increment or decrement one of 16, 16-bit registers

On Sat, 13 May 2017 13:07:40 -0700, Jecel wrote:

> On Thursday, May 11, 2017 at 7:21:33 PM UTC-3, rickman wrote:
>> On 5/11/2017 5:55 PM, Kevin Neilson wrote:
>> > 24 cycles?  Holy smokes.  I remember most of the 6502 instructions
>> > being 2-3 cycles.
>> 
>> No one ever said the 1802 was fast.  If you want slow, you should have
>> seen the 1801!  lol ;)
> 
> Indeed, many early microprocessors looked a lot more impressive until
> you saw how many clock cycles each instruction took.
> 
> But it is important to remember that there were two different clock
> styles and it is complicated to compare them directly.
> 
> The 6502, 6800 and ARM2 used two non overlapping clocks. This required
> two pins and a more complicated external circuit but simplified the
> internal circuit. In a 1MHz 6502, for example, you have four different
> times in which things happen in each microsecond: when clock 1 is high,
> when both are low, when clock 2 is high and when both are low again.
> 
> Many processors had a single clock pin, which allowed you to use a
> simple oscillator externally. But to have the same functionality of the
> 1MHz 6502 this single clock would have to be 4MHz so you could do four
> things in each microsecond. This was the case of the 68000, for example.
> The Z80 only needed to do three things.
> 
> -- Jecel


Motorola's MC6809 was available in both clocking varieties - The 'E' 
suffix part number was the one with the external clock generator and two 
quadrature clock input pins (called E and Q).
The non-'E' suffix part number had one clock input pin (EXTAL) and 
divided by four internally.  E and Q were outputs in this case.
A pin (MRDY) was available to freeze the divide by four counter to insert 
wait states.

It had an 8 bit ALU.  16 bit operations took two cycles, and the 8 x 8 
multiply took 8 cycles.


I vaguely recall wire wrapping one of these as a hobby project in the 
early to mid '80s.

Regards,
Allan

On 5/13/2017 9:16 PM, Allan Herriman wrote:
>
> I vaguely recall wire wrapping one of these as a hobby project in the
> early to mid '80s.

I vaguely recall wire wrapping!

-- 

Rick C

On Wed, 10 May 2017 16:42:59 -0500, Tim Wescott wrote:

> I've been geeking out on the COSMAC 1802 lately -- it was the first
> processor that I owned all just for me, and that I wrote programs for
> (in machine code -- not assembly).
> 
> One of the features of this chip is that while the usual ALU is 8-bit
> and centered around memory fetches and the accumulator (which they call
> the 'D' register), there's a 16 x 16-bit register file.  Any one of
> these registers can be incremented or decremented, either as an explicit
> instruction or as part of a fetch (basically, you can use any one of
> them as an index, and you can "fetch and increment").
> 
> How would you do this most effectively today?  How might it have been
> done back in the mid 1970's when RCA made the chip?  Would it make a
> difference if you were working with a CPLD, FPGA, or some ASIC where you
> were determined to minimize chip area?
> 
> I'm assuming that the original had one selectable increment/decrement
> unit that wrote back numbers to the registers, but I could see them
> implementing each register as a loadable counter -- I just don't have a
> good idea of what might use the least real estate.
> 
> Thanks.

Found the patent for the 1801; it shows the increment unit as separate 
from the registers, and separate from the ALU.  I would assume that if 
it's to be binding, the patent needs to reflect what's actually there, at 
least in the large:

http://www.cosmacelf.com/publications/data-sheets/cdp1802-rca.pdf

-- 
www.wescottdesign.com