Boot process

MVS boot process

• Interrupts are disabled and acknowledged, watchdog is kicked, $4000 in REG_LSPCMODE (Autoanim speed = 64 frames)
• Puts $FF on all 3 LED latches (uses NOP waits)
• Use BIOS vector table and onboard FIX/M1 ROM, write byte to $380065 and $380067 (?)
• Checks the 64KiB of work RAM by filling with $5555 and reading back, filling with $AAAA and reading back, filling with increment and reading back.
• If all dipswitches are down, check backup RAM like before
• If not, check if "BACKUP RAM OK !",$80 is at $D00010
• If string isn't there:
  • Go for full check
  • Init backup RAM by filling the 64KiB with zeroes backwards ($D0FFFF to $D00000)
  • Copy "BACKUP RAM OK !",$80 string
  • Write 01 01 01 02 01 01 01 03 00 00 30 3B 00 (13 bytes) to $D0003A
  • Write $23(35) to $D00122
  • Write 8 times $0000FFFF from $D00124
  • Write $F0F0F0F0 to $D001AC
  • Write $FFFF to $D001B0
  • Reprotect backup RAM
• Checks the 2*8KiB of palette RAM, bank 0 and 1
• Checks the low VRAM ($0000~$7FFF) and high VRAM ($8000~$85FF) with the same $5555/$AAAA/inc method
• Does continuous RAM and backup RAM check if all dipswitches are down ?
• Tells the RTC to output a 1s period pulse, time is measured in frames using the Vblank interrupt between two falling edges, if less than 57 or more than 64 frames: Calendar error. Warning: code can get stuck in a loop if RTC pulse line is cut ?
• Weird check on the Vblank handler pointer in the bios itself, never changes ?
• Computes 16bit bios checksum by adding together 128 bytes from $000000 (vector table) and all the ones from $C00082 to $C1FFFF. The value is then compared against the one at $C00080, if it's different: "SYSTEM ROM ERROR"