From: boylesgj AT lion DOT cs DOT latrobe DOT edu DOT au (Gregary J Boyles)
Newsgroups: comp.os.msdos.djgpp
Subject: Char/unsigned/unsigned char conversion problems.
Date: 16 Feb 1997 10:16:57 GMT
Organization: Comp.Sci & Comp.Eng, La Trobe Uni, Australia
Lines: 62
Distribution: world
Message-ID: <5e6mqp$6ah@lion.cs.latrobe.edu.au>
NNTP-Posting-Host: lion.cs.latrobe.edu.au
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

I am moving characters + text attribute to a window, which is later copied
directly to video RAM.

The window is stored as a 1D array and the text attribute is an unsigned with
the lower byte zeroed.

The function :

void WindowC::Write(char Ch)
{
  <calcualte array index from cursor coordinates>

  Window[Index]=Attribute | (unsigned)Ch;

  <move the window cursor>
}

In one example of using this function I should have seen a light grey
character on a blue back ground.
I.E. 0 001 0111 10110011
     |  |    |      |
     |  |    |      +------ASCII 179
     |  |    +-------------light grey fore ground
     |  +------------------blue back ground
     +---------------------non blinking

Instead I saw a white flashing character on a light grey back ground.
I.E. 0 001 0111 10110011
     |  |    |      |
     |  |    |      +------ASCII 179
     |  |    +-------------white fore ground
     |  +------------------light grey back ground
     +---------------------blinking

I was assuming that the upper byte would be zeroed in the conversion (unsigned)Ch.
I.E.
char : 10110011 -----> unsigned : 0000000010110011

Is this assumption wrong and the upper byte could be anything?




Originally the function was as follows:

void WindowC::Write(unsigned char Ch)
{
  cout<<Ch;

  <calcualte array index from cursor coordinates>

  Window[Index]=Attribute | (unsigned)Ch;

  <move the window cursor>
}

However passing a space character (' ') to this function resulted in a '3'
being written to stdout and passing ASCII 179 to it resulted in '-' being
written to stdout.
Can some one explain what is happening in the conversion from char to
unsigned char.