Compaq Presario CDS 520

In this post I will write some hopefully useful observations from working with the Compaq Presario CDS 520, with the goal of turning it into a MS DOS gaming machine, similar to my Presario 433 I owned as a child.

(This post is updated on a regular basis).

Compaq Presario CDS 520 specifications

  • Year: 1994
  • CPU: 486 SX 66 MHz
  • 4 mb ram (+ 16 mb expansion = 20 mb (max = 64))
  • Cirrus Logic graphics adapter
  • ESS 688 onboard sound (I upgraded to a Soundblaster 16, see below)
  • ~500 mb hdd
  • 3.5¨ 1.44 mb floppy
  • CD-rom
  • MS DOS 6 + Windows 3.1 default

General remarks

  • BIOS: As explained by, these machines does not come with a vast range of configuration options in their BIOS. They autodetect your disks, and can boot from floppy and hdd. Larger drives than 540 mb can be used, although the limit is unclear (2.5 gb confirmed ok). To access BIOS, press F10 when the white rectangle is flashing to the right top of the screen during boot-up. If this fails, you do not have a setup partition on your computer. Then, you need to boot up from a floppy disk to access the bios. To prepare a floppy you need to download the SP1363.exe SoftPaq (see above, I will also provide links here shortly) and make a disk out of it. If you are making a clean install of DOS, you can also use this disk to make a setup partition on the hdd.
  • Integrated builds. One limitation with these all-in-one machines is of course that if something fails, the whole machine might be rendered useless. They also have limited room for additional cards and other upgrades. That is essentially the cost of the compact form factor. My Compaq Presario 433 did not have an integrated CD-rom for example. Due to the slow LPT-port, the performance of parallel port CD-roms were limited, so I used long IDE- and Y-power cables and simply positioned the CD-rom outside of the machine. The advantage with the CDS520 is that it comes with an integrated CD, which is really beneficial when installing games etc. I still keep a lot of CDr’s at home for my Dreamcast, and other consoles.

Installing a Soundblaster 16

My biggest problem with this machine is the ESS688 onboard sound. Although it is soundblaster compatible in games, the sound volume tends to be to loud, and the front volume control does not do a great job adjusting it (neither did the software mixer). So, what I wanted to do first, was to install a faithful soundblaster 16 ISA card while still being able to utilize the internal speakers, and being able to accurately control the volume.

I found a soundblaster 16 CT2960 card on Austrian eBay for a good price. Although reading negative remarks about the VIBRA16-series I am perfectly fine with its performance. To use the internal speakers with this card, I connected them to the 3.5 mm out jack of the soundblaster, with a volume control on the audio cable coming out from the back of the computer. It works very well! Only problem is that the right speaker also served as the internal PC speaker. I solved this by simply using a speaker from a broken Nintendo Gameboy as PC speaker. This speaker had a most pleasant tone to it than the original. So, now I can enjoy games at a pleasant and adjustable volume, with full soundblaster compability. The installation is fully reversable and no harm was done to the machine.

Compaq Presario CDS 520 Sound

Compaq Presario Sound

The VIBRA16 driver installation was done by using the Sound Blaster 16 Value PnP (Vibra16) CD from Since the card is Plug n Play, it installs a TSR program, followed by additional configurations, such as the set blaster environment. Although these take up some memory it was not that bad (616kb of conventional memory left at the moment), they can probably be optimized later (I removed the content the installation added to config.sys).

Coming steps: cpu, memory, storage and network upgrades

The machine could benefit from installing a 486 DX2 66 MHz CPU, and some additional RAM. I also have a 3COM ISA Ethernet card for Internet connectivity, which I would do through Windows 3.11. The current Windows-installation from the former owner contains some interesting programs which I am not prepared to replace with a clean install just yet. Also, sooner or later, I will have to replace the hard drive with something larger, and then, an IDE to compact flash adapter might come in handy. Although, I would miss the sound of the spinning hard drive!

Another option would of course be Windows 95, but for a MS-dos gaming machine I really don’t see the purpose at the moment. Perhaps I will prepare one such solution on a dedicated CF-card in the future.


  1. Joao Cunha

    Lovely post! This was my first computer back in the 90’s (1994, I believe). And I was able to buy one in mint condition last year for 200$.

    I already did some upgrades:

    – Intel DX40DPR100 Overdrive Processor
    – 2x32mb 60ns FPM (gives a total of 68mb ram but the system caps it at 64mb)
    – Atrend ATC-6631 (YMF715B-S) Yamaha OPL ISA sound card

    Some notes:
    – The DX4 overdrive gave some performance improvement but nothing spectacular, I just ordered an Evergreen AMD 5X86 133 MHz and hopefuly this one will bring a noticeable performance kick.
    – Regarding the sound card, I didn’t face your problem regarding the sound volume being too loud, I remember tackling that issue with the ESS sound volume config utility. And don’t forget… the onboard ESS688F actually has an authentic YAMAHA YMF262 (OPL3) chip, which is great for retro gaming, it justs sounds awesome. I only got the Atrend card (which also has real OPL3) just to compare if there is less noise and interference compared to the onboard card.

    BTW, have you considered extending your speakers hack to also use the volume up/down/mute buttons on the chassi?

    I will stay tuned on your next posts. Let me know if you need some help!

  2. Dreamcast (Post author)

    Thank you, Joao! Yes, optimally I would have used the volume buttons on the chassi for the perfect integration of the SB16 card, but that would require a bit more disassembly than the current modification: another day perhaps 🙂

    Regarding the overdrive cpu:s, back in the days they were described as mixed bags due to other limitations / bottlenecks in the systems, although I never tried one myself. How do the DX4 fare for games that would normally run rather slow on the CDS 520, such as Duke Nukem 3D and similar 3D-games?

    1. João Cunha

      Regarding the DX4 there was a small improvement on Duke Nukem 3d, I would say 3/4 fps.
      I just installed the evergreen 5×86 and now Duke Nukem runs awesome! Even Quake became sort of playable 🙂

      One awesome thing that I notice on the compaq motherboard, is the missing pins for the jumper that would allow switching from write-through cache to write-back.

      See this picture:

      This would allow us to use some 486 DX2/4 variants and also the evergreen 5×86 that support internal write-back cache. I would imagine we could squizz an additional 10% performance improvement. I still have to test it whenever I get a soldering kit.

  3. Dreamcast (Post author)

    Ah, well I’ll see what upgrades I come across. Checking Swedish eBay on a regular basis. Old cpu:s seem to be high in demand nowadays 🙂 Interesting hidden gem on the motherboard! I wonder if they planned on supporting it later or why they did not put the jumper there to begin with..

    1. João Cunha

      It’s an interesting dilema… for sure those pins connect to something on the PCB, the traces are not visible on top of the motherboard, but whenever I can, I’ll check the back of it, I’m hopping they connect the cpu to the VLSI chipset. I did some research on the chipset and I saw some references to the write-back cache support for l1 and l2 cache ( missing on the CDS 5xx), so there is hope. But even if the chipset supports it, the WB cache may be disabled at the BIOS level… and as you may know, if we can’t configure much on it 🙁

      1. Dreamcast (Post author)

        Yeah, the bios settings are rather limited. Perhaps you can shorten the pins (“dots”) somehow before soldering, just to see if it works?


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