Vishura Bit 2 enables the receiver line status interrupt. Bit 2 sets the length of datawheet stop bits. At this point the computer thinks the Virtual Modem to which it is connected is ready and has detected the carrier of the other modem. Parity errors Bit 2 can also indicate a mismatched baud rate like the framing errors particularly if both errors are occurring at the same time.
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Introduction[ edit ] Finally we are moving away from wires and voltages and hard-core electrical engineering applications, although we still need to know quite a bit regarding computer chip architectures at this level. Computer designs have evolved quite a bit over the years, and often all three chips are put onto the same piece of silicon because they are tied together so much, and to reduce overall costs of the equipment.
So when I say , I also mean the successor chips including the , , Pentium, and compatible chips made by manufacturers other than Intel. There are some subtle differences and things you need to worry about for serial data communication between the different chips other than the , but in many cases you could in theory write software for the original IBM PC doing serial communication and it should run just fine on a modern computer you just bought that is running the latest version of Linux or Windows XP.
Modern operating systems handle most of the details that we will be covering here through low-level drivers, so this should be more of a quick understanding for how this works rather than something you might implement yourself, unless you are writing your own operating system. For people who are designing small embedded computer devices, it does become quite a bit more important to understand the at this level.
Just like the , the has evolved quite a bit as well, e. Remember as well that this is trying to build a foundation for serial programming on the software side. While this can be useful for hardware design as well, quite a bit will be missing from the descriptions here to implement a full system. The newer CPUs have enhanced instructions for dealing with more data more efficiently, but the original instructions are still there.
When the was released, Intel tried to devise a method for the CPU to communicate with external devices. In the , this meant that there were a total of sixteen 16 pins dedicated to communicating with the chip. The exact details varied based on chip design and other factors too detailed for the current discussion, but the general theory is fairly straightforward.
Since this is just a binary code, it represents the potential to hook up different devices to the CPU. It gets a little more complicated than that, but still you can think of it from software like a small-town post-office that has a bank of PO boxes for its customers.
The next set of pins represent the actual data being exchanged. You can think of this as the postcards being put into or removed from the PO boxes. An pin signals whether the data is being sent to or from the CPU. This was a source of heartburn on those early systems, particularly when adding new equipment. This has some problems, including the fact that it chews up a portion of potential memory that could be used for software instead. And this really is a warning. At the minimum, it will crash the operating system and cause the computer to not work.
Worse yet, in some cases it can cause actual damage to the computer. This means that some chips inside the computer will no longer work and those components would have to be replaced in order for the computer to work again. Damaged chips are an indication of lousy engineering on the part of the computer, but unfortunately it does happen and you should be aware of it.
Finally we are starting to write a little bit of software, and there is more to come. Higher bits of the port number being ignored, this made multiple port number aliases for the same port. In addition, besides simply sending a single character in or out, the will let you send and receive 16 bits at once.
The chips will even let you send and receive bits simultaneously. We will not cover that topic here. The chip designers at Intel got cheap and only had address lines for 10 bits, which has implications for software designers having to work with legacy systems.
This issue would generally only show up when you are using more than the typical 2 or 4 serial COM ports on a PC. Within the , there are two kinds of interrupts: Hardware interrupts and Software interrupts. There are some interesting quirks that are different from each kind, but from a software perspective they are essentially the same thing. The CPU allows for interrupts, but the number available for equipment to perform a Hardware interrupt is considerably restricted.
There are a total of fifteen different hardware interrupts. The point here is that if a device wants to notify the CPU that it has some data ready for the CPU, it sends a signal that it wants to stop whatever software is currently running on the computer and instead run a special "little" program called an interrupt handler.
Once the interrupt handler is finished, the computer can go back to whatever it was doing before. In fact, if you are reading this text on a PC, in the time that it takes for you to read this sentence several interrupt handlers have already been used by your computer.
Every time that you use a keyboard or a mouse, or receive some data over the Internet, an interrupt handler has been used at some point in your computer to retrieve that information. Interrupt handlers[ edit ] We will be getting into specific details of interrupt handlers in a little bit, but now I want to explain just what they are.
Interrupt handlers are a method of showing the CPU exactly what piece of software should be running when the interrupt is triggered. The advantage of going this route is that the CPU only has to do a simple look-up to find just where the software is, and then transfers software execution to that point in RAM. This also allows you as a programmer to change where the CPU is "pointing" to in RAM, and instead of going to something in the operating system, you can customize the interrupt handler and put something else there yourself.
How this is best done depends largely on your operating system. For a simple operating system like MS-DOS, it actually encourages you to directly write these interrupt handlers, particularly when you are working with external peripherals. Other operating systems like Linux or MS-Windows use the approach of having a "driver" that hooks into these interrupt handlers or service routines, and then the application software deals with the drivers rather than dealing directly with the equipment.
How a program actually does this is very dependent on the specific operating system you would be using. If you are instead trying to write your own operating system, you would have to write these interrupt handlers directly, and establish the protocol on how you access these handlers to send and retrieve data. Software interrupts[ edit ] Before we move on, I want to hit very briefly on software interrupts. Indeed, often these subroutines are written directly in assembly language. Depending on the values of the registers, usually the AX register in the in this case, it can determine just what information you want to get from DOS, such as the current time, date, disk size, and just about everything that normally you would associate with DOS.
Compilers often hide these details, because setting up these interrupt routines can be a little tricky. Now to really make a mess of things. The difference here is that software interrupts will only be invoked, or have their portion of software code running in the CPU, if it has been explicitly called through this assembly opcode.
External devices are directly connected to this chip, or in the case of the PC-AT compatibles most likely what you are most familiar with for a modern PC it will have two of these devices that are connected together. The purpose of these chips is to help "prioritize" the interrupt signals and organize them in some orderly fashion. There is no way to predict when a certain device is going to "request" an interrupt, so often multiple devices can be competing for attention from the CPU. Generally speaking, the lower numbered IRQ gets priority.
When it was built, there was only one chip on the motherboard. Since there was still only 1 pin on the CPU at this point the that could receive notification of an interrupt, it was decided to grab IRQ-2 from the original chip and use that to chain onto the next chip. The nice thing about going with this scheme was that software that planned on something using IRQ-2 would still be "notified" when that device was used, even though seven other devices were now "sharing" this interrupt.
This is mainly of concern when you are trying to sort out which device can take precedence over another, and how important it would be to notified when a piece of equipment is trying to get your attention. If you are dealing with software running a specific computer configuration, this priority level is very important. We will visit this concept a little bit more when we get to the chip.
Serial Programming/8250 UART Programming