Re: 1541B ROM not shadowed

From: Marko Mäkelä (
Date: 2002-07-03 07:32:29

William Levak wrote:
> The 1541 rev. B circuit board (250448-01) does not shadow the ROM to
> $8000-BFFF as the other versions of the circuit boards do.  
> This is not much of a difference, but if your program makes use of the
> shadowed data, it will not work correctly on this version of the board.

I think I have seen 1541 clone firmware (maybe from an Oceanic OC-118 or 
Excelerator Plus) where all JMP and JSR labels were shifted by $4000, 
plus minus one.  Most of the code was shifted by one byte, so if the 
1541 ROM did JSR $C123, this clone ROM would do JSR $8122 or $8124.  I 
guess the motivation was to make the ROM image look different from the 
1541 ROM to avoid any copyright lawsuits.  Once I disassembled both ROMs 
and applied some regular expression transformations to the disassembler 
dump of the clone ROM, the dumps became identical.

Some clone drives also twist some data lines going to the EPROM chip. 
I'm not sure if my OC-118 does that; I haven't bothered to desolder the 
chip and to read it out in an EPROM burner.


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