The 8253 (timer) implementation seems downright weird. The timers only count when the low byte of their current counter value is read!? And they are only read when ports 0x40-0x42 are read? So there is no proper timer interrupt?!

... no, wait...

The 8253 calculates a 'timer_period' which is used in the {linux/win/pico}-main files. They read the platform time and call 'doirq(0)' to signal IRQ0 whenever the time is right. The actual counting in the emulated 8253 isn't used and has absolutely no relation to the IRQ0 signals. Quirky.