Here’s another big Macintosh firmware update for the BMOW Floppy Emu disk emulator, and it’s got some good stuff! Writable MOOF images, in-emulator disk formatting, performance improvements, and a significant bug fix. If you’re the impatient type, skip the rest of this text and download the new firmware now from the Floppy Emu project page. And as always, you can also buy a new Floppy Emu Model C at the BMOW Store. Or just sit back and enjoy this completely gratuitous screenshot of a logic analyzer capturing some disk operation.

 
Writable MOOFs

The 221201M update introduced support for MOOF disk images, a new image format that’s designed to capture all the low-level disk information needed for copy-protected software. Today’s update extends MOOF support by adding write capability, instead of treating MOOFs as read-only, which is essential for a few software titles that require writing to the boot disk. Print Shop, Carmen Sandiego… I’m looking at you. This sounds like a small change, but turned into a major effort to get it working reliably.

If you need to format a MOOF (see below), I recommend using the Blank 400K.moof and Blank 800K.moof disk images that are provided in the firmware’s Examples subdirectory, rather than using any random MOOF. The provided blank images have been tweaked to guarantee every track has a number of bits that’s an exact multiple of eight. This results in the best possible SD Card I/O performance, and reduces the chance of a formatting failure.

In the weeks since MOOF support was first introduced, the Moof-a-Day collection at archive.org has also grown from 52 to 78 titles, so check them out!

 
In-System Disk Formatting

All the supported disk image types can now be formatted directly on the Floppy Emu, from the attached Mac or Lisa. MOOF, DiskCopy 4.2, and raw images can all be formatted. In-system formatting normally isn’t necessary – you can usually use a pre-made blank disk image instead. But it’s essential for a few software titles like Seven Cities of Gold that require formatting a save disk directly from within the game. It’s also helpful when using disk copy tools to copy a real disk to a disk image, because most disk copy tools will automatically attempt to format the destination disk.

This is a big change, and the amount of effort that it required is way out of proportion with the value of the new feature, but I stubbornly wanted it anyway. In-system formatting has been a sore point for the Floppy Emu since the very beginning, and for a long time I believed it was impossible for technical reasons, but the work on MOOF writes helped me to realize that was only half-true.

To quote Aladdin’s Robin Williams quoting William F. Buckley, in-system formatting comes with a few, uh, provisos, a couple of quid pro quos. Here they are.

DiskCopy 4.2 and raw disk images can be formatted with a standard format, using standard address marks and data marks, standard sector sizes, standard checksumming… you get the idea. A DC42 or raw disk image format will fail if you attempt to create a non-standard disk format, like a disk that uses D5 18 3F to mark the start of each sector instead of the standard D5 AA 96. That’s because these disk image types don’t actually contain any of this format information, and it’s just implied to use standard defaults. I haven’t yet found any example where this is an issue, but if it ever comes up, formatting a MOOF disk image will allow for esoteric format types that intentionally break the standard.

For those who are paying very close attention, there’s also a subtle difference between 400K/800K formatting and 1440K formatting. The 400K/800K format is a true format, where every sector is written as zeroes, and then a new file allocation table and root directory are created. To employ a more modern term, it’s like doing a secure erase of the disk image. The 1440K format is more like doing a quick erase – it creates a new FAT and root directory, but doesn’t actually modify any of the sectors except the ones in the FAT. The Floppy Emu fools the Mac into thinking it zeroed all the sectors, but the old data is still hiding in there, which you can see if you view the disk image with a hex editor.

Formatting performance may vary depending on your SD card’s write cache behavior. If the format fails, try again or use a different SD card.

Disk images must be formatted as the same size as they were before formatting. When the Mac asks if you wish to format a disk as one-sided or two-sided, choose wisely. Attempting to reformat a 400K disk image as an 800K disk will lead to disappointment.

As mentioned earlier, for best performance when formatting a MOOF disk image, use the blank MOOF images provided with the firmware. These have been tweaked for optimal I/O speed.

 
1440K Disk Emulation Fixes

If you’ve been using 1440K disk emulation before now, I apologize. It’s been semi-broken for years, and I didn’t even realize it. I discovered there was a bug that could sometimes introduce invalid MFM clock bits into the bitstream, causing the Mac’s disk controller to freak out and think an error occurred. In the best case this substantially degraded the speed of disk transfers, and in the worst case disk transfers would fail outright with a message like “a disk error occurred”. After fixing this bug, 1440K disk emulation is dramatically better. It’s a very noticeable difference.

This is a short section with only a single paragraph, so I’ll repeat myself once more. This small fix makes a great difference, and if you use 1440K disk emulation very often then you’ll want this.

 
Performance Optimizations

The MOOF pseudo-random number generator has been greatly improved, and is used to create so-called fake bits or weak bits in certain MOOF disk images. The old random generator was pretty lousy, because I didn’t realize it might be important to get a specific profile of random bits. The bits need to mimic the random-seeming behavior of a real disk drive’s read head when no magnetic flux is present. The poor-quality randomness may have caused problems for some MOOF disks that employ copy-protection based on weak bits. The new pseudo-random number generator is a 16-bit linear feedback shift register, with a cycle length of 65535 that’s about the length of a whole track, and with a post-processing step to ensure that only 30 percent of the random bits are 1s. To maintain real-time bit streaming, the code must be optimized to generate each new random bit in less than two microseconds, along with other housekeeping tasks.

To accelerate data transfers from/to the SD card, Floppy Emu will now ignore the tag bytes contained in DiskCopy 4.2 disk images while it’s in Macintosh floppy emulation mode. The Mac doesn’t use these tag bytes anyway, so they’re just a waste of time. Lisa floppy emulation mode still supports tag bytes, as they’re an essential part of the Lisa filesystem. If you were doing something non-standard involving tag bytes on the Mac with Floppy Emu and DiskCopy 4.2 images, and this firmware update causes you problems, let’s talk.

SD transfers have also been optimized in other ways. If the Floppy Emu is in the midst of loading a track’s worth of data from the SD card, and it suddenly discovers it no longer needs that track because the head stepped to a different track, it will now abort the transfer ASAP instead of finishing the whole transfer and then discarding it. This actually happens a lot. The result is there’s less time on average between when the head steps to a track and the data for that track begins streaming from the Floppy Emu, which helps reduce the chances of disk I/O hiccups.

The raw SPI transfers from/to the SD card have also been substantially accelerated. The actual SPI clock hasn’t changed, and is still 10 MHz, but the dead time between each byte and the next one has been reduced. What dead time, you ask? The microcontroller in the Floppy Emu unfortunately doesn’t have any SPI buffering, so the CPU needs to intervene after every byte to queue up the next byte and start it, which can badly hamstring performance. With a core clock of 20 MHz and an SPI clock of 10 MHz, it should theoretically be possible to transfer a bit every 2 core clock cycles and a full byte every 16 clock cycles. But with a typical SPI polling loop like the one in the SD library I used, it needed 26 clock cycles instead of 16, only about 60 percent of the theoretical best performance. Not great.

I rewrote the innermost loop to do a blind transfer of each byte instead of polling for completion, employing some cycle-counting to ensure that each new byte would be started at exactly the right time. This improved performance to 18 clock cycles per byte, about 90 percent of the theoretical max. The loop could easily be timed for 17 or 16-cycle transfers, but the SPI hardware fails at this speed. I suspect the hardware needs 1 SPI clock (two core clocks) to reset its internal state before the start of the next byte, but the datasheet mentions nothing about this.

I hope you enjoy all these new features and improvements. That’s it for me until sometime next year. Happy holidays!

Download the latest Floppy Emu firmware from the project page.