The IBM Personal Computer 300GL is a model designation that actually covered a pretty wide variety of different models. Machines were built with the 300GL designation with CPUs ranging from Pentium MMX to Pentium IIIs and everything in between. This particular one came with a 300 MHz Celeron (Mendocino). The Mendocino Celerons were essentially Pentium IIs with a small, on die cache. The cache was less than the Pentium IIIs of the time (128K vs. 256K) but it ran at full clock speed instead of half speed. The Celerons were also missing the new SSE instructions that Pentium IIIs had but they were highly overclockable. For instance, a 300 MHz Celeron could typically be easily overclocked to 450 MHz by simply changing the bus speed from 66 MHz to 100 MHz.

I believe that the 300GL iterations that came with Celeron processors were among the latest released even though they weren’t the most capable. The Celeron iterations were designed as an inexpensive way to buy into the IBM ecosystem. They (at least some of them) were crippled with a 66 MHz bus though whereas the vast majority of computers with the BX chipset could also support a 100 MHz bus. This limits the upgrade possibilities significantly. HWiNFO32 and CPU-Z identify the mainboard as having a 440BX/ZX chipset. Maybe it is really a ZX chipset which was a lower cost variant of the BX which sometimes only supported a 66 MHz bus… In any case, this seems to have been an intentional design decision by IBM presumably to keep people from inexpensively upgrading and forcing them to buy a whole new machine instead. Too bad for IBM most people went with less expensive, less crippled hardware moving forward. I’m being a little harsh. While IBMs were typically not the fastest around and not very upgradeable, they did tend to be well built, quiet and reliable. This one is still running strong after all.

_{Celeron 300A “Mendocino” Slot 1 Processor (back)}

No hard drive was included when I got my 300GL. I thought about putting in a compact flash card as I had a couple of compact flash/IDE converters. However, there is only one IDE connector to connect the hard drive and the optical drive in a master slave configuration so the adapter wouldn’t work unless I was willing to give up the optical drive (which I needed for things like installing the OS). Instead, I found a 60 GB hard drive lying around that would work. So step one was to install the hard drive. In addition to installing a hard drive, I also decided to upgrade the RAM. I replaced the single 64 GB module with two 128 MB 133 MHz SDRAM modules (which will only be running at 66 MHz anyway) for a total of 256 MB. In theory, this is the maximum amount of RAM supported but I haven’t tested that theory. It’s possible that up to 512 MB would work but I wouldn’t bet on it.

_{Before CPU and RAM upgrade}

The next decision was which OS to install. I could install some iteration of Linux but my favorite ones won’t run well in 256 MB (or with a 300 MHz CPU for that matter). I decided to keep things a little more retro and installed Windows 98 SE instead. The install went well for the most part but I had trouble getting the network card working. I found a CD image online to download that was supposedly drivers and other software for the 300GL but the network “drivers” didn’t work. Instead, trying to do the setup from that CD prompted me to insert the Windows 98 CD where it couldn’t find them. I ultimately found a couple of other downloads that are supposedly drivers for this card that might work but I had already lost my patience and decided to install Windows XP instead.

_{After CPU and RAM upgrade}

Windows XP was really a big leap over Windows 98. That’s both good and bad. Good in the sense that most drivers for most machines of this era are included, multitasking is better, it’s more stable, etc. Bad in the sense that older DOS games and software don’t always work with it. Anyway, the install went perfectly, including drivers for the network card which Windows XP identifies as an IBM Netfinity 10/100 Ethernet Adapter (which is not exactly what I thought it was so maybe that was part of the problem). I downloaded k-meleon as a browser which works pretty reliably if slowly. But then what do you expect with 300 MHz and 256 MB of RAM? Windows XP itself though is pretty snappy even on such low spec hardware.

_{Celeron 300A “Mendocino” Slot 1 Processor (front)}

The next step in the upgrade process is installing a 533 MHz Celeron processor. This motherboard takes a Slot 1 processor and the fastest Slot 1 processor on a 66 MHz bus is a 433 MHz Celeron. However, with a Socket 370/Slot 1 converter (slotket) faster CPUs can be used. The fastest 66 MHz bus Socket 370 processor that can be used is the 533 MHz Celeron. Most Slotkets cost more money than I am willing to spend for such an upgrade but I found a cheap one on eBay labeled “no jumpers, no Pentium III” or something like that. I took that to mean it would work with Celerons and not Pentium IIIs though I am not sure exactly why. At any rate, I bought that one and a 533 MHz Mendocino Celeron…

_{Ad for the IBM 300GL}

The 533 MHz Celeron is also available as a Coppermine model, the main advantage of which is that it supports SSE instructions. It also runs a little cooler. However, I wasn’t sure it would work with this converter so I stuck with the older Mendocino model. Incidentally, there ended up being two jumpers on the adapter I got but I have no idea what either one of them do. There are also a series of dip switches on the motherboard of the 300GL that are supposedly used to configure processor speed but as the 533 MHz Celeron seems to run fine at 533 MHz without fiddling with them I’m not sure exactly when they are needed.

_{Technical Specs (1)}

The heatsink and clip on the existing processor seems to be a custom job as it wouldn’t fit on the adapter I had. There weren’t the right number of holes in the right places. However, the adapter I have can accept a standard socket 370 heatsink clip and I had one lying around. Unfortunately, the heatsink and fan combined were too big as the memory would be in the way. I took the fan off since some research showed that the Celeron 533 could be run with a passive heatsink in at least some cases. There was a case fan right next to it blowing air over it and it was a fairly large, copper heatsink. The machine booted up right away though it complained about the BIOS not having the right code for it. Even though I updated to the “latest and greatest” BIOS before attempting the upgrade. Pressing cancel allowed booting into Windows XP normally and the new processor seemed to work exactly as expected.

_{Technical Specs (2)}

I’m running BOINC and the milkyway@home project on it as a sort of stress test. That particular project has relatively low memory requirements and seems to work well on this old PC…if slowly. On the 300 MHz processor it took approximately 6 days and 7 hours to complete one task. However, after running for an hour or so with the 533 MHz upgrade the machine froze. I assume due to overheating though there is no temperature sensor to verify that). I added the cpu fan into the case. It sits a few inches away from the cpu heatsink at an angle and blows air onto the memory and heatsink. That seems to be doing the trick though I wish I had a better way to secure it. I’ll have to hunt down a lower profile socket 370 heatsink and fan at some point. I might also try a 100 MHz bus P3 slot processor just to see if one will actually work. Assuming this motherboard can actually do 100 MHz, then at least an 850MHz PIII should work and perhaps up to a 1 GHz processor.

Here are the links for the HWINFO output before and after the CPU and memory upgrade.