PC Hacks

Hack #79. Boost COM Port Performance

If your PC has a COM port, tweak your state-of-the-art UAR/T chip through Windows settings.

Back in the olden days of the early PCs, getting online meant dialing in to CompuServe, Delphi, or The Source with a 300- to 1,200-baud modem, only to arrive at the legacy equivalent of Telnet—plain white characters on a green or black screen background, text menus (online servers were truly mainframes back then), and XMODEM downloads of "huge" 300 KB files. Back then, you could connect at 2,400 baud—or better yet 9,600—if your local node supported it, but this was a real challenge and a technical stress on PC serial ports. Generally, a PC built before 1990 couldn't communicate with the outside world any faster than 9,600 bits per second (bps).

The main component that made up the serial/COM port in the PC was a component called the 8250 Universal Asynchronous Receiver/Transmitter chip (UAR/T). The 8250 was capable of transferring data at a rate of 9,600 bps, roughly 960 ASCII characters per second. In the IBM PC-AT systems, the UAR/T was upgraded to the 16450 chip, which doubled the reliable throughput to 19,200 bps.

Both of these chips generated an interrupt signal to the CPU for every character of data or control information that came across the port. This was highly inefficient and bogged down the PC to a crawl, which didn't matter under single-tasking DOS but had a severe impact on the performance of multitasking Windows.

After becoming aware of this ...