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From: jru@etn-rad.UUCP (John Unekis)
Newsgroups: comp.sys.ibm.pc
Subject: Re: forcing a graphics pixel to on or off, a question
Message-ID: <232@etn-rad.UUCP>
Date: Fri, 31-Jul-87 16:35:46 EDT
Article-I.D.: etn-rad.232
Posted: Fri Jul 31 16:35:46 1987
Date-Received: Sun, 2-Aug-87 08:36:29 EDT
References: <779@cod.UUCP>
Reply-To: jru@etn-rad.UUCP (0000-John Unekis)
Organization: Eaton Inc. IMSD, Westlake Village, CA
Lines: 37
Keywords: hercules, pixel, graphics

In article <779@cod.UUCP> murphys@cod.UUCP (Steven P. Murphy) writes:
>
>Here is a question for all the assembly hacks out there.
>
>the appended Bresenham's algorithm works great for drawing lines
>   ..................(more words)
>may not be a zero)
>
>        mov ch, al
>        and ch, cl
>        sub cl, ch      ; 7 - (x mod 8) = bit within the byte
>        mov ch, color
>        sal ch, cl              ; prepare to look at the pixel
>--->    or es:[bp], ch          ; or the bit with color
>        mov cx, ax
>
... try this ...


        mov ch, al
        and ch, cl
        sub cl, ch      ; 7 - (x mod 8) = bit within the byte
	mov ch,0FEH     ;  11111110 hex
        or  ch, color
        rol ch, cl              ; prepare to look at the pixel
        and es:[bp], ch          ; and the bit with color
        mov ch, color
        sal ch, cl              ; prepare to look at the pixel
        or  es:[bp], ch          ; or the bit with color
        mov cx, ax


... it is a few more instructions, but it does the job. 

P.S. - look for the code I am posting monday, it is the source to a game
       which contains a graphics routine for hercules which is faster
       than this , and is written in `C`     .