DS12887 pcb substitute with battery

[Maciej W. Rozycki]

On Sun, 22 Jan 2017, Tony Duell wrote:

> On most PCs the RTC chip is mapped as I/O ports, not memory, so you couldn't
> execute code from its RAM anyway. I suppose on (say) a 68000 based machine
> you could (there is no separate I/O port address space on that processor) but
> I have never seen it done.

 It's not the I/O vs memory mapping that is the problem with treating the 
data space of the MC146818 and its derivatives as executable code; most 
processor architectures do not have a dedicated I/O address space anyway.  
The issue with these chips is indirect addressing, that is that ordinarily 
they only present a pair of 8-bit registers on the bus they are attached 
to, one being an address register to select the intended location within 
the embedded memory and the other being a data window register which only 
gives access to a single location at a time.

 Direct linear access to data in these chips can be achieved with a small 
amount of glue logic, but this is seldom done.  I only know of a single 
line of computers, namely the MIPS processor based DECstation machines, 
which had such logic so that internal locations of their DS1287 chip 
appeared linearly in its dedicated MMIO space slot, providing for atomic 
RMW accesses from the CPU for example.

 But even there the locations were only mapped onto the least significant 
byte lane of the 32-bit internal I/O bus, so direct execution would not be 
possible due to the unpredictable (or at least uncontrollable, e.g. if 
pull-ups or pull-downs were provided) nature of bit patterns presented in 
the upper 24 bits of each 32-bit memory word.  You'd have to copy data 
first from the chip's internal RAM to main RAM to execute it, just as it 
was done by DECstation firmware with some option board code which was 
contained in 8-bit ROM chips.

  Maciej