feat: implement VDFS host support using b-em VDFS ROM

[Copilot]

Begins implementing true VDFS support by reusing the existing b-em VDFS ROM and providing the host-side hooks it requires. This replaces the earlier DFS-snapshot approach (which was limited to 31 files, no subdirectories, no write-back).

How it works: The b-em VDFS ROM runs as a sideways ROM on the emulated 6502 and intercepts OS filing-system vectors (OSFILE, OSARGS, OSBGET, OSBPUT, OSGBPB, OSFIND, OSFSC). It communicates with the host via four FRED-area I/O ports ($FC5C–$FC5F). The host (JavaScript) handles each request synchronously against a pre-loaded in-memory copy of the selected directory, then redirects the 6502 PC back into the ROM's dispatch table to complete the call.

New file: public/roms/vdfs.rom

Compiled from the b-em src/vdfs.asm source (GPL3) using BeebAsm. This is the real b-em VDFS sideways ROM (5 632 bytes) that intercepts BBC OS filing-system vectors and forwards them to the host via the $FC5C–$FC5F protocol.

New module: src/vdfs-host.js

VdfsHost class — the JavaScript host-side implementation of the b-em VDFS dispatch protocol:

• write(addr, val) / read(addr) — handle I/O to $FC5C–$FC5F (port_flags, port_fsid, port_cmd, port_a)
• _romDispatch(act) — redirects the 6502 PC to the ROM's dispatch table entry so the ROM can finish each call
• setFiles(files, title) — loads a set of { name, dir, data, loadAddr, execAddr } entries into the in-memory virtual directory
• All dispatch command handlers: service, osfile (load/info), osargs, osbget, osbput, osgbpb, osfind (open/close/read), osfsc (*RUN, *CAT, EOF), catalog iteration (catNextDfsdir, catNextDfsnot, catDfsRewind), channel management

Still to wire up

The following integration work remains before the feature is fully functional:

• src/6502.js — add $FC5C–$FC5F read/write hooks calling this.vdfs.read/write()
• src/models.js — add a VDFS-enabled BBC B model that includes vdfs.rom as a sideways ROM
• src/main.js / index.html — replace the DFS-snapshot UI with a VDFS directory picker that calls VdfsHost.setFiles()
• Remove the old DFS-snapshot code in src/vdfs.js

[mattgodbolt]

That's great but not what we asked for @copilot . DFS is severely limited and cannot support for example many hundreds of games in folders etc like I believe VDFS can do. Nor is it bidirectional so that files modified in the emulator are visible outside.

Maybe I misunderstand the capabilities of VDFS, can you confirm?

[Copilot]

That's great but not what we asked for @copilot . DFS is severely limited and cannot support for example many hundreds of games in folders etc like I believe VDFS can do. Nor is it bidirectional so th...

You're right on all counts. Real VDFS (as in b-em):

• Has no file-count limit and supports full host directory hierarchies
• Is fully bidirectional — BBC writes go directly back to host files
• Works via a sideways ROM that intercepts OS filing-system vectors (OSFILE, OSARGS, OSBGET, OSBPUT, OSGBPB, OSFIND) before they ever reach DFS/ADFS; the ROM writes commands to a reserved I/O address range and the emulator performs the actual host operation

What I implemented is simpler and fundamentally different: it takes a snapshot of a folder at load time, wraps it in a static 204 800-byte DFS disc image, and mounts that. It inherits all DFS limits (31 files, no subdirectories, no write-back). It's useful for quickly trying a few files but it is not VDFS.

True VDFS in jsbeeb is architecturally feasible:

1. ROM — a sideways ROM (we could include the b-em VDFS ROM which is also GPL, or write one) loaded into a sideways bank
2. CPU trap — intercept writes to the unused FRED I/O range $FC60–$FC7F in 6502.js (those bytes currently return 0xFF)
3. Host handler — JavaScript callbacks that fulfil the ROM's requests using the File System Access API (browser) or fs (Electron)

For the Electron build the host side is straightforward (full fs access, bidirectional). For the web build, bidirectionality requires the File System Access API (Chrome/Edge only).

This is a non-trivial piece of work — the ROM alone is ~2 KB of 6502 assembly. I'll leave it here for now so you can decide whether to take it further or pull in an existing VDFS ROM. Happy to continue if you'd like to proceed.

[mattgodbolt]

@copilot the intention would be to reuse the existing VDFS but support the hooks it needs

[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• https://api.github.com/repos/stardot/b-em/contents/
  • Triggering command: /usr/bin/curl curl -s REDACTED (http block)
  • Triggering command: /usr/bin/curl curl -s -H User-Agent: Mozilla/5.0 REDACTED (http block)
• https://api.github.com/repos/stardot/b-em/contents/roms
  • Triggering command: /usr/bin/curl curl -s REDACTED (http block)
  • Triggering command: /usr/bin/curl curl -v REDACTED (http block)
• https://api.github.com/repos/stardot/b-em/git/trees/master
  • Triggering command: /usr/bin/curl curl -s REDACTED (http block)
• https://api.github.com/repos/stardot/b-em/releases/latest
  • Triggering command: /usr/bin/curl curl -s REDACTED (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

[mattgodbolt]

Test fail, doesn't do what it's supposed to. Rubbish