Section I Section Real Time Systems. Processes. 1.6 Input/Output Management. (Textbook: A. S. Tanenbaum Modern OS - ch. 5)

Transcription

1 EE206: Software Engineering IV 1.6 Input/Output page 1 of 9 Section I Section Real Time Systems. Processes 1.6 Input/Output Management (Textbook: A. S. Tanenbaum Modern OS - ch. 5) Input/Output Overview Input and Output operations allow the process to interact with the outside world. Process I/O Outside world IPC Other processes Usually this involves data exchange between: CPU and memory and Diverse I/O devices (keyboard, mouse, disk drives, video card, network card, etc.)

2 EE206: Software Engineering IV 1.6 Input/Output page 2 of 9 These I/O devices allow: to get input to produce output to store information I/O management is complicated: Must issue commands to the devices, catch interrupts, and handle errors. Must provide the same interface (device independence) between the devices and the rest of the system which should also be simple and easy to use I/O Device Classification 1. Block devices Store information in fixed-size blocks. Each block has its address. Common block sizes: 128 bytes to 1K bytes. Each block could be read or written independently of the other ones (i.e. random access). E.g.: Disks.

3 EE206: Software Engineering IV 1.6 Input/Output page 3 of 9 2. Character devices Delivers or accepts a stream of characters, without regard to any block structure. It is not addressable. Does not have any seek operation (i.e. no random access). Most devices that are not disk-like can be seen as character devices. E.g.: Printer, terminal, plotter, network interfaces, tape. 3. Other devices Note: Some devices are neither character nor block type. E.g.: Clocks (cause interrupts at well-defined intervals). Although not perfect, the model of block and character devices is usually enough. It can be used as a basis for making some of the operating system software dealing with I/O device independent.

4 EE206: Software Engineering IV 1.6 Input/Output page 4 of 9 I/O Hardware Architecture I/O units typically consist of: a mechanical component (the device itself) and an electronic component (device controller or adapter). Controllers can handle multiple identical devices, (e.g. floppy disk controllers usually handles two floppy disks) Operating system nearly always deals with the controller, not with the device. CPU MEM SYSTEM BUS FLOPPY HARD DISK TTY CLOCK "CONTROLLER" A KEYB CRT B X

5 EE206: Software Engineering IV 1.6 Input/Output page 5 of 9 Nearly all micro- and minicomputers use the single bus model above for communication between the CPU and the controllers. Large mainframes often use a different model, with multiple buses and specialised I/O computers called I/O channels taking some of the load off the main CPU. Controller/CPU Interface 1. A dumb controller with no internal buffer causes an interrupt every time it gets a byte (or a word) from the device (e.g. disk), to tell the CPU to fetch it. CPU CONTROLLER interrupt (time critical) This kind of interface is time critical. Once a disk transfer has started, the bits may arrive from the device at a constant rate, whether the controller is ready for them or not.

6 EE206: Software Engineering IV 1.6 Input/Output page 6 of 9 For each byte (word) transferred the CPU would have to be interrupted and system bus acquired. If the CPU or the system bus were busy (e.g. interrupts temporally disabled by a system process, or the bus busy due to some other device using it) the controller would have to wait. If the next byte (word) arrived before the previous one had been transferred, this would cause a problem, because it has no place to store it. Overwriting the previous one before it had been transferred means loss of data. In reality a controller has an internal buffer to store the bytes (words), thus avoiding the time critical problem. 2. A programmed CPU loop allows read the bytes one at a time from the controller's buffer. CPU CONTROLLER address/ data

7 EE206: Software Engineering IV 1.6 Input/Output page 7 of 9 The CPU gives the controller the address of the block and then does other things. The controller reads the block from the drive serially, bit by bit, until the entire block is in the controller's internal buffer. Next, the controller computes the checksum to verify that no read errors have occurred and if so it causes an interrupt. The CPU can then read the block from the controller's buffer by executing a loop, with each iteration reading a byte (or a word) from the controller and storing it in memory. It wastes CPU time! 3. DMA (Direct Memory Access) Previously the CPU read the bytes (words) one at a time from the controller. DMA was invented to free the CPU from this lowlevel work.

8 EE206: Software Engineering IV 1.6 Input/Output page 8 of 9 DMA is a technique for data transfer which provides a directed path between the I/O device and the memory without CPU intervention. With this path, a peripheral device has "Direct Memory Access" and can transfer data directly to or from the memory. When it is used, the CPU gives the controller two items of information, in addition to the address of the block: Memory address where the block comes from or goes to. Number of bytes to transfer.

9 EE206: Software Engineering IV 1.6 Input/Output page 9 of 9 After the controller has read the entire block from the device into its buffer and verified the checksum, it copies the first byte or word into the main memory at the address specified by the DMA memory address. Then it increments the DMA address and decrements the DMA count by the number of bytes just transferred. This process is repeated until the DMA count becomes zero, at which time the controller causes an interrupt indicating that the transfer has ended. The CPU does not have to copy the block to memory: the controller supervised its copy. When the block is buffered internally, the bus is not needed until the DMA begins, so the design of the controller is much simpler (compare with DMA without internal buffering) because the DMA transfer to memory is not time critical. Many controllers, especially those for block devices, support DMA.