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I am trying to find a way to dump the rom contents of several MAB8421 microcontrollers. I know that on many 8048 MCUs, you can use the external access pin to dump the rom, but the 8421 doesn't have any external memory pin. I was wondering if there was possibly another way to dump the rom, but there is little to no documentation I could find on a process of doing so.

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If you're really asking about the 8421 mask-programmed MCU (and not about one of its brothers that expose address lines for external ROM support), the answer is simple: There is no way to read the ROM contents from the outside.

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    After all, not being able to access the code is a major way to prevent any cloning, isn't it :))
    – Raffzahn
    Commented Sep 19, 2022 at 20:08
  • @Raffzahn - That was probably not the main reason for not providing access. There's just no need to spend some expensive external pins to read the ROM when the guy who bought the chip already knows what's in there ;) (Because he's asked you to put it there)
    – tofro
    Commented Sep 19, 2022 at 22:04
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    Well, I would think the same. But at the same time I would consider his competition, being a great reason to limit ROM access. THere is a reason why read-disable-'fuses' have been implemented early on with (E)PROM based microcontrollers as well. A 'tradition' continued with CPLD, FPGA and modern FLASH based controllers.
    – Raffzahn
    Commented Sep 19, 2022 at 22:24
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The MAB8421 is a version with the factory mask ROM, so while the MCU core is the same as with the ROM-less version which requires external ROM, the pins for the ROM are used internally and there is no access to these pins.

So without hacking into the chip and looking or debugging the silicon, there is no way to dump the ROM contents. Unless the software has a feature to transmit out the ROM contents somehow, but that ìs unlikely.

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