>> I did that at university.  We had a hex keypad to write machine code 
>> instructions to RAM.  IIRC we had to write a program to do some 
>> thermostat control of a heater or something - extra marks for 
>> proportional control. But then this was just part of the CPU course to 
>> get us familiar with instructions and stuff.
>
> Right. So you type stuff in hex rather than raw binary, and it 
> automatically advances to the next address?

Yeh something like that, I don't remember the exact details.  I think you 
could enter a memory address and hit the "address" button or something and 
it would then accept instructions from that point, and then there was a 
"run" button.  Pretty basic, but armed with the instruction set datasheet 
you could actually write stuff that worked.  IIRC they had a few channels of 
analogue and digital IO mapped to certain addresses that were connected to 
useful stuff like LEDs, temperature sensors and heaters.

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