System self-test

AES and MVS system ROMs compute their own checksums and compare them against the 16bit value at address $000080. The checksum is a simple byte sum of the whole system ROM except locations $000080 and $000081.

Checks order ?

Cartridge systems

Information by Channelmaniac.

AES
MVS	Crosshatch	WORK RAM ERROR	COLOR RAM BANK0/1 ERROR	VIDEO RAM ERROR (VRAM)	SYSTEM ROM ERROR	MEMORY CARD ERROR
Meaning	All tests passed (AES does not test the Z80). Cartridge not detected or not present.	Work RAM error ($100000 region)	Palette RAM error ($400000 region)	Video RAM error	System ROM checksum mismatch (0xC00000 region)	Memory card error (only tested if card present and blank)

The UniBios case

The Universe BIOS does not use the above color codes to indicate errors.

MVS checks

Additionnal checks are performed on MVS systems:

• Backup RAM: BACKUP RAM ERROR (Battery-backed RAM)
• RTC chip: CALENDAR ERROR
• Z80: Z80 ERROR

When a MVS board is powered up without a cartridge inserted, the border color around the edges of the crosshatch screen indicates the system ROM region:

• Green: 6 slot (ROM name ?)
• Cyan: USA, SPU2
• Blue: Spanish, SPS2
• Red: Japanese, SPJ2