The simplest boot ROM and BIOS uses a serial port as a user interface. This connects to a terminal or a terminal emulator.
There are source code files for generating software that uses either of the two text-based video boards, the MVS1 (48x16) and the MVS2 (80x20).
An operating system needs a real-time clock for time-stamping files. Without one, the user needs to enter the date and time at start-up. The Z80 processor boards did not have an RTC onboard, but there was a battery-backed CMOS RAM board with an RTC on it, called the SCRAM.
The source files include TIME.ASM (no clock) and TIMES.ASM (time from SCRAM board).
I shall make a project for the SCRAM board soon.
It had up to 48K of CMOS RAM (six 8K chips), a NiCAD battery and an MC146818 clock. This has a large DIP24 footprint, hence it often got left off designs.
These day designers have RTC chips in DIP8 packages. The BIOS might be modified to use such a device.
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