9 Apr
2002
9 Apr
'02
5:41 p.m.
Richard Erlacher wrote:
I've built CPU's using standard TTL MSI/SSI
devices, without any sort of
microcode, and, since that was in the days before PALs, bipolar PROMs were the
only handy programmable device that allowed simple localized decoding and
steering without discarding resources. The fact they can easily provide a
preset to a cycle counter, based on their inputs even though there might be a
"sparse" lookup table for that function, saved both time and real estate,
often rendered PROMs the vehicle of choice.
The main reason I have never played with them is I could never find a
Fuse Prom Burner schematic that looked reasonable. I still would like to
do a TTL computer with fused based proms ( or EEPROM's as modern
substitute )for control logic. I am just finishing up a nice FPGA design
but thinking this is going to be a pain to get a serial prom and have it
burned too. 50,000 sure ! quantity 1, HA-HA you must be kidding.
(EPC1441LC20 altera 440,800 bits -- any place in Canada). While TTL is
low density you don't need to pay $$$ for a license for modern
programiable logic, have the software needed 5+ years down the road! (
That is assuming TTL is still around 10 years from now )
> The PDP11/05 was 2 full hex-height boards just
for the CPU, so around 200
> chips. It used mostly TTL, but also some PROMs containing the microcode.
> And the TTL included chips rather more complicated than just gates --
> things like 16*4 RAMs, 4 bit latches, multiplexers, etc.
This was ball park figures. A 4 bit shift register ,4 bit up counter
,dual 4-1 muliplexer would be the typical ALU parts. I was looking at
74HCXX chips as 74LSXX is harder to find and lots more power. How ever a
74LS382 style alu would be used rather than a 74LS181 if LS parts were
used. A 8 bit x 1 register file is a 8 bit addressable latch and a 8/1
multiplexer. The alu design is for a undefined 24 bit processor with a
classic style of architecture.A PDP-11 has byte operations and a complex
opcode where this architecture would be more PDP-8 style on a single 24
bit word.
--
Ben Franchuk - Dawn * 12/24 bit cpu *
www.jetnet.ab.ca/users/bfranchuk/index.html