optiroc/superfamicheck

Simple Super Nintendo ROM image inspection utility

superfamicheck's Issues

Fixing checksum on ExHiROM games can take two iterations

Hi again!

When you have an ExHiROM game where the checksum bytes themselves are corrupt (more precisely, when checksum != (checksum complement XOR $FFFF), it takes two iterations to actually fix the checksum.

To reproduce, with Tales of Phantasia as an example:

• Go to offset $40FFDC, and overwrite the checksum bytes (should be A8 83 57 7C) with, say, 12 34 56 78
• Check and fix the ROM. SuperFamicheck will output the following:

superfamicheck tales.sfc -f
ROM info for file "tales.sfc"

 Header at   0x40ffb0
 Title       TALES OF PHANTASIA   
 Game code   ATVJ
 Maker code  AF
 Country     0x00 (Japan)
 Version     1.0

 ROM size    0x0d (8192KB, actual size 6144KB)
 RAM size    0x03 (8KB)
 Map mode    0x05 (Extended HiROM)
 Chipset     0x02 (ROM, RAM, Battery)
 Speed       120ns

 Checksum    0x7856
 Complement  0x3412

The following issues were found:
 Checksum/complement should be 0x7b6d/0x8492

Writing ROM image to file "tales.sfc"
 Fixed checksum

• The new checksum is obviously bad, too. However, running the command once more yields the correct result:

superfamicheck tales.sfc -f
ROM info for file "tales.sfc"

  Header at   0x40ffb0
  Title       TALES OF PHANTASIA   
  Game code   ATVJ
  Maker code  AF
  Country     0x00 (Japan)
  Version     1.0

  ROM size    0x0d (8192KB, actual size 6144KB)
  RAM size    0x03 (8KB)
  Map mode    0x05 (Extended HiROM)
  Chipset     0x02 (ROM, RAM, Battery)
  Speed       120ns

  Checksum    0x7b6d
  Complement  0x8492

The following issues were found:
  Checksum/complement should be 0x7c57/0x83a8

Writing ROM image to file "tales.sfc"
  Fixed checksum

This means that fixing the checksum within one iteration only works if the checksum is bad but its complement still equals checksum XOR $FFFF.

This issue only seems to be affecting ExHiROM games. I also tried it with Super Mario World (4 Mbit LoROM) and Chrono Trigger (32 Mbit HiROM), and it fixed the checksum in both cases immediately.

Thanks! :-)

"Fixed checksum" output when removing copier header

Hi!

Upon removing a copier header, the tool reports the checksum was fixed although it wasn't, nor was it bad in the first place.

superfamicheck TALES.SWC -f
ROM info for file "TALES.SWC"

  Header at   0x40ffb0
  Title       TALES OF PHANTASIA   
  Game code   ATVJ
  Maker code  AF
  Country     0x00 (Japan)
  Version     1.0

  ROM size    0x0d (8192KB, actual size 6144KB)
  RAM size    0x03 (8KB)
  Map mode    0x05 (Extended HiROM)
  Chipset     0x02 (ROM, RAM, Battery)
  Speed       120ns

  Checksum    0x7c57
  Complement  0x83a8

The following issues were found:
  File has a copier header (0x200 bytes)

Writing ROM image to file "TALES.SWC"
  Removed copier header
  Fixed checksum

Thanks! :-)