"DoN. Nichols" wrote in message
...
On 2016-09-05, Jim Wilkins wrote:
"DoN. Nichols" wrote in message
...
There were 8 of the 2102s in my Altair 680b (kit computer based
on the Motorola 6800, not the Intel 8080 which the first Altair
had).
While not too fast, they were still a lot faster than the machine,
simply because they pulled the clock down to 500 KHz instead of 1
MHz
(the max for the CPU chip) because they used 1702a EPROMs for the
monitor, and did not bother implementing a stretchable clock for the
system, so it had to be slowed down to the 1702a's speed.
Otherwise, it
could have been 1 MHz -- or later, 2 MHz for the 6800B, which I
wire-wrapped into a replacement CPU card for the SWTP 6800 (moving
the
baud-rate clock off the CPU board, because the original of that
system
had something like a 768 KHz CPU clock to divide down to match baud
rates. :-) (I was also using the 6116s in that system at its end of
life. A lot more reliable than the dynamic RAM chips which were
used on
some boards. So -- the replacement CPU board was wire-wrap too.
They had
samples of 6116 2K x 8 and 6264 8K x 8 CMOS static memory but IIRC
they weren't fast enough, so I was given them for my wirewrap 8080
computer. At first it had 256 bytes of memory.
Four of the 6264 chips would have saturated the address space of
the 6800. :-)
I scratch-built my wirewrapped computer to learn how to design and
program them instead of to use it, though the editor / assembler I
wrote for it worked well enough to compose and print my resume. The
I/O circuits were simplified versions of those in the IBM PC and the
RS Color Computer.
It had a minicomputer switch panel with enough circuitry to write and
display memory and vary the CPU clock, down to 1 cycle per second for
debugging. At first I had to toggle in a boostrap loader that read in
the monitor program (a mini OS) from a Teletype tape.
That got old very quickly so I added NiCads to keep the 6116 alive. As
more slightly used sample RAM became available I installed more 6116s
and rewired the lowest socket to take a 2816 EEPROM. We had been
experimenting with adaptive algorithms to program them rapidly in
production.
Eventually my growing code collection crashed into the 8080's lack of
relative jumps and I stopped working on it. By then better CPUs were
coming along too rapidly to know which to upgrade to. I would have bet
on the 6809, 68020 or 8086 instead of the 8088 that soon dominated in
the IBM PC. The company chose the DEC LSI-11 and then the TI TMS9900.
At later jobs the TMS320 DSP family was THE choice for fast dedicated
systems, an early color scanner and Mitre's digital radios. The DRAM
controller I designed was for the scanner, to prioritize competing
memory requests from the A/D, DSP and the IEE488 controller and then
try to fit in refresh cycles. During a scan the data stream from the
A/D converter couldn't wait, but it effectively kept the memory
refreshed by hitting every row repeatedly.
https://en.wikipedia.org/wiki/Memory_refresh
" A normal read or write cycle refreshes a row of memory, but normal
memory accesses cannot be relied on to hit all the rows within the
necessary time, necessitating a separate refresh process."
Actually DRAM holds data for several seconds without a refresh. If you
have a $100,000 IC tester to play with you can write a pattern and
then read it after 1 second, then repeat for 2 seconds, etc. I saw the
first bit drop out at 2.
The color printer & scanner company was also driven under by a
lawsuit. I barely dodged having to testify because I knew that the
thermistor circuit I used which the plaintiff claimed as their 1980
trade secret was in my 1978 car.
--jsw