PT501 - an 8049 based teaching computer, 1979


The UK’s Open University provided the PT501 for the “Microprocessors and Product Development” course in 1979. Intel’s 8049 was an 8-bit microcontroller with 128 bytes of RAM and 2k of mask ROM on chip.

Photo credited to Chris Millard here on

There’s more info here
and another photo on this wiki:

Although the 8048 datasheet tells us we can attach external RAM and ROM, there doesn’t seem to be any on this board. Which is a conundrum: it’s not clear we can execute code from RAM. Is it possible the OU created custom firmware which includes an interpreter for a simple virtual machine which does execute from RAM? In any case, 128 bytes of RAM isn’t much. The examples given are a traffic light controller and a temperature display - the board has the peripherals to support those.

This machine is a very rare find because the OU loaned it as course materials and then took it back.


The 8048 / 8051 have a Von Neumann architecture and so can run code from RAM, unlike its rivals PIC and Avr8 which have a Harvard architecture. But like you said, you can create a virtual machine in a Harvard computer which can then run its code from RAM (I did it on a PIC16F84).


The 8048 / 8051 is actually also Harvard with separate code and data space. The only exception I know is the very modern single-cycle DS89 series that has 1k “external” memory on the chip in addition to the built in 256 bytes of ram that can be either code or data. I’m also guessing this 8049 is running a virtual machine unless one of those small chips is a tiny amount of ram set up as code memory that you can key a few instructions into.

Edit: a clue about the virtual machine is if you zoom in on the keypad. It has Ld and St which could be the load and store instructions of the virtual machine. Assembly just has one instruction called Mov.


Hmmm, you are right that the 8048 is Harvard. I wrote an 8048 assembler in 1988 and did a project with an 8748 soon after that so should have known better. Unlike the other architectures I mentioned both memories are 8 bits wide and both can be expanded externally as described in this PDF.


The '51 is Harvard, with separate code and data, but they can be combined externally so they appear as one memory. I think the '48 can do that, too, but I am not certain off the top of my head.