This is another quick post about an unfinished (but working) software which I wrote around 8 years ago. It is an Intel 8085 microprocessor simulator, with a text based interface. The objective was to simulate the microprocessor, along with a minimal interface which closely resembles the microprocessor kit with the 7-segment displays, hex keyboard and minimal debugging features.
In our undergraduate computer science degree, we had a few subjects on microprocessor architecture. One of the subjects focused on the Intel 8085 microprocessor architecture in great details, Intel 8086 architecture, interfacing, etc. Along with the detailed architecture, we also had to do some assembly code for 8085. It was fun because, we had to use a physical 8085 microprocessor kit with a hex keyboard and just those 7-segment displays.
To write a code in the kit, you need to scan through the memory and enter the values of the assembled code. Who assembles it? We had to do that manually. We would have a table of all the instructions and the hex value for the op-code. More essentially it is very important to know the precise operations for each instruction. What operation it performs, which registers are accessed, what memory locations are accessed and how does it change the flags.
It gets more interesting (sometimes painful) when you first write your code in assembly in a white sheet of physical paper, then you refer the table and convert the assembly code to machine code, basically an entire page of hex values. Next, you get to your 8085 microprocessor kit, start from a memory location and keep on typing these hex values like a maniac. There were 8085 microprocessor kits which had some “debugging” facility, but essentially, if something goes wrong, it was extremely difficult to find, given our skills.
Although we were required to use the physical 8085 microprocessor kit in the exams, but for practice, we used 8085 microprocessor simulators. There are quite a few 8085 microprocessor simulators available. One of them was provided with one of the text books. There were simulators with text interface, some with text and some with nice GUI interfaces. I used one of them, the GNUSim8085. You can find an in depth review of GNUSim8085, which I wrote for OpenSource For You long ago, also posted here: Reviewing the GNUSim8085 (v1.3.7).
I personally did not like most of the simulators or the interfaces. All of them involved a lot of mouse-clicking, which slows you down a lot and does not reflect the actual 8085 trainer kit experience. There were a few which had full keyboard control, but somehow we felt that they were cumbersome. The Intel 8085 trainer kit experience was highly required inorder to timely and correctly finish the tasks given in the examination. The good thing was, I (and a few of my friends) knew exactly what I was looking for. Therefore I tried to make one … (drum roll) … Dirty8085.