Sleep 10: SemiDisk Emulator

1 February, 2021


The SemiDisk Emulator was one of the first true RAM Drives for PCs. An article from a self-proclaimed speed-freak from the April 1983 edition edition of the Creative Computing Magazine.

Transcript available
[Music] Helping you fall asleep. I'm John Chidjie. You can follow me on the Fediverse at, on Twitter at John Chidjie, all one word, or the network at engineered_net. Sleep is supported by you, our listeners. If you'd like to support the show, you can do so via Patreon, with a thank you to all of our patrons and a special thank you to our Patreon silver producers, Mitch Bilger, John Whitlow, Kevin Koch, Oliver Steele, Lesley Law Chan, Hafthor and Shane O'Neill. And an extra special thank you to our Gold Producer known only as R. Visit to learn how you can help. Thank you. So now that that's out of the way, let me talk to you. Just for a few minutes. Semi-Disk Disk Emulator by Glenn A. Hart. Most computerists are speed freaks. No, that does not mean we take drugs. It just indicates that we have an almost unnatural hang-up about how fast our machines execute. Witness the continuing popularity of benchmarks that purport to measure how fast some standardised programs will run on a given collection of computer equipment. Most of the tests don't really provide meaningful information on how a computer will run real-world applications, But they are fun anyway. Sometimes the gloating of the winners, and the rationalisations of the losers remind me of children claiming that "my dad is bigger than your dad". This emphasis on speed is doubly odd considering that even a slow microcomputer is blindingly fast in human terms. Nevertheless, raw speed is an almost intangible element in the human-machine interaction. anything that can make a 30-second process take half or a quarter of that brief period seems like a tremendous advance. It makes the machine feel more responsive and much less sluggish. There are two main avenues to speed enhancement. The clock rate of the central processor is the main determinant of pure processing speed for any given processor chip. A faster clock executes more quickly. Since the clock speed is designed into the system, it cannot be readily changed. So there is not much the average user can accomplish with processor speed. The road to higher speeds leads to mass storage. I recently wrote an article for Creative chronicling my "growth" from cassette to 5-inch floppy disk to 8-inch floppy to Winchester hard disk. Each upgrade resulted in a significant speed increase and made my evolving systems far more pleasant and powerful. Now there is a mass storage device that is a speed freak's dream with speed that exceeds even the hard disks. The Semidisk is a very high capacity RAM memory board that emulates a disk drive. The Semidisk is available in either 512 kilobytes or 1 megabyte versions, all on one standard S100 board. New versions are also available for the IBM Personal Computer and the TRS-80 Model 2. Such an incredible memory density is made possible by the new 64K memory chips. 4864s on the Semidisc. Data are transferred to and from the Semidisc at the rate of 110 kilobytes per second. Power consumption is extremely low for the amount of memory involved. about 0.6 amps for the 512k board and 0.9 amps for the full megabyte board. Such figures would have been almost unthinkable only a short while ago. Installing the Semidisk is absolute simplicity. The Semidisk requires four sequential I/O ports and any addresses can be selected with a DIP switch. Other switches and jumpers can configure various operating characteristics, but the board is set up at the factory for the most normal configuration. Multiple semi-disk boards can reside in a system to provide up to an unbelievable 8 megabytes. Even with multiple semi-disks, the same four ports are all that's necessary. Onboard switches set up the added boards to simply expand the disk capacity of the semi-disk. Like most RAM, the semi-disk cannot store data when power is removed, however, the board includes provisions for battery backup. An 8-12V battery, preferably nickel-cadmium, is trickle-charged by the SemiDisc. Special arrangements are made for power down and power up so data are retained. SemiDisc Systems claims that the software provided with the board allows the SemiDisc to run with any standard CPM system. Two main installation paths are possible. The supplied drivers can built into the BIOS like any other disk driver if the user knows how to do this, or the file can be executed., for which source code is also provided, allows complete control over the operation of the hardware. Table 1 details the various options available. The semidisk driver is positioned immediately below the normal CPM console command processor. The Hyperboot option speeds execution by preventing the CCP from being reloaded on every warm boot. can optionally check the hardware configuration and abort if there is an I/O conflict or request the address of the Semidisk if it is not where the program has been instructed to look. This option is quite useful when first installing the system. Other options control parity checking, what disk to call the semi-disk, etc. The changes made by the user can be temporary to check operation or can permanently modify the program itself for everyday operation. A few other utilities are provided to test the board, brute clear and format the disk, etc. They are not normally needed, a program which allows the customized semi-disk driver to execute upon cold boot to make initialization of the SemiDisk board proceed automatically is also supplied. Otherwise, the user must explicitly execute the to clear and format the board before use. My computer is complex, with a card in nearly every slot of the 20-slot motherboard. Operating software is accordingly so rather complicated and has been extensively patched. Thus, I was both skeptical and a bit apprehensive when trying the SemiDisk for the first time. I've tried some other hardware and software which guaranteed compatibility and had been disappointed. After adjusting the SemiDisk address to non-conflicting ports but not changing anything else, I ran The SemiDisk worked perfectly the very first time. The memory clears and formats in a bit less than ten seconds and that's it. From then on, you have a half a megabyte disk drive that runs fast. How fast? I ran four tests to get a feel for the speed of the semi-disk compared to 8-inch single and double density on my fast Inertronics drivers and my 40 megabyte Quantum Winchester. IMS's CPM 2.24e was the operating system. The results are shown in Table 2. I'm not reading that. First, I tried a linkage with Microsoft's L80 of a long Fortran program. The L80 linker is terribly slow. And I had always thought it must have something to do with disk access. Well, it does. But not to the degree I had expected. The semi-disk was about twice as fast as a single-density floppy. The advantage over my hard disk was only about 20%. Next I loaded WordStar and a 70k text file and did a Alt Q C to go to the end of the file and an Alt Q R to get back to the beginning again. Now the differences really began to show up. The advantage of the hard disk over floppies was significant but the semi-disk was more than twice as fast again. Comparing the semi-disk with floppies provided no contest. Then I piped a 224k data file from the hard disk to each destination device with Verify. Copying from one area of the hard disk to another was disappointingly slow, but this time varied somewhat depending on the surface being copied to. Again, the semi-disk was more than twice as fast as the hard disk. Finally, I wrote a basic program which generated 1000 sequential records, wrote them to disk, and then read them back in again to simulate a business application. The semi-disk wasn't quite twice as fast as the hard disk, but its advantage was still noteworthy. In all cases, the speed gained through the use of the semi-disk compared to hard disk was significant. And even double-density floppies couldn't come close. Perhaps even more important, the feel of the system was dramatically improved. When I got my hard disk, I was quite pleased with the response time. But now I have been spoiled even more by the semi-disk. Are there any disadvantages? Yes, two. One is the volatile nature of RAM. Without battery backup, work-in-progress is liable to total loss if there is a power failure or serious system crash. Saving files to the semi-disk occasionally as one would do with a normal disk just doesn't provide real security. Even though power failures are rare in my area and my system is very reliable, I sometimes get a bit nervous when working on the semi-disk and back up to hard disk or floppy. This can negate at least a small part of the time advantage provided by the board. The second problem is cost. The retail price of the 512K Semi-Disk is $1,995 and the 1MB version costs $2,995. These prices can be considered in several lights. Compared to normal S164K memory cards, the Semi-Disks are a bargain. IBM PC memory on the other hand seems to be available for as little as $600-$700 for 256K. Finally, 5.25 inch Winchester hard disks have come so far down in price that 5 or even 10 megs of hard disk may be available for less than the semi disk. In a system without hard disk, many people would think the hard disk a better first purchase. But even if the semi disk is a bit of a luxury, it is simply a delight to use. As advertised. If you work with long compilations, large text files and other serious applications, the Semidisc will make your life a lot more pleasant. Semidisc Systems, P.O. Box, GG, Beaverton, Oregon 97075. [BLANK_AUDIO]
Duration 11 minutes and 48 seconds Direct Download

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John Chidgey

John Chidgey

John is an Electrical, Instrumentation and Control Systems Engineer, software developer, podcaster, vocal actor and runs TechDistortion and the Engineered Network. John is a Chartered Professional Engineer in both Electrical Engineering and Information, Telecommunications and Electronics Engineering (ITEE) and a semi-regular conference speaker.

John has produced and appeared on many podcasts including Pragmatic and Causality and is available for hire for Vocal Acting or advertising. He has experience and interest in HMI Design, Alarm Management, Cyber-security and Root Cause Analysis.

You can find him on the Fediverse and on Twitter.