Dirty8085: An Intel 8085 Microprocessor simulator

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.

Quick story

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.

8085 Microprocessor trainer kit

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.

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CPULeash: A tool for capping per-process and process group CPU utilisation from userspace

It’s now mid October, there are 0 posts in 2017. Therefore I think it’s time to loosen up posting constraints and try make a bit more casual posts. For a start I will dig up one of the unfinished works. I will start with a quick overview of an old piece of tool (unmaintained) I wrote quite a few years ago. The tool caps the maximum CPU utilisation of a process from userspace without requiring root access. One instance of the process can cap multiple processes individually, in a group or it can also cap an entire process sub-tree. I intended to post about it in great detail, but this time I will keep it brief, and dig deeper in the next opportunity. Let’s get into it.

Motivation (from agony)

A few years ago, I was used to run different CPU intensive experiments on my old laptop, which would sometime run weeks. The CPU temperature used to go up to 95 Deg C. Sometimes it went upto 99 Deg C and then it used to throttle down the cores to get the temperature down. This is definitely not a healthy temperature, therefore I feared of hardware damage. The laptop had a metal case, which became so hot that it left red marks on my thighs. One time an entire chocolate bar melted into a pool of liquid chocolate (someone left it near the exhaust fan). Therefore it was time that I figure something out, such that I can run the experiments, but keep the laptop cool (relatively).

The obvious idea was to cap the CPU usage for the processes. An easy solution is to install a virtual machine and run the code inside the virtual environment. Note that, in my case nice does not work, as it does not help control how much of the CPU a processes utilises when it acquires it. I could have used cgroups, but I didn’t want to (I don’t know why). At that point of time I was feeling the need for Solaris Zones.

Essentially what I wanted is something portable in any *nix system, works completely from userspace and also does not need root privilege. Therefore I made a small tool which does in. It was written in C and POSIX compliant libraries.

Also note that, there is a similiar kind of tool called cpulimit, which I didn’t know existed when I wrote this. This tool works well too.

Given this situation, it was time to write something in-house … drum rollCPULeash.

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A wide character trie implementation

It’s been a long time since I posted here. Therefore I decided to dig out some quick and straightforward stuffs from my disk which I previously decided should’t be in this blog.

I have posted multiple trie and dictionary search based programs in C, C++ and Perl before Jumble Work Solver and Jumble Work Solver Again. This time (again!) it’s about a trie. Although this time there was a specific requirement from a group who needed to implement a trie based word distance counting for bangla language, therefore Unicode support. This was supposed to be modified more and plugged into a spelling correction for scanned OCR text in the Bengali language.

Initially I suggested that a ternary tree would be more appropriate as the memory cost for standard trie (not compressed) would be huge. Although, finally the decision was to go with plain and simple trie. I know it is mad, but this is what it is :D.

Also, before going into the implementation, I should note that there is an efficient implementation of Radix Tree present in libcprops library. Though which we won’t be using for this one. I would recommend you people to have a look into this library if you already haven’t seen it yet.

Let’s say, we have a word “hello” and a node with an array of pointers of length 26 representing each character of the English language, each of which indicates that if the ith character follows the character represented by this node. Therefore for this example “hello”, the head node’s array of pointers will have the location 7 pointing to another node, which will have the 4th location of the pointer pointing to another node, whose pointer array’s 11 th location will point to another node and so on. Note here the indexing starts from zero. When the word ends, then the next pointer can be pointed to a special terminal marker node, which can be common to all, and the node is marked as a terminal node. This is essential as a valid word can be a substring of another valid word, in which case the shorter would need to be decided. When another new word like “help” comes in, we will follow the same path upto “hel” created by the word “hello”, and as we find there are no pointer for “p” pointing to any node, a new node will be created as explained before. Continue reading “A wide character trie implementation”

An overview of the PC Real Time Clock (RTC)

Introduction

Have you ever thought how the computer is able to display the correct time after you power on the system? There is a component called the RTC in the computer which stores this date and time information when the computer is turned off.

I will talk about how to read/write the date and time and use other features of the RTC using command-line tools, Linux RTC driver and also a low level direct access to the RTC internal registers.
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Generate the process tree of a Linux system

This is a quick post on how to generate a process tree Linux (and *nix) operating systems.

The idea is the same, as in the previous posts: Finding overall and per core CPU utilization and Find process IDs of a running process by name. Read the information present in the /proc/ directory. To get which processes are running we can read the directories with numbers as their names in the /proc/ directory. To generate a process tree we need to establish a process child relationship within the running processes. Each process has a parent (the first generated process is an exception), and it is stored in the process table entry of that process. We need to fetch the parent process id for each running process inorder to establish the tree. Here’s the plan. Continue reading “Generate the process tree of a Linux system”

Find process IDs of a running process by name

In this post I will talk about a procedure to find the process IDs of a running process by name, which can then be used to send signals or do other stuffs. For example if you have multiple instances of bash opened, this should be able to get you the list of process IDs (PIDs) of the bash instances.

Firstly, a shell utility is already available called pidof which is a part of the sysvinit-tools package. There are a whole bunch of tools in this package which lets you query PID based on different requirements, send signals to set of processes, etc. Just check out the stuff.

I will only mention the outline of how this is done and post the sourcecodes to do it. After that this can be extended to have many features just like the tools of sysvinit-tools package or more.
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Finding overall and per core CPU utilization

Today I will post about how to monitor CPU usage by processor in Linux. As you might have expected this will simply access the CPU time information from /proc pseudo-filesystem and report the results.

The proc filesystem or the Procfs is a special filesystem which gives you a view into the kernel data. No files in procfs exists actually in the disk. There is no disk inodes and thus storage related to the files. Instead of going into procfs I will redirect you to wikipedia: http://en.wikipedia.org/wiki/Procfs. Let’s get in.
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Change Useragent in Firefox

Firefox LogoThere are some websites which require you to visit them using Internet Explorer only, I don’t know why but there are some. My friend had to download an important document from a site which insisted on using Internet Explorer. We both are in Linux, Fedora. Is there a way? Yes there is. The browser version and certain system details are stored in the browser configuration, and this sting is called the useragent string. The information in this string is used to detect what browser you are using. Therefore if we can modify this string such that it looks like we are using Internet Explorer instead of Firefox, then we can fool the other end. After doing this the server side will believe that you are using an InternetExplorer browser or any other browser (also a different OS), depending on what the useragent string you are using to override the original.

Instagram does not allow you to upload pictures from desktop/laptop. Using this method you can set the useragent string of your desktop/laptop browser to an andriod or iOS browser and upload pictures to Instagram from your desktop.

What we need to do is simple. Just create a new entry in firefox browser configuration and set the value to appropriate useragent string. So let’s get started with this short and quick guide.
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Fix dark video in Skype for Linux

fedora_logoI was very much disappointed when i installed Skype in Fedora, three causes which are as follows in the order. First, Skype for Linux is very old version (2.2 beta) than the Windows Skype, second, the video was too dark or black, and third cause surfaced after i installed Fedora 16 x86_64, no 64 bit binary for Fedora was available. The third one was solved as described in this post: Running Skype for Fedora x86 in Fedora x86_64. For the first one i can’t do anything, but there should be some solution for the second problem: dark video. Cheese (a video capture application) works fine and video is good. Searching in the skype forums got a lot of mumbo jumbo solutions of which none worked. After a lot of search i got a very simple solution.

Edit (16.06.2012): The first problem seems to solve atleast upto some extent as Skype 4.0 is now out and can be downloaded here.

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