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The ram is external in the 17b/27s/42s, not the rom. Its a masked rom processor chip :(
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Alas the ROM and processor are the same IC (which also contains the display and keyboard interfaces). The only other chip in the machine is (standard) RAM chip.
However, there is a way to make the 42S (and therefore, I assume the 17B-II) processor run from external ROM. HP development machines had a socket on the back which held a standard EPROM. From what I remember (and it will take me some time to find out), you tie one pin on the processor chip to one of the power lines to disable the internal ROM, then hang an EPROM off the address and data lines, with one more pin of the processor being the chip-enable line.