How to set up an old PC to run LAPDOS

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Creating a PC for use with LAPDOS and PDD.EXE


From time to time I have needed to put together a PC capable of running both LAPDOS and PDD.EXE, for accessing data on a TPDD.

Typically, I have some Pentium class computer lying around, so that is what I have used. I cannot confirm that anything newer that this will work, but I would bet that a Pentium II would still work. My standard needs are 1-2 serial ports, a small hard drive, and a floppy, and I typically use DOS 6.22, but DOS 5 seems good also.

My current system for this is a K6-3 400 on a Super 7 PCB (100MHz clock rate). Ridiculous overkill. I have it slowed down to run the CPU at 166MHz with a bus of 66MHz.

The key thing to do, is to slow down the CPU so that it runs at a similar pace to a PC from 1987 ish.

Hardware setup:

  • Slow down clock rates on the motherboard. Sometimes you can select slower than max clock rates. Running 66M MHz is a good starting point.
  • Minimize the CPU multiplier. I have my K6-3 running at 166 MHz.
  • I disable the on-chip cache on the CPU. This is done from the bios of the PC.
  • Sometimes I must also disable level 2 cache. Again, a bios switch.
  • Hard disk - using an old hard disk may be required, but I find that setting up a small (<20MB) partition, and formatting in FAT16 works best.
  • using SLOWDOWN utility to further slow things down.

I notice two things that tell me LAPDOS is not working right- 1) starting the software fails at "determining system timings..." - if this happens things are running too fast 2) LAPDOS cannot recognize the available space on the hard disk. This means you need to run it on a smaller partition.


If need be, I also run SLOWDOWN V 3.10 from Bret Johnson. See the link below.

For settings, I use the following command in autoexec.bat-

slowdown /a

That is about it. If when running either LAP.EXE or PDD.EXE, you have timeout problems when accessing the TPDD, or the TPDD cannot be found, try slowing down the system further with a smaller number than 100.

Give it a try.

My current set up:

  • K6-3 400 running at 166 MHz with no L1/L2 cache
  • main board clock rate 66 MHz
  • SLOWDOWN /A running, making the machine work as if it was a PC/AT.

This set up runs both PDD.EXE and LAP.EXE.

cheers, Steve Adolph.