From: peuha AT cc DOT helsinki DOT fi (Esa A E Peuha)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Char/unsigned/unsigned char conversion problems.
Date: 19 Feb 1997 09:20:08 GMT
Organization: University of Helsinki
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References: <5e6mqp$6ah AT lion DOT cs DOT latrobe DOT edu DOT au>
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To: djgpp AT delorie DOT com
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Gregary J Boyles (boylesgj AT lion DOT cs DOT latrobe DOT edu DOT au) wrote:
: 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

The binary values here are identical (the latter should be 1 111 1111 ...).

: 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?

It's *not* anything. What happens here is that your (signed) char is
first *implicitly* cast to (signed) short and then *explicitly* cast to
unsigned short, so the upper byte is all zeros or all ones depending on
the highest bit of the lower byte. You should use (short)(unsigned)Ch.

--
Esa Peuha
student of mathematics at the University of Helsinki
http://www.helsinki.fi/~peuha/