The save routine is the routine that, well, saves the game. The routine begins at $009BC9. The documentation here was done by p4plus2.
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
- 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).