Upgrade to 512k memory board upgrade; HwNote06.01

[Tom Tkacik]

In article <441 at uncle.UUCP> jbm at uncle.UUCP (John B. Milton) writes:
>I have had questions from people who couldn't quite figure out how to do the
>hardare patch to the 512k memory boards, so this is as upgrade to HwNote06.
>I will call this HwNote06.01. Keep the old 06 around, just append this to it.
>
>Once again, this concerns using the 512k (0.5M) RAM ONLY BOARD with a FULLY
>populated EIA/Combo board. If you don't have both of these ingredients, you
>can't make this batch of cookies. According to the manuals, there is no way to
>have both a fully populated (i.e. 1.5M) Combo Card in your system AND a 512k
>memory board, which would give you a complete 2M of expansion RAM. Well, as I
>pointed out in HwNote06, this is not quite true, if you're willing to get out
>the solding iron...
>
The problem is because of the way the EIA/Combo board maps the
expansion memory.  The 2M of expansion memory is mapped to addresses
0x200000 to 0x3fffff.  The expansion bus is designed to use slot dependent
addressing, with each slot at addresses:

	SLOT		.5M Memory Board	1M Memory Board
	 1		0x200000-0x27ffff	0x200000-0x2fffff
	 2		0x280000-0x2fffff	0x200000-0x2fffff
	 3		0x300000-0x37ffff	0x300000-0x3fffff


Which is why if two 1M memory boards are used they cannot be placed into
slots 1 and 2, (or they would have the same addresses).

A 1.5M or 2M memory board must use hardwired slot independent addressing.
A 1.5M memory board (like the EIA/Combo baord) can have two possible
address ranges  0x200000-0x37ffff, or 0x280000-0x3fffff.
If it has the first, then there are no slots to put a .5M memory board,
(any would have overlapping address ranges).  However, if the second
address range were used for the 1.5M memory board, then it could be placed
in slots 2 or 3, and a .5M memory board could be placed into slot 1.

The .5M board would have addresses 0x200000-0x27ffff, and the 1.5M board
would have addresses 0x280000-0x3fffff.  John's hardware mod would be
unnecessary.  AT&T must have designed it with the wrong address range! :-(
(Their own S4BUS spec. documentation even describes this.)  So why did
they get it wrong?

---
Tom Tkacik
GM Research Labs,  Warren MI  48090
{umix, uunet!edsews}!rphroy!megatron!tkacik