HP-21MX bootstrapping?

Tim Riker Tim at Rikers.org
Thu Feb 21 13:44:26 CST 2008

J. David Bryan wrote:
>> 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  It was superceded because 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.

sorry, yes. I meant 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?

32kw installed. I have tried some simple write/read tests all the way 
through memory and they worked. I have not touched the range settings on 
the card, but it sounds like a good thing to try is just remove 3 of the 
4 cards and see if that changes anything.

> If overflow is setting, the ROM isn't being read, because the microcode is 
> bailing out before that.

Agreed. The question is why. :)

FYI: I'm started taking notes here:


I note the original installed boards there. At present just the labels, 
I'll add part numbers soon. I have moved the IO cards around too.

Tim Riker - http://Rikers.org/ - TimR at Debian.org
Embedded Linux Technologist - http://eLinux.org/
BZFlag maintainer - http://BZFlag.org/ - for fun!

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