Very broad question but I see where you're going.
With older CPUs when you see the ability to write a CPU family for one vendor and not others, usually the issue is simply that the pins or memory layout need to be adapted to the different ECU brands, or that not all of the "same CPU" are the exact same CPU - for example, if a different flash chip is used or different pins are used to bring up a part of the board. In the case of "SH705x" I think this is what's going on for you, depending on the vendor and chip revision, different pins are used for serial comms (I think some chips use UART0 and some use UART1 for example?) and there are different flash sizes / layouts at play.
With some newer CPUs, yes, sometimes the mask ROM is customized by manufacturer, and even the first stage of the bootloader / Mask ROM or SBOOT has some kind of security system that needs to be cracked on a per-manufacturer basis.
And in some cases with BDM, which is a bit of a different beast because it's generally handled by a monitor firmware that runs on the application CPU, the application software itself can actually tamper with the interface and prevent or alter its use.
Hey thanks for info!
okay this would explain why many flashing tools are so limited.
I found a flasher that claims to read and write SH705x on board / in circuit with 512kb or 1064kb memory size. Manufacturer says Hitachi ECU work but has no info for other brands. With a look on what you wrote above I think that this flasher will probably not work e.g. on a Denso ECU with SH705x, even if the pinout is done correctly...?
