recreating the PDP-11

From: Ben Franchuk <>
Date: Sat Apr 6 15:15:26 2002 wrote:
> I know it's not a conventional way to do it, but if I design my own CPU I
> might just use a few EPROMS - at least for multiplicaion and divison, and
> perhaps for addition and subraction. If I only do an 8-bit proc (very
> likely), that's doable in a 256k*8 and a 128k*8 EPROM, running me only
> less than $12 for 120ns parts and only 64pins in two chips to implement
> all of the difficult ALU functions. AND, OR, etc should be easy and
> cheaper to do with 74* TTL or CMOS IC's. Either battery-backed SRAM or
> (E)EPROM for the state controller should be cheap, easy, and effective.

While 8 bits are OK for a 8 bit data path, you still need more than 8
bits for a practical program counter address space. That is where 12
bits are practical has room for useful instructions. Here is a nice
small 8 bit FPGA design that could be easily adapted to TTL.
A 8 bit FPGA computer

Simplex real a 8 bit TTL computer with about 120 chips.

> I might think about using an 80xx series DMA controller and a
> semi-standard WDC floppy controller like those used in PeeCees so I can
> get easy acces to docs for them. However, I might not bother adding in
> the fdd stuff or hdd stuff - as long as I can get the code to fit onto an
> SRAM or something I should be OK. I just have to figure out what I want
> to do before I go any further.

A IDE disk interface may be a simpler design, and faster than a floppy.
> That's OK, I'll deal with the non-authentic feel of battery-backed SRAMs.
> Anyhow, the SRAM will act more like some core memory. :)

I plan later to use a EEPROM bootstrap that I can load from the front
panel, once I decide on a I/O device -- floppy disk or IDE drive. I got
a nice hardware bootstrap that can load from serial console/paper tape
(If I had a reader/punch that is), front panel or a 4 bit wide prom.

Ben Franchuk - Dawn * 12/24 bit cpu *
Received on Sat Apr 06 2002 - 15:15:26 BST

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