fx-5800p Internal Information
SOC: OKI ML610901B (CPU, RAM, Mask ROM, LCD Driver)
Flash: 29LV800CBT (Only half of the Flash ROM is used, weird)
Oscillator Type: Unknown, probably embedded.
The oscillation frequency can deviate by a huge amount, probably RC or on-chip oscillator.
2. OS Update
“OS” refers to the binary data residing in the external Flash ROM. In fact, most of the core math, hardware-access, communication, and maintenance functions are hard-wired inside the SOC (Mask-ROM). The font set and the power-off screen are also in the Mask-ROM.
From what I know so far, the UI, statistics, and programming functions are probably in the external flash, which can be reprogrammed.
To enter the OS Update menu:
1. Shutdown the device using [SHIFT]+[AC/ON]
2. Hold [MODE]+, Press [AC/ON]
3. Press [⬅️], then 
You’re now in the maintenance menu, press  for OS Update functions. (Warning: you may accidentally delete your OS, be careful. Self-test or OS Update will cause a memory clear)