The story behind the 55ms timer tick rate goes all the way back to the original IBM PC BIOS. The original IBM PC used a 1.19MHz crystal, and 65536 cycles at 1.19MHz equals approximately 55ms. (More accurately, it was more like 1.19318MHz and 54.92ms.)<\/p>\n
But that just pushes the question to another level. Why 1.19…MHz, then? <\/p>\n
With that clock rate, 216<\/sup> ticks equals approximately 3600 seconds, which is one hour. (If you do the math it’s more like 3599.59 seconds.) [Update: 4pm, change 232<\/sup> to 216<\/sup>; what was I thinking?] <\/p>\n What’s so special about one hour? <\/p>\n The BIOS checked once an hour to see whether the clock has crossed midnight. When it did, it needed to increment the date. Making the hourly check happen precisely when a 16-bit tick count overflowed saved a few valuable bytes in the BIOS. <\/p>\n Another reason for the 1.19MHz clock speed was that it was exactly one quarter of the original CPU speed, namely 4.77MHz, which was in turn 4\/3 times the NTSC color burst frequency of 3.5MHz. Recall that back in these days, personal computers sent their video output to a television set. Monitors were for the rich kids. Using a timer related to the video output signal saved a few dollars on the motherboard. <\/p>\n Calvin Hsia has another view of the story behind the 4.77MHz clock<\/a>. <\/p>\n