> The standard clock for the system timer runs at 1,193,182 Hz, derived > for the original IBM PC as the 4.77 MHz processor clock divided by 4.
Digging a bit deeper clarifies it: http://sos.enix.org/lxr/source/hwcore/i8254.c?v=6.5 |* Ahhh PC systems are nice toys: this maximum "strange" frequency |* equals that of the NTSC clock (14.31818 MHz) divided by 12. In |* turn, the famous 4.77 MHz cpu clock frequency of the first IBM PC |* is this same NTSC frequency divided by 3. Why the NTSC frequency as |* a base "standard" ? Because the 14.31818 MHz quartz were cheap at |* that time, and because it allows to simply drive altogether the |* cpu, the "time of day" timer, and the video signal generators. So, the base frequency actually was a 14.318 Mhz quartz, not a 13.125 as I assumed in my previous post. But that's enough history lessons for a day. ;) Vinzent. -- GRATIS für alle GMX-Mitglieder: Die maxdome Movie-FLAT! Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome01 _______________________________________________ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
