How were 32-bit minis built in the 70s/80?

Dave Wade dave.g4ugm at
Sun May 12 02:06:01 CDT 2019

> -----Original Message-----
> From: cctalk <cctalk-bounces at> On Behalf Of ben via cctalk
> Sent: 12 May 2019 01:48
> To: cctalk at
> Subject: Re: How were 32-bit minis built in the 70s/80?
> On 5/11/2019 5:14 PM, Warren Toomey via cctalk wrote:
> > I'm building my own 8-bit CPU from TTL chips, and this caused me to think:
> > how were 32-bit minis built in the late 70s and early 80s? In
> > particular, how was the ALU built? I know about the 74181 4-bit ALU,
> > and I know (from reading A Soul of a New Machine) that PALs were also
> used.
> >
> > Did companies get custom chips fabricated, or was it all off-the-shelf
> > chips with a few PALs sprinkled in?
> >
> > Thanks, Warren
> >
> 8 bit computers are EVIL. REPENT DEAR BROTHER.
> 24 bit computers are HOLY AND DIVINE.
> Building a  12/24 BIT CPU with 8 bit I/O.
> (back on topic)

They certainly thought that at Jodrell Bank, an outpost of the University of Manchester

in 1964 they started using Ferranti Argus 100 computers to control the radio telescopes.

these were programmed in FORTH and the 24 bit word was ideal for storing astronomical co-ordinates as sixed packed decimal digits.
So much so that when Ferranti stopped making the Argus 100 they created bit-slice emulations of the machine using AMD2900 chips 

> Early 70's computers other than IBM used TTL, and fast core memory with
> mostly a 16 bit word width. Other than the PDP 11, most computers where
> adapted from the transistor era with tweaks added for banks of
> memory.When the late 70's came around commercial customers had a large
> main frame computer or small control computer from a few years earlier with
> FAST TTL (S)logic, PDP 11's, IBM 360's or clones,or TTL standard/H like PDP 8
> or NOVA computer.
> Bit slice logic like the 2901 alu, (1975) would make for nice low cost 16/32 bit
> cpu with byte load/store.
> The market for 32 bit computers was decided however to sell FAST LARGE
> systems (floating point/64K+ memory) like the VAX (S TTL) or upgrade other
> designs like the NOVA computer with Custom or semi-custom (PAL logic)
> logic.
> INTEL being slow with the forgotten APX 432 design came out with 8086
> leaving us with the defective CPU's of today.

The AMD2900 could be used to build 24 bit CPUs as well....

> Ben's view point.
> I am doing  my computer with a FPGA development system for design logic
> and testing and later using 2901's and LS TTL with 3 proms used for the
> alu/control cards.
>   I have A nice 8/16/32 cpu design with 512KB of memory
> (2901 alu )but I can't get it to route correctly. The 12/24 bit cpu just fits with
> the FREE develpment software.
> For a few K $ I can get the better version with being able route by hand my
> logic to meet timing specs.
> Once hardware SD card/serial port and software are working I then will port
> the design to TTL.
> I may need to write my own tiny langage to boot strap my system.
> Ben.

Best of luck

> PS:
> 16 bit computer format
> [op 3..1][ac 3..1][mode 3..1][ix 3..1][aux][k 3..1] The tricky part is K is the
> upper 3 address bits to extend 16 bit offset to 19 bits or a auto indexing
> mode. This would be valid memory for the late 70's early 1980's but not for
> today.

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