Problems
In addition to the unreliable hard disk drive, the high-density floppy disk drives turned out to be problematic. Some ATs came with one high-density (HD) disk drive and one double-density (DD) 360 kB drive. High-density floppy diskette media were compatible only with high-density drives. There was no way for the disk drive to detect what kind of floppy disk was inserted, and the only clue the user had was the disk label and an asterisk molded into the 360 kB disk drive faceplate. If the user accidentally used a high-density diskette in the 360 kB drive, it would sometimes work, for a while, but the high-coercivity oxide would take a very weak magnetization from the 360 kB write heads, so reading the diskette would be problematic.
A different problem occurred when using a double-density diskette in the 1.2 MB drive; the high-density drive's heads had a track width half that of the 360 kB drive, so they were incapable of fully erasing and overwriting tracks written by a 360 kB drive. Therefore, overwriting a DD disk that had been written to in a DD drive with an HD drive would result in a disk perfectly readable on an HD drive, but producing many read errors in a DD drive. Whereas a HD read head would only pick up the half track that drive had written, the wider DD read head would pick up the half-track written by the HD drive mixed with the unerased half-track remnant of the track written earlier by a DD drive. So, the DD drive would end up reading both new and old information together, causing it to "see" garbled data.
The combination of the faster clock rate, fewer clock cycles per instruction, and the 16-bit bus led to a computer that was in the marketing sense too fast. IBM was protective of their lucrative mainframe and minicomputer businesses and consequently ran the original PC/AT (139 version) at a very conservative 6 MHz with one wait state. They also used a three-to-one interleave on the hard disk, even though the controller supported two to one. Many customers replaced the 12 MHz crystal (which ran the processor at 6 MHz) with a 16 MHz crystal, so IBM introduced the PC/AT 239 which would not boot the computer at any speed faster than 6 MHz, by adding a speed loop in the ROM. This also introduced the Baby AT motherboard form factor. The final PC/AT, the 339, ran the processor at 8 MHz with one wait state, and was built as IBM's flagship microcomputer until the 1987 introduction of the PS/2 line.
Read more about this topic: IBM Personal Computer/AT
Famous quotes containing the word problems:
“I believe that if we are to survive as a planet, we must teach this next generation to handle their own conflicts assertively and nonviolently. If in their early years our children learn to listen to all sides of the story, use their heads and then their mouths, and come up with a plan and share, then, when they become our leaders, and some of them will, they will have the tools to handle global problems and conflict.”
—Barbara Coloroso (20th century)
“Currently, U.S. society has been encouraged by its political and subsidized mass-media intelligentsia to view U.S. life as a continual “morning in America” paradise, where the only social problems occur in the inner cities. Psychologists call this denial.”
—Ishmael Reed (b. 1938)
“If when a businessman speaks of minority employment, or air pollution, or poverty, he speaks in the language of a certified public accountant analyzing a corporate balance sheet, who is to know that he understands the human problems behind the statistical ones? If the businessman would stop talking like a computer printout or a page from the corporate annual report, other people would stop thinking he had a cash register for a heart. It is as simple as that—but that isn’t simple.”
—Louis B. Lundborg (1906–1981)