FPGA VAX update, now DIY TTL computers - test-drb@ccmp.vtda.org

3 Nov 2005

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Subject: Re: FPGA VAX update, now DIY TTL computers
   From: Jos Dreesen <jos.mar at bluewin.ch>
   Date: Thu, 03 Nov 2005 07:00:20 +0100
     To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at
classiccmp.org>

 What surprises me about the home-built computers from scratch is the 
 lack
 of imagination when it comes to architecture.  Most are basically
 one-address-cum-accumulator designs.  It would seem that larger 
 register
 files are much easier to build nowadays and would open up the door to 
 some
 2 and 3-address designs. 
The reason is the effort it takes......
Ask anybody who actually  made a CPU  ( I myself did a 12 bit
  TTL based, single address single accumulator machine).
The real effort is actually building and debugging the unit.
  There is only so much time you have....

 I've always thought that 24 bits is a nice word size for a small 
 computer. 
Much too big to actually build in TTL.

In 1974 Elektor magazine started a series of DIY articles that described
an extensible ( 12 bit or 16 bit width ) three address, clockless 
computer.
TTL based, with a shiftregister based memory.  (6 or 8 512x2 
shiftregisters .)
An expensive extra wasa 2102 based RAM.
The machine did hardware multiply and divide +
Instructions were build into the address map .i,e, address  XX is the 
adder, adress yy is a shift and so on..

The start of this series actually predates the Mark8  computer !

					Jos Dreesen 
With current generation MSI TTL functions the highest parts consuming
section the ALU is reduced to a few peices.  Registers with tristate 
outputs are easy now.  Those things were not easy to find back in 1973-4.
The whole point and problem solution of serial machines was flipflops
were 6 trasistors each and a logic gate could be one transistor and 
many diodes.  Now a 4 bit multifuction arithmetic slice is 74181 
(transistor and diode equivelent may have numberd in hundreds).

Another area that was tedious to the max was memory.  In 1974 the 2102
was a over 10 dollar part and you needed 8 for a byte parallel ram.
By 1980 1kx8 parts could be had for 3 dollars.  Now 32kx8 parts are
cheap with far larger available.  So building a memory system is 
simplified as well.

So building a larger machine is not as daunting a task as in 1974 or even
1980.  Even without FPGAs.

Allison