Font display and text editing system with character overlay feature

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings wherein like numerals indicate like elements, there is shown in FIG.
1 a font display and text editing system constructed in accordance with the principles of the present invention and designated generally as 10.
MAIN SYSTEM The heart of font display and text editing system 10 is a microprocessor 24 which may be an 8086 microprocessor manufactured by Intel Corporation.
A complete description of the structure and operation of this microprocessor, as well as various applications thereof, is described in Intel's "iPX 86, 88 User's Manual" dated Aug.
21, 1981.
The disclosure of this manual is incorporated herein by reference.
Throughout the following description, reference will be made to signals which are either active low or active high.
An active low signal will be indicated by the presence of a line over the signal (e.
g.
, DEN).
An active low signal will be referred to as being set or generated when it is at the binary "0" level and reset when it is at the binary "1" level.
An active high signal will be referred to as being set or generated when it is at the binary "1" level and being reset when it is at the binary "0" level.
In addition to active low and active high signals, various elements of system 10 have active high and active low inputs and outputs.
An active low input or output will be indicated by the presence of a small circle at the input or output of the element.
For example, each of the outputs of the 3 to 8 decoder 44 ar active low outputs.
An active low input will be activated by the presence of a binary "0" on its input.
An active low output will place a binary "0" on its output when it is activated.
Any input or output which is not indicated to be active low is active high.
Microprocessor 12 communicates with the remaining elements of system 10 by writing address information onto address bus 14 and by both writing information onto and reading information off of data bus 16.
Microprocessor 12 has a common set of input/output ports A0-A19 which are connected to both address bus 14 and data bus 16 through address latch 18 and transceiver 20, respectively