Speaking of 6502s, was Re: 70's micros still available - was 1802 problems

[Roy J. Tellason]

On Tuesday 03 January 2006 01:43 pm, Chuck Guzis wrote:
> On 1/3/2006 at 12:42 PM Roy J. Tellason wrote:
> >Any of you guys have advice on how to get a grip on the weirdnesses of
> >these chips,  when what I'm used to is 8080/8085/z80 assembler?
>
> With the 6502, the big lmitation to me was the very small fixed stack (with
> no overlow-underflow traps) and the heavy reliance on "direct" low memory
> cells.

Yeah,  those are the two things that jump out at me all right...

I used to think that a bigger stack was a good idea.  After playing around 
some with some monitor code,  I'm not so sure any more,  and for a simple 
embedded application that might not be as much of a problem as I'd thought.

As for that "low page" stuff?  I guess that could work out fairly well if you 
thought of it as a whole lot of registers or something.  :-)

> No 16 bit operations per se

That could make things a bit more complicated.

> and almost everything you do affects the condition codes.

And that as well.

> You tend to think about everything in terms of 8 bits, even computing
> addresses.   Things that involve more than byte quantities tend to get very
> wordy--for example, try writing a subroutine to move an arbitrary number (up
> to 65K) bytes and compare it with the same in Z80 or even 8080 code.  To me,
> the 6502 has more of a microcontroller instruction set than the 8080/Z80.

One of my favorite approaches to using a monitor type setup was to use a 
dispatch table,  which was trivially easy in the z80 environment,  just take 
the "op" code you're passed,  and index into the table (and I don't even mean 
using an index register per se,  I never had much use for those).  The other 
thing I did nontrivially often was to take a parameter off the stack as 
passed,  that whole cp/m convention of sticking things in registers always 
struck me as rather silly by comparison.  You wanna pass a parameter to 
something,  stick it on the stack,  have the called code remove it,  and 
adjust as necessary.  I haven't looked yet at how easy it would be to do that 
on a 65xx chip.

More of a microcontroller?  Yeah,  maybe.  I have some of those I haven't 
played with much either,  80xx parts mostly,  some with rom but that's easy 
enough to disable and use an external eprom if you don't mind losing a bunch 
of pins.  I just haven't gotten around to them yet.  But ever scrapped 
keyboard has one of those in there.  :-)

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