Unknows S-100 System

[Chuck Guzis]

On 23 Sep 2007 at 12:35, Roy J. Tellason wrote:

> > without accessing the internal register at 0BBh.
> 
> That's odd.  :-)

In a way, it makes sense.  Consider that they needed *some* way to 
access the mask register.  They could perhaps take one of the "no op" 
instructions, such as "ld c,c" and detail it off for special duty, 
but that's taking an awful gamble that the code doesn't actually 
appear anywhere.  The 8085 RIM/SIM opcodes are already taken by the 
relative jumps, so what to do?  Reserve an I/O address and do your 
best to make sure that 8085-type I/O doesn't inadvertently access it 
by restricting the access to Z80 indirect I/O.  At least if one hits 
the port with some legacy Z80 I/O, it's easy to find.
 
> I never really made use of those,  myself,  nor saw much source code for stuff 
> that did.

This comes in very handy for stack-relative and table addressing.  
For the 8085:

    LXI		HL,offset on stack
    DAD		SP
    MOV		C,M				(5 bytes)

(repeat for every reference)

For the Z80:

    LD		IX,0
    ADD		IX,SP   (done only once at function entry)

    LD		C,[IX+offset]   (3 bytes)

and it leaves HL free for other uses.  The limitation, of course, is 
that the displacement for the Z80 case can't exceed 255 locations--
but that generally isn't a big issue with a lot of x80 C code.
 
> What are these?  I remember some stuff in the magazines early on (probably 
> Byte,  in its first year of publication or so) and have run across some stuff 
> on the 'net about undocumented z80 opcodes,  most of which don't seem to be 
> terribly useful,  but this is the first I've heard of undocumented 8085 
> codes.

Make up your own mnemonics for these:

    08		HL <= HL-BC	All flags affected
    10		Rotate HL 1 bit right   No flags affected
    18		Rotate DE 1 bit left    Carry = old bit 15
    28		Add immediate 8 bit value to HL   All flags affected
    38		Add immediate 8 bit value to SP   All flags affected
    CB		If the V (overflow) flag is set, CALL 0040H
    DE		Load HL with the 16-bit value pointed to by DE
    D9		Store HL into the 16-bit location pointed to by DE

In addition, some 8 bit instructions affect bit 5 of the flags 
register (in a way that I don't completely understand).  DD xx xx 
will jump to xxxx if this bit is clear, ED xx xx will jump to xxxx if 
this bit is set.  

>From my viewpoint, some of these instructions look like afterthoughts-
-but some would have been useful in a couple of instances.

The biggest implication of the above, however, is that there's an 
ugly trap for the lowly coder who decided that 08/10/18/28/38 was a 
no-op.
 
> Do you know of any handy data online for the part?  A basic overview and maybe 
> some info on the instruction set and hardware would be nice.  Somehow or 
> other I never managed to get a hold of any when that chip hit the market.

http://www.datasheetarchive.com/preview/2602635.html
 
> Are those easy enough to find these days if one wanted to play with 'em?

Not terribly difficult--there were a few commercial personal 
computers that used them, mostly for the  CMOS power advantage.   
There's an eval kit from NSC on ePay right now.

Cheers,
Chuck