P21 Boot Help (fwd)

[Penio Penev <penev@xxxxxxx>]

Dear MISC readers,

There have been some problems with an OS install on pisa, which I hope to
have been resolved alseady.

I am resubmitting a message that did not ge through to all of us.

Yours,

--
Penio Penev <Penev@pisa.Rockefeller.edu> 1-212-327-7423

---------- Forwarded message ----------
Date: Wed, 9 Apr 1997 10:35:00 -0700
Subject: P21 Boot Help
From: Dave Lowry <lowry@htc.honeywell.com>
To: MISC@pisa
ReSent-From: Penio Penev <penev>

MISC Readers-

I've been going through the ROM from Ting's '21H kit, trying to understand
how P21 boots.

There is a chunk of 8 bit code that builds the following 20 bit code into
the first 16 words of DRAM:

( byte ) 
2* 2* 2* 2*
2* 2* 2* #
( ff )
@+ and xor ;

( word' )
a push nop a!
@+ 2* nop nop
call byte
2* call byte
a pop nop a!
push !+ pop ;

( boot )
# com # nop
( a8030 -# )
( 800 )
( begin ) push call word'
pop # nop nop
( 80 )
( MISSING: + -until )

Then the 8 bit code does a ; to boot.  Now, here's where it gets tricky.
Per the OK listings, boot ends with a "+ -until" which branches back
to "push call word'".  But, the 8 bit code doesn't build that into DRAM.
So, where does it come from?

Is the "+ -until" the first instruction fetched by boot from ROM into
DRAM?

And, what's the deal with the a8030 that's stored by -# and then complemented?
Is that to get a 1 in the carry bit to address SRAM?

Thanks.

-Dave