NMI (Non-Maskable Interrupt) requests for RS-232 processing. This allows background RS-232 processing to take place during BASIC and machine language programs. There are built-in hold-offs in the KERNAL, cassette, and serial bus routines to prevent the disruption of data storage or transmission by the NMIs that are generated by the RS-232 routines. During cassette or serial bus activities, data can NOT be received from RS-232 devices. But because these hold-offs are only local (assuming you're careful about your programming) no interference should result.

There are two buffers in the Commodore 64 RS-232 interface to help prevent the loss of data when transmitting or receiving RS-232 informa- tion.

The Commodore 64 RS-232 KERNAL buffers consist of two first-in/first- out (FIFO) buffers, each 256 bytes long, at the top of memory. The OPENing of an RS-232 channel automatically allocates 512 bytes of memory for these buffers. If there is not enough free space beyond the end of your BASIC program no error message will be printed, and the end of your program will be destroyed. SO BE CAREFUL!

These buffers are automatically removed by using the CLOSE command.

OPENING AN RS-232 CHANNEL

Only one RS-232 channel should be open at any time; a second OPEN statement will cause the buffer pointers to be reset. Any characters in either the transmit buffer or the receive buffer will be lost.

Up to 4 characters can be sent in the filename field. The first two are the control and command register characters; the other two are reserved for future system options. Baud rate, parity, and other options can be selected through this feature.

No error-checking is done on the control word to detect a nonimplemented baud rate. Any illegal control word will cause the system output to operate at a very slow rate (below 50 baud).

BASIC SYNTAX:

   OPEN lfn,2,0,"<control register><command register><opt baud low><opt
 baud high>"

lfn-The logical file number (lfn) then can be any number from 1 through 255. But be aware of the fact that if you choose a logical file number that is greater than 127, then a line feed will follow all carriage returns.