machine said:
Computers built using TTL devices (the 74 and 54 series of TTL) was common prior to useful microprocessors. They are called 'Discrete Processors'.
But it is not an easy task to build your own discrete processor. It takes a lot of skill and it will take probably 200 to 400 chips. The board would be large and just making the PCB will cost a lot as it will be so large. Doing the PCB layout by hand will simply be practically impossible, you would need software to do this like Protel.
Humm, I did it and it was less than 200 chips. A really interesting design was EGO presented in BYTE Sept 1985, a 16 bit machine. It used no PALS
and the densest devices (other than rams) were 74381 ALU and 74189
register array. Rough guess is under 100 TTL packages. It's not microcoded.
I have a very crude machine I did in less than 50 chips. Didn't have Protel nor use it. The base design is 16bit word and a vague resemblence to PDP-8 but a few things like the Data counters in ram left out. All ttl and
fast 400ns fetch/execute cycle. No gals/pals no microcode. Paper,
Logic maps, And the TTL data book.
There are several minimalist CPU designs like TOY, VSC and others. See
http://www.homebrewcpu.com/links.htm One desing listed was only 16
pieces of logic if PALs are not used. Granted it's a trivial machine but
if you can conceive and program that Colossis is just a big wirewrap
project away.
machine said:
You would need to know how to implement 'microcode' also which is the code embedded in all microprocessors which tells the chip how to behave internally. Microcode is not the program you apply externally to the microprocessor to get it to do useful things.
Microcode once you understand the simplest forms is trivial and can be
easier than a sequence engine. However it's not required. The upside of microcode is it can be easier to move bits than wires. The simplest uCode
machine is two parts. A clock source like a 555 timer chip, 74374 octal latch and a EPROM of atleast 256x8 in size. Code optional. Using just a pair of 374s and a 2732 I can easily make a state machine that can alter it's state based on inputs. Thats the core of a programable system.
I once believed as an engineer designing computers was black art. One day I decide to found out how they work and found out that taken one step at a time they are less forbidding than many say. What was missing for me was all those tutorials of how gates and FFs work were only a prelude
and then they skipped over how sequential systems happen to work. Computers are only very big sequential systems and in some cases
sequential systems within larger sequential systems. The latter being
very terse descritption of a microcoded computer.
Allison