80-Bus News


January–February 1983 · Volume 2 · Issue 1

Page 39 of 56

The Keyboard

If the CPU card is a GM813 then bit 1 of location 0003 (IOBYTE) is automatically set to 1 on power-on or reset. This disables the operation of the keyboard attached directly to the CPU card, since this is not provided for on the GM813.

Screen edit mode is entered by Control-‰ (Ed. – ‰=‘at’) instead of DEL to be consistent with the various disk BIOSes.

Screen dump operates only within screen edit mode to be consistent with SYS and to allow the control code previously used to be available for program use.

Resetting RP/M

On reset the system is tested to see if a disk card is present. If it is then an attempt is made to boot from the disk. If this fails an appropriate message is output as from the B command.


The D command may be cancelled during output by typing a Space.

Screen Editing

Use Control-‰ [‰ = ‘at’] (Null) to enter screen edit mode. The cursor changes to a blinking block while in this mode. Entering Control-C exits screen edit returning a Control-C and a carriage return, instead of performing a warm boot.

Screen Dump

The contents of the screen may be printed by pressing Control-B while in screen edit mode. When the screen has been printed screen edit mode is terminated automatically. The output is directed via the I/O jump table, allowing users who modify this table to obtain screen dumps.

Printer Support

Both serial and parallel printers are supported. To use a parallel printer use the S command to change location 0003 (IOBYTE). Bit 7 is tested to determine which type of printer to use. If it is 1 then the parallel printer is selected, so change the value from 01H to 81H or 03H to 83H. The parallel printer interface uses Centronics conventions. Port B is the output data port, and port A is used to control the data transfer. Bit 0 of port A is connected to the BUSY line from the printer, and bit 1 is the strobe which indicates to the printer that data is available.

Operation Without Video Card

RP/M is designed to be used with the Gemini GM812 intelligent video card. However for some applications it is useful to be able to operate the system with minimal hardware. RP/M will now operate without a video card if bit 0 of location 0003 (IOBYTE) is set to 0. Since this needs to be set on power-on, this bit is automatically flipped if a link has been installed on the CPU card. This is the Ring Indicator link attached to the Modem Status Port. If this link is made then the serial port is used as the console device. Note that serial handshaking is used, so ensure that this is provided. If there is a video card in the system it will be reset but it will not be used and its keyboard will not operate. If there is a disk card then an attempt will be made to boot a disk, as normal. All messages are output to the serial printer, and input may be from a serial keyboard or from a keyboard on the CPU card (GM811 only).

Page 39 of 56