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<HTML>
<HEAD>
<META name=vsisbn content="1576101746">
<META name=vstitle content="Michael Abrash's Graphics Programming Black Book, Special Edition">
<META name=vsauthor content="Michael Abrash">
<META name=vspublisher content="The Coriolis Group">
<META name=vspubdate content="07/01/97">
<META name=vscategory content="Web and Software Development: Game Development,Web and Software Development: Graphics and Multimedia Development">
<TITLE>Michael Abrash's Graphics Programming Black Book Special Edition: Reading VGA Memory</TITLE>
<!-- HEADER -->
<!-- Empty Reference Subhead -->
<!--ISBN=1576101746//-->
<!--TITLE=Michael Abrash's Graphics Programming Black Book Special Edition//-->
<!--AUTHOR=Michael Abrash//-->
<!--PUBLISHER=The Coriolis Group, Inc.//-->
<!--CHAPTER=28//-->
<!--PAGES=526-530//-->
<!--UNASSIGNED1//-->
<!--UNASSIGNED2//--></HEAD><BODY LINK=#0000FF ALINK=#000099 VLINK=#0000FF BGCOLOR=#FFFFFF>
<CENTER>
<TABLE BORDER>
<TR>
<TD><A HREF="28-01.html">Previous</A></TD>
<TD><A HREF="index.html">Table of Contents</A></TD>
<TD><A HREF="28-03.html">Next</A></TD>
</TR>
</TABLE>
</CENTER>
<P><BR></P>
<P><B>LISTING 28.1 L28-1.ASM</B></P>
<!-- CODE //-->
<PRE>
; Program to illustrate the use of the Read Map register in read mode 0.
; Animates by copying a 16-color image from VGA memory to system memory,
; one plane at a time, then copying the image back to a new location
; in VGA memory.
;
; By Michael Abrash
;
stacksegmentword stack 'STACK'
db512 dup (?)
stackends
;
datasegment word 'DATA'
IMAGE_WIDTHEQU 4 ;in bytes
IMAGE_HEIGHT EQU 32 ;in pixels
LEFT_BOUND EQU 10 ;in bytes
RIGHT_BOUND EQU 66 ;in bytes
VGA_SEGMENT EQU 0a000h
SCREEN_WIDTH EQU 80 ;in bytes
SC_INDEX EQU 3c4h ;Sequence Controller Index register
GC_INDEX EQU 3ceh ;Graphics Controller Index register
MAP_MASK EQU 2 ;Map Mask register index in SC
READ_MAP EQU 4 ;Read Map register index in GC
;
; Base pattern for 16-color image.
;
PatternPlane0 label byte
db 32 dup (0ffh,0ffh,0,0)
PatternPlane1 labelbyte
db 32 dup (0ffh,0,0ffh,0)
PatternPlane2 labelbyte
db 32 dup (0f0h,0f0h,0f0h,0f0h)
PatternPlane3 labelbyte
db 32 dup (0cch,0cch,0cch,0cch)
;
; Temporary storage for 16-color image during animation.
;
ImagePlane0 db 32*4 dup (?)
ImagePlane1 db 32*4 dup (?)
ImagePlane2 db 32*4 dup (?)
ImagePlane3 db 32*4 dup (?)
;
; Current image location &amp direction.
;
ImageX dw 40 ;in bytes
ImageY dw 100 ;in pixels
ImageXDirection dw 1 ;in bytes
dataends
;
code segment word 'CODE'
assume cs:code,ds:data
Start proc near
cld
mov ax,data
mov ds,ax
;
; Select graphics mode 10h.
;
mov ax,10h
int 10h
;
; Draw the initial image.
;
mov si,offset PatternPlane0
call DrawImage
;
; Loop to animate by copying the image from VGA memory to system memory,
; erasing the image, and copying the image from system memory to a new
; location in VGA memory. Ends when a key is hit.
;
AnimateLoop:
;
; Copy the image from VGA memory to system memory.
;
mov di,offset ImagePlane0
call GetImage
;
; Clear the image from VGA memory.
;
call EraseImage
;
; Advance the image X coordinate, reversing direction if either edge
; of the screen has been reached.
;
mov ax,[ImageX]
cmp ax,LEFT_BOUND
jz ReverseDirection
cmp ax,RIGHT_BOUND
jnz SetNewX
ReverseDirection:
neg [ImageXDirection]
SetNewX:
add ax,[ImageXDirection]
mov [ImageX],ax
;
; Draw the image by copying it from system memory to VGA memory.
;
mov si,offset ImagePlane0
call DrawImage
;
; Slow things down a bit for visibility (adjust as needed).
;
mov cx,0
DelayLoop:
loop DelayLoop
;
; See if a key has been hit, ending the program.
;
mov ah,1
int 16h
jz AnimateLoop
;
; Clear the key, return to text mode, and return to DOS.
;
sub ah,ah
int 16h
mov ax,3
int 10h
mov ah,4ch
int 21h
Startendp
;
; Draws the image at offset DS:SI to the current image location in
; VGA memory.
;
DrawImageprocnear
mov ax,VGA_SEGMENT
mov es,ax
call GetImageOffset ;ES:DI is the destination address for the
; image in VGA memory
mov dx,SC_INDEX
mov al,1 ;do plane 0 first
DrawImagePlaneLoop:
push di ;image is drawn at the same offset in
; each plane
push ax ;preserve plane select
mov al,MAP_MASK ;Map Mask index
out dx,al ;point SC Index to the Map Mask register
pop ax ;get back plane select
inc dx ;point to SC index register
out dx,al ;set up the Map Mask to allow writes to
; the plane of interest
dec dx ;point back to SC Data register
mov bx,IMAGE_HEIGHT ;# of scan lines in image
DrawImageLoop:
mov cx,IMAGE_WIDTH ;# of bytes across image
rep movsb
add di,SCREEN_WIDTH-IMAGE_WIDTH
;point to next scan line of image
dec bx ;any more scan lines?
jnz DrawImageLoop
pop di ;get back image start offset in VGA memory
shl al,1 ;Map Mask setting for next plane
cmp al,10h ;have we done all four planes?
jnz DrawImagePlaneLoop
ret
DrawImageendp
;
; Copies the image from its current location in VGA memory into the
; buffer at DS:DI.
;
GetImage proc near
mov si,di ;move destination offset into SI
call GetImageOffset ;DI is offset of image in VGA memory
xchg si,di ;SI is offset of image, DI is destination offset
push ds
pop es ;ES:DI is destination
mov ax,VGA_SEGMENT
mov ds,ax ;DS:SI is source
;
mov dx,GC_INDEX
sub al,al;do plane 0 first
GetImagePlaneLoop:
push si ;image comes from same offset in each plane
push ax ;preserve plane select
mov al,READ_MAP ;Read Map index
out dx,al ;point GC Index to Read Map register
pop ax ;get back plane select
inc dx ;point to GC Index register
out dx,al ;set up the Read Map to select reads from
; the plane of interest
dec dx ;point back to GC data register
mov bx,IMAGE_HEIGHT ;# of scan lines in image
GetImageLoop:
mov cx,IMAGE_WIDTH ;# of bytes across image
rep movsb
add si,SCREEN_WIDTH-IMAGE_WIDTH
;point to next scan line of image
dec bx ;any more scan lines?
jnz GetImageLoop
pop si ;get back image start offset
inc al ;Read Map setting for next plane
cmp al,4 ;have we done all four planes?
jnz GetImagePlaneLoop
push es
pop ds ;restore original DS
ret
GetImageendp
;
; Erases the image at its current location.
;
EraseImage proc near
mov dx,SC_INDEX
mov al,MAP_MASK
out dx,al ;point SC Index to the Map Mask register
inc dx ;point to SC Data register
mov al,0fh
out dx,al ;set up the Map Mask to allow writes to go to
; all 4 planes
mov ax,VGA_SEGMENT
mov es,ax
call GetImageOffset ;ES:DI points to the start address
; of the image
sub al,al ;erase with zeros
mov bx,IMAGE_HEIGHT ;# of scan lines in image
EraseImageLoop:
mov cx,IMAGE_WIDTH ;# of bytes across image
rep stosb
add di,SCREEN_WIDTH-IMAGE_WIDTH
;point to next scan line of image
dec bx ;any more scan lines?
jnz EraseImageLoop
ret
EraseImage endp
;
; Returns the current offset of the image in the VGA segment in DI.
;
GetImageOffset proc near
mov ax,SCREEN_WIDTH
mul [ImageY]
add ax,[ImageX]
mov di,ax
ret
GetImageOffset endp
code ends
end Start
</PRE>
<!-- END CODE //-->
<P><BR></P>
<CENTER>
<TABLE BORDER>
<TR>
<TD><A HREF="28-01.html">Previous</A></TD>
<TD><A HREF="index.html">Table of Contents</A></TD>
<TD><A HREF="28-03.html">Next</A></TD>
</TR>
</TABLE>
</CENTER>
<hr width="90%" size="1" noshade>
<div align="center">
<font face="Verdana,sans-serif" size="1">Graphics Programming Black Book &copy; 2001 Michael Abrash</font>
</div>
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