Strange peripherals… JiffyDOS

In the last couple of weeks, I’ve occasionally mentioned JiffyDOS as an important component in really getting the full benefits of peripherals like the 1581 drive and the HD20 hard drive.
It’s only fitting that in this week’s episode of “Stranger peripherals…”, I’ll elaborate a bit more on this great piece of hardware.

JiffyDOS was released onto the market in 1989 (if memory serves me well), by the company that delivered the first hard disk (the HD20) for the C64: Creative Micro Designs.
It came as a DIY-kit that provided two new ROM-chips that replaced the Kernal-ROMs from both the C64 (or SX-64 and even the C128), so you had to open both systems and replace these yourself.  Luckily you didn’t have to be an electronics whizz to do that J
Since it operated in serial mode, there was no need to have parallel cables hooked up between your Commodore and the drive.  No external wiring was needed, so it provided a nice and clean solution to speeding up your drives.

So, what’s the secret to the speed-gains that JiffyDOS was able to provide?  Since we’re talking about a complete ROM replacement for both the C64 and your drive, we can assume that the JiffyDOS ROMs make better use of timings, handshakes etc. to create a higher throughput of the data on the serial bus.  If you really want the nitty-gritty details, there’s a document online that actually has a resourced JiffyDOS-Kernal, in which almost all routines are explained in detail.
Here we read for instance that the since JiffyDOS performs its own timing and handshaking, the system can use both the data AND the clock lead to transfer data, giving two distinct advantages: Two bits can be sent/received at the same time and because of the heavy timing that is done before the bits are sent, we don’t need any timing for the next bytes, and we can send 4*2 bits (ie. the entire byte) without any more timing.

So, if you want to get into things like at which address the system accesses the serial devices (i.e.  through the CIA at $DD00), then this document is for you!
If you want more background on the operation of JiffyDOS, then be sure to check out the original manuals than can be found here.

Next to all the technical innovations JiffyDOS brought, it also enhanced the Commodore DOS with additional disk commands and added commands to the function keys on the Commodore.
It could also change the interleave-type (which is the distance between the sectors on a track on the disk), again contributing to speed-gains.

So how fast is JiffyDOS?  As we’ve already seen in the previous articles, the speed-gains are significant.  The manual lists some interesting speed comparisons, as does the May ’91 issue of the German 64’er magazine.  The numbers speak for themselves:

  1541 1571 1581
Original JiffyDOS Original JiffyDOS Original JiffyDOS
202 Blocks (Load) 124 sec. 12 sec. 124 sec. 9 sec. 102 sec. 8 sec.
100 Blocks (Save) 75 sec. 24 sec. 75 sec. 20 sec. 40 sec. 15 sec.
125 Blocks sequential (Load) 84 sec. 15 sec. 84 sec. 13 sec. 63 sec. 9 sec.
100 Blocks sequential (Save) 81 sec. 27 sec. 81 sec. 24 sec. 44 sec. 17 sec.

Surely, speeding up the disk-access must has its drawbacks, like compatibility issues, right?  Well, in all the years, I haven’t come across any incompatibility, and Creative Micro Designs was so confident it would not pose any problems they even mentioned the high compatibility of their ROMs in the manual.  Even GEOS did not have any issues whatsoever.
Nevertheless, in the unlikely event of an incompatibility, a small built-in switch allowed you to switch to an original Commodore ROM.

In conclusion the JiffyDOS ROMs were an essential means of speeding up the disk access of the C64, and for a mere € 60 – €70 it was well worth the investment – and still is if you want to have quick access to that old 80’s nostalgia: great applications and great games on the best home computer ever built!

Share This Post


3 Responses to Strange peripherals… JiffyDOS

  1. Pingback: Laptops For Dummies |

  2. Pingback: Strange peripherals… the RAMLink | MOS 6502

  3. We had JiffyDOS installed on our c64 and 2 1541s back in the 1980s when it first came out. We still have that c64 and I remember that later when we acquired a fastload cartridge and did some speed comparisons, JiffyDOS still beat the pants off the fastload.

    We only ran into about 5-10 different occasions where we had to flip the switch back into c64 mode, but mainly it worked to do this just on the 1541 vs the c64. I think there were only 2 games/programs that need to be back to fully stock c64 to work properly.

    It’s amazing to see how much JiffyDOS is such a household name now. It’s more popular today than back when it was available and c64s were everywhere!

Leave a Reply

Your email address will not be published. Required fields are marked *