RE: [Sdcc-user] How to Get Memory Size in Real Time

[Simon H. <Sim...@co...>]

Simon Hosie:
> If there's a risk that RAM will map to multiple addresses by not
> decoding enough address bits then it's tricky.  You'd have to make =
sure=20
> that a write to a particular location doesn't get mirrored in any =
lower
> location.  I've also seen people say that they've managed to detect a
> fully populated address space just by storing the test data in the
> capacitance of the tracks and connectors.  Use a complex, multibyte
> pattern to avoid that.

Olaniyi Bajomo:
> Sorry I don't get what you mean by that last part can you please
> explain further,

Well, if you don't decode all of the address bits, then a RAM chip could =
appear (for example) at 0x0000, 0x4000, 0x8000, and 0xC000 and behave =
just the same no matter which base address you use.  If you're scanning =
up from zero writing data and then reading it back to see if it was =
stored in something, you may cross from 0x3FFF to 0x4000 successfully, =
and then when you test 0x4000 you'll get a valid result because it's =
just the same as 0x0000 - but if you actually tried to use address you'd =
be destroying other data.

The other point, regarding capacitance, is where you write data out to =
an address, and so all the tracks and connectors up to where RAM would =
be plugged in go to the appropriate voltages to represent your test data =
in the write cycle, and then when you read the data back they're still =
in that state because there hasn't been any influence to change.  So it =
looks like your data was saved in RAM when really it's just the =
capacitance of the circuitry.

I think something like this might work, in an 8/16 bit system and the =
mapping is contiguous and no device is going to be less than 256 bytes =
and there is at least one device attached, but I haven't really thought =
it through.

char xdata *end_of_xdata(void)
{
	unsigned char offset =3D 0xFF;
	do
	{
		*(unsigned char xdata *)((offset << 8) + 0xFF) =3D offset;
		offset--;
	} while (offset !=3D 0);

	return (char xdata *)(*(unsigned char xdata *)0xFFFF << 8) + 0xFF;
}