The Zana Zen

This is the second part of my introduction to PCI article. If you haven’t read the first part, it is highly recommended that you’ll read it first.

So, in part I, we learned that all our PCI devices are connected through this main bus line to the CPU and memory. Any entity in the system can use the PCI bus, but it will first ask the bus arbiter to do so. A bus master is any entity that holds the bus and has an ability to write to it.

Any device can hold memory resources that are being used to transfer data between the driver and the device. Those addresses may be used for storing the current display buffer for example. This memory resource is part of the main computer’s RAM, so if the driver will want to copy data to the buffer, it will just copy the data to the RAM, to the address where the BIOS mapped the resource to.

BIOS stands for Basic Input and Output System, it is reliable to boot the system and provide it with some basic capabilities like primitive display adapter support. One part of the bios, moves through the existing PCI devices and supplies them with the memory resources that they require. After doing so, it boots the operating system which enumerates the PCI devices and existing memory resources.

Each PCI device is identified by a device ID and a vendor ID, both are part of the standard 64 bytes configuration space that each PCI device holds. So after the operating systems probes for this info, it searches for an appropriate driver to handle this device, this is being done by matching the device/vendor IDs.

In windows there is a kernel module which is called the Plug and Play manager that loads the right driver and let it initialize itself. There is also a file, for each driver, that describes these IDs. These files are called INF files and they provide info for the PnP manager for loading and installing the appropriate driver. To look at some examples, you can dig in the %windir%\inf directory.

Let’s go back to the memory resources. As I said, these are just some memory regions from the computer’s RAM that are being mapped to the PCI device as well. When a driver needs to write data it may utilize the main CPU for this task or it may use a special hardware unit which is called DMAC.

DMAC stands for Direct Memory Access Controller and it’s basically a processor that knows how to copy data from one place to another. Drivers are using this controller to carry out copying operations thus letting the main CPU be free to carry out other tasks.

DMA may be operate in two ways, the standard one, which by the DMAC reads each time the following byte from the source (the PCI device for example) and copy it to it’s destination (the RAM).

More sophisticated DMACs are using a method which is called flow-through. By which the controller just playing with signals over the bus and holds no actual data by itself. It operates in this manner: on the same time it issues a read operation to the device and a write operation to memory, letting the signals flow smoothly over the bus, the DMAC will keep doing this until it will finish its transaction.

I hope that this series of articles where informative and enjoyable for you. Keep to check out for new posts on this cool blog!