23 Sep
2007
23 Sep
'07
11:40 a.m.
On 23 Sep 2007 at 12:35, Roy J. Tellason wrote:
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.
without
accessing the internal register at 0BBh.
That's odd. :-) 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