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Q35651: ASCII Codes That Do Not Output Using SCRN:, CONS:, or PRINT

Article: Q35651
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 13-MAR-1990

Most ASCII values display graphics or alphanumeric symbols when sent
to the screen. However, there are some ASCII character codes for which
BASIC displays nothing on the screen of a PC. The list of excluded
characters for the PRINT statement is the same as for the PRINT#
statement sending output to a file opened with the "SCRN:" device
name. The list of excluded characters is different for the "CONS:" and
"SCRN:" device names, as shown below.

Note that you can display all the excluded character codes by directly
poking them into video memory (under MS-DOS only, not in OS/2), as
shown farther below.

This information applies to QuickBASIC Versions 4.00, 4.00b, and 4.50
for MS-DOS and to the Microsoft BASIC Compiler Versions 6.00 and 6.00b
for MS-DOS.

For a table of the graphics and alphanumeric symbols associated with
ASCII bytes, please refer to Appendix A of the BASIC language
reference manuals for QuickBASIC Versions 4.00, 4.00b, and 4.50 for
MS-DOS or the language reference manual for Microsoft BASIC Compiler
Versions 6.00 and 6.00b for MS-DOS and MS OS/2. See also the QB
Advisor online Help system for QuickBASIC 4.50.

The following ASCII values cannot be output with PRINT, or with PRINT#
to either "SCRN:" or "CONS:"

   Decimal  Hex    ASCII  Description
   -------  ---    -----  -----------

   007      7H     BEL    Bell
   009      9H     HT     Horizontal Tab
   010      AH     LF     Linefeed
   013      DH     CR     Carriage Return

PRINT# to "SCRN:" and PRINT output the following characters, but
PRINT# to "CONS:" does not:

   Decimal  Hex    ASCII  Description
   -------  ---    -----  -----------

   027      1BH    ESC    Escape
   127      7FH    (The DOS device CONS: recognizes code 127 as DEL.)

PRINT# to "CONS:" outputs the following control characters, but PRINT#
to "SCRN:" and PRINT do not:

   Decimal  Hex    ASCII  Description
   -------  ---    -----  -----------

   011      BH     VT     Vertical Tab
   012      CH     FF     Formfeed
   028      1CH    FS
   029      1DH    GS
   030      1EH    RS
   031      1FH    US

There is a method in QuickBASIC of displaying every character,
including those on the above lists. This involves using the POKE
statement to send the appropriate ASCII code into the even-numbered
byte in video memory that corresponds to a particular screen position.
(The color attribute (default=7) is POKEd into the odd byte that
follows the even byte.) This requires knowing the starting address for
the correct page of screen memory (as shown on Page 85 of the "Peter
Norton Programmer's Guide to the IBM PC"), as well as the desired row
and column position.

Code Examples
-------------

The following code POKEs a given ASCII character at a specific row and
column position on a Hercules-compatible monochrome monitor:

   DEF SEG = &HB000 'start of Hercules memory page 0.
   'DEF SEG = &hB800 'start for EGA or CGA Cards page 0
   DEFINT A-Z
   row = 10
   column = 30
   attribute = 7 'normal white-on-black
   Character = 7 'The bell, normally
   CharPos = 2 * (row * 80 + column)
   POKE CharPos, Character
   POKE CharPos + 1, attribute
   ' Additional characters can be POKEd here as desired.
   END

The following code verifies which characters can be displayed in
QuickBASIC. You can change "SCRN:" to "CONS:" and rerun for
comparison. You can also change the < PRINT#1, > statement to the
< PRINT > statement for comparison.

   CLS
   OPEN "scrn:" FOR OUTPUT AS #1
   FOR k = 0 TO 255
      WHILE INKEY$ = "": WEND
      PRINT #1, k; CHR$(k)
   NEXT

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