SMW Save routine

The Super Mario World save routine is the routine that, well, saves the game. The routine begins at $009BC9. The documentation here was done by p4plus2. The Routine

save: ; PHB  ;\ Switch to current data bank PHK  ; | PLB  ;/ LDX.w $010A  ; Load save game LDA.w high_byte,X  ;\ Get SRAM index XBA  ; | LDA.w low_byte,X  ; | REP #$10  ; | TAX  ;/ Transfer to X .write_file  ; LDY.w #$0000  ; Initalize the buffer index to 0 STY $8A  ; Zero the checksum counter .copy_buffer  ; LDA.w $1F49,Y  ;\ Copy one byte over STA.l $700000,X  ;/ CLC  ;\ Add the value of the mirror to the checksum ADC $8A  ; | STA $8A  ; | BCC .no_carry ; | Handle overflow INC $8B  ;/ .no_carry ; INX  ; Increment the SRAM index INY  ; Increment the save buffer index CPY.w #$008D  ;\ Check if the entire buffer is copied BCC .copy_buffer  ;/ Loop again if not REP #$20  ; 16 bit A LDA.w #$5A5A  ; Load the default checksum value SEC  ;\ Subtract to get the correct complement SBC $8A  ; | STA.l $700000,X  ;/ CPX.w #$01AD  ;\ If at the end of SRAM BCS .return  ;/ TXA  ;\ Offset the file and duplicate it ADC.w #$0120  ; | TAX  ;/ SEP #$20  ; 8 bit A BRA .write_file ; Duplicate file(used to finish the checksum)

                                       ;

.return ; SEP #$30  ;\ Restore data banks and return PLB  ; | RTL  ;/

high_byte: db $00,$00,$01

low_byte: db $00,$8F,$1E

Explanation

   Get the file index.
   Index a table to get the SRAM address.
   Copy the SRAM buffer to SRAM ($1F49, which is actually a mirror of $7E:1EA2-$7E:1F2E).
   While doing this it keeps a sum of the buffers contents, this is used to calculate the check sum complement.($8D contains the sum).
   The check sum is subtracted from #$5A5A (the default checksum value) and written to SRAM as the last byte of the file.
   Make a duplicate copy of the file, presumably this was to ensure data integrity. (Basically repeated from step 3, however 288 bytes are added to the SRAM index).