FPGA VAX update, now DIY TTL computers

[Allison]

>
>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