drive 0 (your working drive).

Format the Disk

You can format (initialize) it with:

*FORM 40 0

or

*FORM 80 0

You’ll see a message confirming the format is complete — now you have a fresh disk ready to store files.

Write a Simple BASIC Program

Press SHIFT+BREAK to return to BASIC mode, then type:

10 PRINT "HELLO, WORLD!"
20 END

Save the Program to Disk

Save it to your newly formatted disk:

SAVE "HELLO"

Check it’s there:

*CAT

You should see HELLO listed.

Run Your Program

You can run it two ways:

From BASIC:

CHAIN "HELLO"

Or directly from MMFS:

*RUN HELLO

That’s it you have your own BBC BASIC disk program, fully stored on your SD card and ready to load anytime.

Core System Commands

CommandDescriptionExample
*HELPLists installed ROMs and available star commands.*HELP MMFS
*OPT n,mSets system or filing options (4 = boot, 5 = filing).*OPT 4,3 → auto-runs !BOOT
*KEY n commandAssigns a command to a function key.*KEY 0 *CAT
*LOAD / *SAVELoad or save binary data at memory locations.*LOAD CODE 3000
*CHAIN "file"Load and run a BASIC program.*CHAIN "MENU"
*RUN fileRun a machine-code program.*RUN GAME
*INFO fileDisplay load/exec addresses and size.*INFO DEMO
*DELETE, *RENAME, *COPYManage files directly.*RENAME OLD NEW

MMFS Disk Commands

CommandDescriptionExample
*DCATLists disks in the .MMB image.*DCAT or *DCAT 1 10
*DIN <drive> <disk#>Mounts a disk image into a virtual drive.*DIN 0 5
*DOUT <drive>Unmounts a disk.*DOUT 0
*CATLists files on the mounted disk.*CAT
*FREEShows remaining disk space.*FREE
*TITLE [name]Sets or views the disk title.*TITLE GAMESDISK
*FORM <drive>Formats (initializes) a blank or corrupted disk.*FORM 0
*OPT 5,1Ensures MMFS handles all disk operations.*OPT 5,1

After mounting a disk, use *CAT then CHAIN "MENU" to start the main program, or *RUN for machine code titles.

Utility & Diagnostic Commands

CommandDescriptionExample
*ROMSLists installed sideways ROMs (MMFS should appear).*ROMS


*VERSIONDisplays OS version and build date.*VERSION
*FX 200,1Resets the filing system (fixes crashes or hangs).*FX 200,1

Common Abbreviations

ShortcutExpands ToUse
*L.*LOADLoad file
*SA.*SAVESave file
*/*RUNRun last file
*CH.*CHAINChain BASIC file
*EX.*EXECExecute script file

Everyday Workflows

Browsing & Loading Software

*DCAT
*DIN 0 1
*CAT
CHAIN "MENU"

Managing Files

*RENAME OLD NEW
*COPY SOURCE DEST
*DELETE TEMP

Auto-Boot Setup

*OPT 4,3

Reset if Stuck

*FX 200,1

Wrapping Up

You now have every practical command for BBC Model B + MMFS, including how to create and format disks, write and save programs, and manage your virtual collection all directly from your SD card.

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