J. David Bryan
jdbryan at acm.org
Thu Feb 21 12:47:36 CST 2008
On 21 Feb 2008 at 10:21, Tim Riker wrote:
> Thanx for the help! :)
> It has the soldered in default rom (1816-0420), as well as another that I
> have not yet identified (1816-0639).
The latter is an early 7900/7905 disc boot ROM. It was superceded because
it will not boot a multiple-drive MAC (7905/06/20/25) system.
> I have tried setting memory for the address range 177700-177777 and I
> can set/reset values everywhere I've tried.
Note that memory ranges go up to 077777 octal (32K words). Bit 15 is
reserved for indirection. So the upper range would be 077700-077777.
> Preset clears overflow, and hitting IBL (with S as 001100) sets it....
The IBL microcode sets overflow as an error indication. This occurs in two
cases: either it cannot find any read/write location in memory, or the
select code entered in the S register is less than 10 octal. (11 octal
should be OK. I presume that you're hitting STORE and that if you select
another register and then S again, the correct value returns?)
The memory check starts at 077700 (32K - 64) and decreases by 4K each time
until the read data matches the written data (the data used is the two's
complement of the address). How much memory is in the machine, is the
address range configured correctly on each memory card, is the memory
controller good, and is there a MEM card in place or not?
If overflow is setting, the ROM isn't being read, because the microcode is
bailing out before that.
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