The HP-71 and the Ubiquitous Binary File

[Christoph Giesselink]

(08-15-2020 04:19 PM)J-F Garnier Wrote:  The HHP and CMT 32K and 64K EPROM modules are using one logical chip (and a single EPROM actually) so the checksums of each 16KB block were no more really needed, and the CMT tools are using a single checksum byte that is simply the last byte of the EPROM.
I found this checksum scheme much better and simpler.

I don't agree with this. IMHO a single 1 byte checksum over a size of 32KB or more makes no sense in my eyes any more. The risk of multiple errors resulting into a correct checksum grows with the size of the checksum area.

Just think about transfer protocols. One of the easiest ones is the X-Modem protocol with a block size of 128 byte. With a block size of 128 bytes you can protect the data block using an 8 bit checksum or a 16 bit CRC. The 16 bit CRC should be prefered by the receiver. When you use X-Modem-1K with a block size of 1024 bytes, the use of the 16 bit CRC is mandatory.

So lets have a look back into the HP calculator world end of 80'ies. The next calculator generation, still without a CRC generator, is the 18C and 28C. Here a 32KB ROM block is protected by a 16 bit checksum. Using checksums in the Saturn CPU family ended with the Low End Pioneer series calculators with the 1LU7 Bert chip. There for the 10KB ROM a 16bit checksum is used. All others and later ones used one or more 16 bit CRC's.

There's one exception, I found no protection in the HP39/40 ROM. A self test on this calculator shows no result of a ROM check and even more, changing larger parts of the ROM, don't bother the self test. The HP49G is better in this case, here I know that the 1st 32KB containing the BOOT block has no CRC. All other Flash pages use CRC's also the objects in the Port2 memory.