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frequency generation and what frequency the generated was anyones guess. In general the 32.768kHz RTC clock x-tal was the best, because that was a regular wrist-watch Xtal, whereas the X-tal generating the ISA bus frequency was much lower quality, often costing as much as several cents a piece, so it made good sense to check the ISA bus frequency against the RTC clock. The other relevant property of those machines, is that they typically had no more than 16MB RAM. These days, CPU chips croak if their clocks are not tightly within specs and all necessary frequencies are derived from the master crystal by means if PLL's. Considering that it takes on average 1.5 second to calibrate the frequency of the i8254 counter, that more likely than not, we will not actually use the result of the calibration, and as the final clincher, we seldom use the i8254 for anything besides BEL in syscons anyway, it has become time to drop the calibration code. If you need to tell the system what frequency your i8254 runs, you can do so from the loader using hw.i8254.freq or using the sysctl kern.timecounter.tc.i8254.frequency. |
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| .. | ||
| atpic_vector.S | ||
| atpic.c | ||
| clock.c | ||
| elcr.c | ||
| icu.h | ||
| isa_dma.c | ||
| isa.c | ||
| isa.h | ||
| nmi.c | ||