IBM-PC/XT BIOS is also missing some services from the later PC/AT BIOS #1
Description
One of the most prominent missing things in the original IBM-PC and "younger brother" PC/XT is RTC and BIOS calls, which use it. One such example is function 86h from INT 15h. On XT class machine is no good method to determine the... time. All time measurement is related to the CPU clock.
Most PC benchmarks I've tried, haven't noticed that I was running my XT clone with 1/3 of the original clock...