Once a device has been opened, you use it by passing the I/O request to it. When the device processes the I/O request, it acts on the information the I/O request contains and returns a reply message, i.e., the I/O request, to the reply port specified in the I/O request when it is finished. The I/O request is passed to a device using one of three functions, DoIO(), SendIO() and BeginIO(). They take only one argument: the I/O request you wish to pass to the device. * DoIO() is a synchronous function. It will not return until the device has responded to the I/O request. * SendIO() is an asynchronous function. It can return immediately, but the I/O operation it initiates may take a short or long time. Using SendIO() requires you to monitor the message port for a return message from the device. In addition, some devices do not actually respond asynchronously even though you called SendIO(); they will return when the I/O operation is finished. * BeginIO() is commonly used to control the quick I/O bit when sending an I/O request to a device. When the quick I/O flag (IOF_QUICK) is set in the I/O request, a device is allowed to take certain shortcuts in performing and completing a request. If the request can complete immediately, the device will not return a reply message and the quick I/O flag will remain set. If the request cannot be completed immediately, the QUICK_IO flag will be clear. DoIO() normally requests quick I/O; SendIO() does not. DoIO() and SendIO() are most commonly used. An I/O request typically has three fields set for every command sent to a device. You set the command itself in the io_Command field, a pointer to the data for the command in the io_Data field, and the length of the data in the io_Length field. SerialIO->IOSer.io_Length = sizeof(ReadBuffer); SerialIO->IOSer.io_Data = ReadBuffer; SerialIO->IOSer.io_Command = CMD_READ; SendIO((struct IORequest *)SerialIO); Commands consist of two parts - a prefix and the command word separated by an underscore, all in upper case. The prefix indicates whether the command is an Exec or device specific command. All Exec commands have CMD as the prefix. They are defined in the include file exec/io.h. Amiga Exec Commands Amiga System Device Command Prefixes and Examples
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