Modem Command Protocol (MCP)
This section describes the Modem Command Protocol. All of the commands which the Serial Adapter will accept are listed below. Each command begins with a command byte which tells the Serial Adapter which command the message contains.
Some commands cause the Serial Adapter to produce a response packet at a later time. The format of all possible Serial Adapter responses is listed in the section entitled “MCP Response from the Serial Adapter.” The first byte of each response generated by the Serial Adapter also contains a “command” byte which the Exter- nal Computer can use to distinguish different responses from each other.
When the External Computer sends a command to the Serial Adapter which will cause the Serial Adapter to generate a re- sponse, the External Computer need not wait until the response is sent by the Serial Adapter before sending the Serial Adapter another command. In other words, the External Computer may have multiple commands outstanding to the Serial Adapter at once. If the Serial Adapter has been sent multiple commands, it will not, in general, respond to the commands in the same order that they are received. The reason for this is that some commands take much longer to execute than others. The Serial Adapter attempts to respond to each command as rapidly as possible.
If the External Computer elects to take advantage of this feature, the software in the External Computer must be able to unambigu- ously associate responses with their corresponding commands. The Serial Adapter Command Protocol contains a feature that makes this process easier.
Each Serial Adapter command for which there is a Serial Adapter response contains an extra field known as the seqno field. When
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