Why does a GCU have an integer counter, does it need to count something to measure time or cycles of something?
The purpose of an integer counter is to provide a standard measurement of time.
Remember that hardware can run at varying speeds, so we can't rely on hardware cycle speed to measure time. E.g., suppose today a CPU runs at 1 GHz, but tomorrow a replacement CPU comes out at 2 GHz, so each hardware cycle is now twice as fast. We don't want all of our time measurements to be suddenly be off by a factor of two!
Therefore a counter is provided which always increases at a predictable, set time period (called the
tick time period) regardless of the underlying hardware speed.
A common tick period is 100 Hz. I.e., the time counter will always increment once every 1/100th of a second, regardless of the speed of the hardware. An elapsed time of 100 ticks means 1 second has passed, on any hardware.
Most real-time systems are completely tick based. At each and every tick, the system "kernel" is activated and every running task re-scheduled for execution based on their priority and allocated processing time budget (also measured in ticks).
If all modern computers are coded in 64 bit sizes, why did Boeing stick with 32 bit.
Boeing probably had little to do with this bug. The affected GCUs would have been supplied by a third-party company.
And that third-party company probably used a Real Time Operating System (RTOS) supplied by yet another company.
My guess is this integer overflow is probably in the RTOS or related code. The bug might have been discovered in some completely unrelated software (maybe not even aviation software) using the same RTOS.
The speculation is that the buggy code is a 32-bit
signed counter measuring 100 Hz ticks. So with one bit taken for the sign (+/-), that leaves 31-bits for the counter and 2^31/(60*60*24*100) = 248.55 days.