Q44237: Hercules Page Support for SCREEN 0 and SCREEN 3

Article: Q44237
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S890428-106 B_BasicCom | mspl13_basic
Last Modified: 15-DEC-1989

The following information summarizes the use of screen pages in
Microsoft QuickBASIC for MS-DOS with a Hercules monochrome graphics
adapter.

SCREEN mode 3 supports two screen pages, page 0 and page 1. However,
SCREEN 3 has only one screen page if there is another graphics adapter
installed.

SCREEN mode 0 with a Hercules adapter has only one screen page, even
though other adapters support up to eight pages in text mode. This is
not a problem with QuickBASIC or a limitation of the adapter.
QuickBASIC was designed to conform to the behavior of other IBM
monochrome adapters.

This information applies to Microsoft QuickBASIC 4.00, 4.00b, and 4.50
for MS-DOS, Microsoft BASIC Compiler Versions 6.00 and 6.00b for
MS-DOS and MS OS/2, and Microsoft BASIC PDS Version 7.00 for MS-DOS
and MS OS/2.

All IBM monochrome adapters assume that there is enough memory to
support only one screen page in text mode, SCREEN 0. QuickBASIC, being
a well-behaved application, makes this assumption so that its
applications can run correctly on both IBM and Hercules monochrome
systems in text mode without any detectable difference to you.

The ROM BIOS function, Interrupt 10 hex with function 5 hex, "Set
Display Page," cannot be used to work around this problem. This BIOS
function correctly changes to another display page, but QuickBASIC
does not utilize the BIOS and continues to write text directly to
video memory to the first display page. Therefore, calling this BIOS
function results only in a blank screen, which may appear to hang the
machine. Setting the page back to page 0 restores the screen.

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