Q45169: PCOPY Causes Screen Snow on Some CGA Video Cards

Article: Q45169
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: 20-DEC-1989

The program below demonstrates that the PCOPY statement may produce
"snow" on some CGA video cards. This is caused by a timing-dependent
refresh problem common to many CGA adapters.

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

The following program is PCOPY.BAS, which uses PCOPY to copy several
lines of text to the second page of video memory. When run on a CGA
system, screen "snow" will appear during PCOPY.

SCREEN 0
PRINT "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
PRINT "bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"
PRINT "Watch the snow"
PCOPY 0, 1
SCREEN 0, , 1, 1

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