Re: ROM Question II More Info

[dempson@actrix.gen.nz (David Empson)]

Bryan Parkoff <BParkoff@satx.rr.com> wrote:

> ROM Question II More Info
> 
>     I have studied Apple I, II and II+.  It shows that all Apple I/II family
> have the EXACT same 6502 microprocessor that they do not get involved with
> 65c02 nor 65xxx (IIgs).  I don't believe that Apple I has 6501.  It does not
> exist, but it always appear to show 6502.

Noone was suggesting that the Apple I had a 6501 processor.  I've never
even seen one.  According to David Wilson's post in the earlier thread,
it had the same core as the early 6502, but had a different pinout,
being able to be used as a plug-in replacement for a 6800.

I was speculating whether the Apple I might have used one of the
earliest 6502 processors which was missing the ROR instruction.

>     I have reviewed a very few routines between Apple I and II.  Only very
> few are very similar, but more codes have added over existing routines to
> expand more memory for the graphic such as lo-res and hi-res.  $400-7FF are
> the same for text.
>     Yes, it can limit up to 32KB for Apple I, but it is possible to break
> the barrier beyond 32KB however Apple II/II+ can reach 64KB -- Good!

I'll have to take your word for all that - it has been a long time since
I last saw a reasonably detailed description of the Apple I
architecture.

I was under the impression that it used a different mechanism to
implement the text screen, and its maximum RAM wasn't as high as 32KB.

(Also note that the II/II+ only reaches 64KB through the use of bank
switching in the ROM space.  Its main RAM is limited to 48KB.)

>     I always like to use monitor routine with only 256 bytes that it helps
> us to study back to original before we can expand to understand enchanced.

It would certainly take less effort to understand the operation of a 256
byte monitor, but it doesn't necessarily follow that you need to do this
before being able to understand the Apple II 2KB monitor.  I certainly
didn't.

Source code is available for the Apple II monitor, so you can read that
and deal with it in pieces.  Some of the code deals with lo-res
graphics, which isn't relevant to the monitor itself.  Another large
chunk is taken up with the disassembler, which is mostly a lookup table.

Another point to consider is that if the 256 byte monitor and 2KB
monitor use completely different principles, then understanding the
small one won't help much toward understanding the big one.

> For example, can you understand advanced complex alegbra?  You can't unless
> you can understand basic alegbra before you can continue to understand from
> the beginning to the complex.

That is a flawed analogy.  In the case of a simple and complex monitors,
the building blocks are the same (6502 instructions) - there are just
more of them.  If you understand the instruction set, then understanding
the monitor is only a question of time.

By comparison, advanced algebra requires the introduction of new
techniques for solving more complicated problems.  If you don't know
these techniques, you are stuck.

-- 
David Empson
dempson@actrix.gen.nz