each save ram slot is 1280 bytes, slot 1 begins at 00h, slot 2 at 500h, slot 3 at A00h.
each slot is then duplicated F00h later, slot 1 F00h, slot 2 1400h, slot 3 1900h

checksum routine:

first checksum, 4FEh
take 90, subtract each even byte from it (00h, 02h, 04h,) if at any time it goes below 0, add 256, and subtract 1 from the second checksum.

second checksum 4FFh
take 90, subtract each odd byte from it (01h, 03h, 05h,) if at any time it goes below 0, add 256.


C source to calculate the checksum

int temp=0;
int upper=90;
int lower=90;

for(temp=0;temp<1278;temp+=2){
  upper-=file.get();
  if(upper<0) {upper+=256;lower--;}
  lower-=file.get();
  if(lower<0) {lower+=256;}
  }
