FF4 SNES SRM Checksum

I checked this out since I'm also curious.

The two bytes at the end $07FE and $07FF are set to #$1BE4 if the save slot is in use.

The checksum is calculated word by word but each word will overlap as the checksum counter is only increased by one byte. This is why both bytes of the checksum increase when one byte is increased.

Partial disassembly

Code:

Transfer A to X through $43
0187B4     85 43         STA $43
0187B6     A6 43         LDX $43
0187B8     60            RTS 

Sanity check a save slot
01995D     BF FE 07 70     LDA $7007FE.l,x        ; SRAM $07FE for save slot X
019961     A8            TAY             ; A to Y
019962     E2 20         SEP #$20        ; 8-bit A
019964     C0 E4 1B     CPY #$1BE4        ; Is save slot in use?
019967     F0 04         BEQ $996D        ; if so get checksum
019969     E2 20         SEP #$20        ; 8-bit A
01996B     18            CLC             ; Carry clear, no save file
01996C     60            RTS             ; Return

Checksum check a save slot
01996D     A5 51         LDA $51    
01996F     1A            INC a    
019970     20 5F CC     JSR $CC5F        ; Calculate checksum to X
019973     C2 20         REP #$20        ; 16-bit A
019975     8A            TXA             ; Checksum to A
019976     A6 45         LDX $45
019978     DF FC 07 70     CMP $7007FC.l,x        ; SRAM checksum for save slot
01997C     D0 EB         BNE $9969        ; exit if no match, with carry clear
01997E     E2 20         SEP #$20        ; 8-bit A
019980     38            SEC             ; Carry set, save file is ok
019981     60            RTS             ; Return

Calculate checksum for save slot
01CC5F     85 4E         STA $4E            ; Save slot index
01CC61     0A            ASL a            ; Multiply by 2
01CC62     65 4E         ADC $4E            ; Multiply by 3
01CC64     20 B4 87     JSR $87B4        ; TAX through $43
01CC67     BF 86 CC 01     LDA $01CC86.l,x        ; High byte of pointer to save slot
01CC6B     85 4E         STA $4E            ; store it
01CC6D     C2 20         REP #$20        ; 16-bit A
01CC6F     BF 87 CC 01     LDA $01CC87.l,x        ; Low 2 bytes of pointer
01CC73     85 4F         STA $4F            ; store it
01CC75     A5 41         LDA $41
01CC77     A0 FA 07     LDY #$07FA        ; Size of save slot
01CC7A     18            CLC             ; Carry clear
01CC7B     67 4E         ADC [$4E]        ; Add next byte and carry
01CC7D     E6 4E         INC $4E            ; next byte
01CC7F     88            DEY             ; decrease size left
01CC80     D0 F9         BNE $CC7B        ; Loop for all bytes
01CC82     AA            TAX             ; Set checksum to X
01CC83     E2 20         SEP #$20        ; 8-bit A
01CC85     60            RTS             ; Return

Pointers to save slots
01CC86     00 10 7E 
          00 00 70     ; Slot 1 at $700000
          00 08 70     ; Slot 2 at $700800
          00 10 70     ; Slot 3 at $701000
          00 18 70     ; Slot 4 at $701800

Note: CPU bank 01 is at 00 in the ROM because FFIV is LoROM.

Good luck and happy hacking!