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27-02.md
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@ -4,7 +4,7 @@
#### Copying Chunky Bitmaps to VGA Memory Using Write Mode 2 {#Heading5}
Let’s take a look at two examples of write mode 2 in action. Listing
Let's take a look at two examples of write mode 2 in action. Listing
27.1 presents a program that uses write mode 2 to copy a graphics image
in chunky format to the VGA. In chunky format adjacent bits in a single
byte make up each pixel: mode 4 of the CGA, EGA, and VGA is a
@ -25,7 +25,7 @@ discussed in Chapter 25, the bit mask makes it possible to narrow the
effect of the CPU write down to a single pixel.
Given the above, conversion of a chunky 4-bit-per-pixel bitmap to the
VGA’s planar format in write mode 2 is trivial. First, the Bit Mask
VGA's planar format in write mode 2 is trivial. First, the Bit Mask
register is set to allow only the VGA display memory bits corresponding
to the leftmost chunky pixel of the two stored in the first chunky
bitmap byte to be modified. Next, the destination byte in display memory
@ -45,14 +45,14 @@ image.
**LISTING 27.1 L27-1.ASM**
; Program to illustrate one use of write mode 2 of the VGA and EGA by
; animating the image of an “A” drawn by copying it from a chunky
; animating the image of an "A" drawn by copying it from a chunky
; bit-map in system memory to a planar bit-map in VGA or EGA memory.
;
; Assemble with MASM or TASM
;
; By Michael Abrash
;
Stack segment para stack ‘STACK’
Stack segment para stack ‘STACK'
db 512 dup(0)
Stack ends
@ -64,15 +64,15 @@ image.
GRAPHICS_MODE equ 5 ;index of Graphics Mode reg
BIT_MASKequ 8 ;index of Bit Mask reg
Data segment para common ‘DATA’
Data segment para common ‘DATA'
;
; Current location of “A” as it is animated across the screen.
; Current location of "A" as it is animated across the screen.
;
CurrentX dw ?
CurrentY dw ?
RemainingLength dw ?
;
; Chunky bit-map image of a yellow “A” on a bright blue background
; Chunky bit-map image of a yellow "A" on a bright blue background
;
AImage label byte
dw 13, 13 ;width, height in pixels
@ -91,7 +91,7 @@ image.
db 000h, 000h, 000h, 000h, 000h, 000h, 000h
Data ends
Code segment para public ‘CODE’
Code segment para public ‘CODE'
assume cs:Code, ds:Data
Start proc near
mov ax,Data
@ -105,7 +105,7 @@ image.
mov [CurrentY],200
mov [RemainingLength],600 ;move 600 times
;
; Animate, repeating RemainingLength times. It’s unnecessary to erase
; Animate, repeating RemainingLength times. It's unnecessary to erase
; the old image, since the one pixel of blank fringe around the image
; erases the part of the old image not overlapped by the new image.
;
@ -113,10 +113,10 @@ image.
mov bx,[CurrentX]
mov cx,[CurrentY]
mov si,offset AImage
call DrawFromChunkyBitmap ;draw the “A” image
call DrawFromChunkyBitmap ;draw the "A" image
inc [CurrentX] ;move one pixel to the right
mov cx,0 ;delay so we don’t move the
mov cx,0 ;delay so we don't move the
DelayLoop: ; image too fast; adjust as
; needed
loop DelayLoop
@ -207,7 +207,7 @@ image.
inc dx ; to the Bit Mask register
RowLoop:
push ax ;preserve the left column’s bit mask
push ax ;preserve the left column's bit mask
push cx ;preserve the width
push di ;preserve the destination offset
@ -251,7 +251,7 @@ image.
CheckMoreScanLines:
pop di ;get back the destination offset
pop cx ;get back the width
pop ax ;get back the left column’s bit mask
pop ax ;get back the left column's bit mask
add di,SCREEN_WIDTH_IN_BYTES
;point to the start of the next scan
; line of the image