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James Gregory
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353 changed files with 5091 additions and 5091 deletions
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35-07.html
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35-07.html
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@ -39,9 +39,9 @@
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<P><B>LISTING 35.3 L35-3.ASM</B></P>
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<!-- CODE //-->
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<PRE>
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; Fast assembler implementation of Bresenham’s line-drawing algorithm
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; Fast assembler implementation of Bresenham’s line-drawing algorithm
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; for the EGA and VGA. Works in modes 0Eh, 0Fh, 10h, and 12h.
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; Borland C<SMALL>++</SMALL> near-callable.
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; Borland C<SMALL>++</SMALL> near-callable.
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; Bit mask accumulation technique when |DeltaX| >= |DeltaY|
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; suggested by Jim Mackraz.
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;
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@ -109,7 +109,7 @@ LINE1 macro MOVE_LEFT
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local MoveToNextByte, ResetBitMaskAccumulator
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mov cx,bx ;# of pixels in line
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jcxz Line1End ;done if there are no more pixels
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; (there’s always at least the one pixel
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; (there’s always at least the one pixel
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; at the start location)
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shl si,1 ;DeltaY * 2
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mov bp,si ;error term
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@ -122,7 +122,7 @@ LINE1 macro MOVE_LEFT
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; for the initial pixel
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LineLoop:
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;
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; See if it’s time to advance the Y coordinate yet.
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; See if it’s time to advance the Y coordinate yet.
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;
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and bp,bp ;see if error term is negative
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js MoveXCoord ;yes, stay at the same Y coordinate
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@ -136,7 +136,7 @@ LineLoop:
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;load latches and write pixels, with bit mask
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; preserving other latched bits. Because
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; set/reset is enabled for all planes, the
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; value written actually doesn’t matter
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; value written actually doesn’t matter
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add di,EVGA_SCREEN_WIDTH_IN_BYTES ;increment Y coordinate
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add bp,si ;adjust error term back down
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;
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@ -148,7 +148,7 @@ if MOVE_LEFT
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else
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ror ah,1 ;move pixel mask 1 pixel to the right
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endif
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jnc ResetBitMaskAccumulator ;didn’t wrap to next byte
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jnc ResetBitMaskAccumulator ;didn’t wrap to next byte
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jmp short MoveToNextByte ;did wrap to next byte
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;
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; Move pixel mask one pixel (either right or left, depending
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@ -169,7 +169,7 @@ endif
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;load latches and write pixels, with bit mask
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; preserving other latched bits. Because
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; set/reset is enabled for all planes, the
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; value written actually doesn’t matter
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; value written actually doesn’t matter
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MoveToNextByte:
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if MOVE_LEFT
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dec di ;next pixel is in byte to left
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@ -191,7 +191,7 @@ Line1End:
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;load latches and write pixels, with bit mask
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; preserving other latched bits. Because
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; set/reset is enabled for all planes, the
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; value written actually doesn’t matter
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; value written actually doesn’t matter
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endm
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;
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@ -225,10 +225,10 @@ LINE2 macro MOVE_LEFT
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;load latches and write pixel, with bit mask
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; preserving other latched bits. Because
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; set/reset is enabled for all planes, the
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; value written actually doesn’t matter
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; value written actually doesn’t matter
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LineLoop:
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;
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; See if it’s time to advance the X coordinate yet.
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; See if it’s time to advance the X coordinate yet.
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;
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and bp,bp ;see if error term is negative
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jns ETermAction ;no, advance X coordinate
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@ -260,7 +260,7 @@ MoveYCoord:
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;load latches and write pixel, with bit mask
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; preserving other latched bits. Because
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; set/reset is enabled for all planes, the
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; value written actually doesn’t matter
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; value written actually doesn’t matter
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;
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loop LineLoop
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Line2End:
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@ -290,7 +290,7 @@ _EVGALine proc near
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mov al,SET_RESET_INDEX
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out dx,al
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inc dx
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mov al,[bp+Color]
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mov al,[bp+Color]
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out dx,al
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dec dx
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mov al,ENABLE_SET_RESET_INDEX
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@ -301,20 +301,20 @@ _EVGALine proc near
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;
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; Get DeltaY.
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;
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mov si,[bp+Y1] ;line Y start
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mov ax,[bp+Y0] ;line Y end, used later in
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mov si,[bp+Y1] ;line Y start
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mov ax,[bp+Y0] ;line Y end, used later in
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;calculating the start address
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sub si,ax ;calculate DeltaY
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jns CalcStartAddress ;if positive, we’re set
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jns CalcStartAddress ;if positive, we’re set
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;
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; DeltaY is negative — swap coordinates so we’re always working
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; DeltaY is negative — swap coordinates so we’re always working
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; with a positive DeltaY.
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;
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mov ax,[bp+Y1] ;set line start to Y1, for use
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mov ax,[bp+Y1] ;set line start to Y1, for use
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; in calculating the start address
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mov dx,[bp+X0]
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xchg dx,[bp+X1]
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mov [bp+X0],dx ;swap X coordinates
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mov dx,[bp+X0]
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xchg dx,[bp+X1]
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mov [bp+X0],dx ;swap X coordinates
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neg si ;convert to positive DeltaY
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;
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; Calculate the starting address in display memory of the line.
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@ -329,7 +329,7 @@ CalcStartAddress:
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shl ax,1 ;Y0 * 32
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shl ax,1 ;Y0 * 64
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add di,ax ;Y0 * 80
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mov dx,[bp+X0]
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mov dx,[bp+X0]
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mov cl,dl ;set aside lower 3 bits of column for
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and cl,7 ; pixel masking
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shr dx,1
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@ -351,8 +351,8 @@ CalcStartAddress:
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;
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; Calculate DeltaX.
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;
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mov bx,[bp+X1]
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sub bx,[bp+X0]
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mov bx,[bp+X1]
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sub bx,[bp+X0]
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;
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; Handle correct one of four octants.
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;
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@ -411,8 +411,8 @@ _EVGALine endp
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<!-- END CODE //-->
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<P>An explanation of the workings of the code in Listing 35.3 would be a lengthy one, and would be redundant since the basic operation of the code in Listing 35.3 is no different from that of the code in Listing 35.1, although the implementation is much changed due to the nature of assembly language and also due to designing for speed rather than for clarity. Given that you thoroughly understand the C implementation in Listing 35.1, the assembly language implementation in Listing 35.3, which is well-commented, should speak for itself.
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</P>
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<P>One point I do want to make is that Listing 35.3 incorporates a clever notion for which credit is due Jim Mackraz, who described the notion in a letter written in response to an article I wrote long ago in the late and lamented <I>Programmer’s Journal</I>. Jim’s suggestion was that when drawing lines for which |<B>DeltaX</B>| is greater than |<B>DeltaY</B>|, bits set to 1 for each of the pixels controlled by a given byte can be accumulated in a register, rather than drawing each pixel individually. All the pixels controlled by that byte can then be drawn at once, with a single access to display memory, when all pixel processing associated with that byte has been completed. This approach can save many <B>OUT</B>s and many display memory reads and writes when drawing nearly-horizontal lines, and that’s important because EGAs and VGAs hold the CPU up for a considerable period of time on each I/O operation and display memory access.</P>
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<P>All too many PC programmers fall into the high-level-language trap of thinking that a good algorithm guarantees good performance. Not so: As our two implementations of Bresenham’s algorithm graphically illustrate (pun not originally intended, but allowed to stand once recognized), truly great PC code requires both a good algorithm <I>and</I> a good assembly implementation. In Listing 35.3, we’ve got y-oh-my, isn’t it fun?</P><P><BR></P>
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<P>One point I do want to make is that Listing 35.3 incorporates a clever notion for which credit is due Jim Mackraz, who described the notion in a letter written in response to an article I wrote long ago in the late and lamented <I>Programmer’s Journal</I>. Jim’s suggestion was that when drawing lines for which |<B>DeltaX</B>| is greater than |<B>DeltaY</B>|, bits set to 1 for each of the pixels controlled by a given byte can be accumulated in a register, rather than drawing each pixel individually. All the pixels controlled by that byte can then be drawn at once, with a single access to display memory, when all pixel processing associated with that byte has been completed. This approach can save many <B>OUT</B>s and many display memory reads and writes when drawing nearly-horizontal lines, and that’s important because EGAs and VGAs hold the CPU up for a considerable period of time on each I/O operation and display memory access.</P>
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<P>All too many PC programmers fall into the high-level-language trap of thinking that a good algorithm guarantees good performance. Not so: As our two implementations of Bresenham’s algorithm graphically illustrate (pun not originally intended, but allowed to stand once recognized), truly great PC code requires both a good algorithm <I>and</I> a good assembly implementation. In Listing 35.3, we’ve got y-oh-my, isn’t it fun?</P><P><BR></P>
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<CENTER>
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<TABLE BORDER>
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<TR>
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