Product Reviews


By Gene Gould
Reprinted with permission from Monitor, CPCUG

There is a thing about speed that we sometimes forget. The development of PCs has hinged around the quest for more central processing unit (CPU) speed, but to some of us it has been a mixed blessing. Two years ago when I upgraded from the Intel 486/33 to the AMD K5 PR-133, I lost the ability to play my favorite flight simulator game, Battle of Britain, because the new CPU was simply too fast to run the older software.

Some months ago I contemplated resurrecting the 486 so I could play that game again, and had actually bought a floppy drive and a hard drive to use in the restoration. I received an e-mail message from a friend suggesting that a program might be useful to me. I then went to the website and downloaded a trial copy of Mo’Slo. I can now play Battle of Britain on a Pentium II 350 running Windows 98!

Mo’Slo was created by David Perrell of Hearn/Perrell Art Associates (hence the hppa in the web address). He describes its functioning as follows:

Mo’Slo sort of "treads water" for a specific time slice of each system clock tick. Mo’Slo does not directly alter any hardware registers or use any undocumented tricks. This is why it runs on everything from a 4.77-MHz 8088 (which it can choke to about 5 percent of normal speed) up through a Pentium III at 500 MHz.

This is not a new idea. Back in 1989 I acquired a shareware program called Varislow that enabled you to slow down or speed up a DOS program by approximately 10 percent. It was primarily designed to give you a better chance in playing arcade games, but, of course, will no longer work on the CPUs we have now.

The background of Mo’Slo is interesting. Perrell wrote the original version in 1990 in response to a complaint that games writtedn for the original IBM PC could not be played on the "new" 80386 processor. Mo’Slo’s adjustable slowdown rate and memory transience were unique at the time, and it quickly became the most widely used utility of its kind.

Mo’Slo can speed compensate hundreds of MS-DOS-based games, both classic and obscure, and has been recommended by support technicians at many major game software companies, including Apogee, Electronic Arts, Interplay, Lucas Arts, MicroProse (Spectrum Holobyte), and Sierra. I will comment that when I visited the Lucas Arts website looking for solutions to the Battle of Britain column, I was unable to find any reference. However, I did not call the technical support phone number.

When CPU speeds exceeded 200 MHz, it became clear that timing sensitive code was not confined to recreational software. Some examples for which Mo’Slo can compensate include ProComm Plus for DOS, Q & A for DOS, FoxPro, Clipper, Carpe Diem, Btrieve, and Framework.

Because Mo’Slo uses only documented MS-DOS and BIOS functions, even the original version remains compatible with current Intel-compatible CPUs and MS-DOS compatible operating systems. Mo’Slo Deluxe version 2.1 was released in 1997, adding keyboard control of slowdown and a "quick-load" option to run opening screens at full speed. I am currently using version 2.1, and it performs perfectly well on the AMD K5 processor as well as on my new Intel Pentium II 350.

Mo’Slo 4Biz was designed as a convenient solution to the speed problem in multiuser networked environments. It provides command-line adjustment of its system-emulation speed, thus allowing it to be deployed on a variety of systems without having to determine the correct percent-of-system speed for each system. When run on a system slower than the system-emulation speed, no slowdown is applied. Because Mo’Slo 4Biz’s default emulation speed approximates a 166-MHs Pentium CPU, many of the business applications previously mentioned require no command switches at all.

Mo’Slo 4Biz includes additional cmmand-line options suited to use in batch files, such as slowdown delay and slowdown duration. The keyboard control is a command-line option, so it can be enabled for testing and disabled for general deployment. It also includes an alternate slowdown method that has proven more useful for games than business applications. The alternate method can slow Windows 32-bit applications under Windows 95 and 98. This alternate slowdown method will be incorporated into the next Deluxe version of the basic Mo’Slo.

Under Windows 9x, each DOS session gets a different virtual machine, meaning that it thinks it is running in its own separate computer. When Mo’Slo uses its standard slowdown method, no DOS session affects any other, and you can switch between DOS sessions using Mo’Slo and those not using it without their affecting each other. There’s only one 16-bit virtual machine under Windows NT, but Mo’Slo 4Biz still works in multiple windows.

Using the alternate slowdown method in Mo’Slo 4Biz, you can set shortcut properties such that slowdown in one DOS box slows all applications running in your Windows 9x system even when the DOS box does not have focus. So, if you slow using this method, Windows 32-bit applications run slowed also, even when the DOS box is minimized. I’m not sure how this would be useful to anyone, but the capability is there. Windows NT doesn’t support the alternate slowdown method.

Mo’Slo runs under OS/2 as well as under DOS emulators on Macintosh and Unix machines, and is being used on Netware networks.

Mo’Slo rates a position on my computer right along with WinZip and Lankford Screen Copy Utility. I’m very happy to have Battle of Britain working on my 350-MHz Pentium II. The Deluxe version is $20, and Mo’Slo 4Biz is $23. Anyone with favorite DOS programs that have problems with Windows 95/98 and NT4+ would do well to give it a try.

Gene Gould retired from the Boeing Company in 1991. He is a member of the Capitol PC Users Group and can be contacted via e-mail at