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Display Double-Size Characters On Personal Computer

IP.com Disclosure Number: IPCOM000100302D
Original Publication Date: 1990-Mar-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 4 page(s) / 123K

Publishing Venue

IBM

Related People

Glatz, MA: AUTHOR [+2]

Abstract

Described is a software package which enables the display of double-size characters on a personal computer. This is accomplished by splitting the normal display character set into two character sets (fonts). One font contains the double high upper half of the characters and the other font contains the double high lower half of the characters. Each character write command is processed as two write commands. The first write command writes the upper half of the character from the upper font, and the second write command, on the next line, writes the lower half of the character from the lower font. This produces double high characters. Operating in this mode is referred to as 480 mode since the resulting screen size is 40 columns by 12 rows.

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Display Double-Size Characters On Personal Computer

       Described is a software package which enables the display
of double-size characters on a personal computer.  This is
accomplished by splitting the normal display character set into two
character sets (fonts).  One font contains the double high upper half
of the characters and the other font contains the double high lower
half of the characters.  Each character write command is processed as
two write commands. The first write command writes the upper half of
the character from the upper font, and the second write command, on
the next line, writes the lower half of the character from the lower
font.  This produces double high characters. Operating in this mode
is referred to as 480 mode since the resulting screen size is 40
columns by 12 rows.

      Each character in a default personal computer character font is
defined in an 8x16 character box.  To generate the 480 mode character
fonts, each row of the 8x16 character boxes is duplicated, forming
8x32 character boxes.  The 480 mode upper font contains the top 16
rows of these character boxes and the lower font contains the bottom
16 rows.  When displayed on the screen, horizontal stretching is
accomplished by the computer VIDEO hardware by setting the display
into 40 column mode.  The resultant character on the display screen
is double size, contained in a 16x32 character box.

      Fig. 1 shows the general structure and information paths of the
480 mode device driver.  Communication between an application and the
device driver 1 is accomplished through the BIOS (Basic Input/Output
System) INT 10H interface 2.  When the application requests 480 mode,
the device driver loads the upper and lower fonts into the personal
computer character generator 3.  Write character commands from the
application then cause device driver 1 to write upper and lower font
characters, as required, into computer display buffer 4.  These
characters are then displayed on screen 5 by the computer.  Although
the device driver processes several INT 10 commands from the
application, only the write command and load font process are
described in this article.

      When the application requests 480 mode, and then sets the
display into 40 column mode, the device driver loads the upper and
lower 480 mode character fonts into the character generator.  The
following assembler instruction sequence illustrates how the device
driver loads these fonts.
MOV AX,1110H
MOV BP,OFFSET UPFONT      set pointer to upper font address
MOV DX,0                  data offset is 0
MOV BX,1000H              each character is 16 rows
MOV...