Acorn Computers Limited PLUS 1, User Manual

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Using a printer 15
The internal printer buffer
Text sent to the printer is first stored in an area of computer memory known as the internal
printer buffer, and only sent on to the printer when the printer can handle it, usually a line or a
character at a time. If you wish to remove all remaining text from the buffer, you can usually
either press ESCAPE (which empties all the buffers on the system) or use the command
*FX21,3
(which empties the printer buffer only). You can disable the emptying of all
buffers on pressing ESCAPE with the command
*FX230,1
and enable it again by
*FX230,0
.
Your printer may also have its own printer buffer so that printing may continue for a short while
after using ESCAPE.
Printing from assembly language
The *FX commands given above may be accessed from assembly language in the usual way, ie
by loading A with the command number and X with the number following the comma, then
calling OSBYTE (&FFF4), eg to flush the printer input buffer (
*FX21,3
):
LDA#&15
LDX#3
JSR &FFF4
Text is printed by selecting printed output (with the assembly language equivalents of
*FX3,0
or
*FX3,10
) and then sending the characters to be printed, one at a time, by loading the
accumulator with the ASCII code and calling OSWRCH (&FFEE) for each character in turn. If
output to screen and printer is selected (
*FX3,0
), the text to be printed must be preceded by
ASCII 2 (CTRL B) and followed by ASCII 3 (CTRL C) as when using BASIC.