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
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.
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.
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
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.
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
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).