
|
 |
|
Michael Abrash's Graphics Programming Black Book, Special Edition
by Michael Abrash
The Coriolis Group
ISBN: 1576101746 Pub Date: 07/01/97
|
LISTING 25.3 L25-3.ASM
; Program to illustrate operation of set/reset circuitry to force
; setting of memory that already contains data.
; By Michael Abrash.
;
stack segment para stack STACK#146;
db 512 dup(?)
stack ends
;
EGA_VIDEO_SEGMENT equ 0a000h ;EGA display memory segment
;
; EGA register equates.
;
SC_INDEX equ 3c4h ;SC index register
SC_MAP_MASK equ 2 ;SC map mask register
GC_INDEX equ 3ceh ;GC index register
GC_SET_RESET equ 0 ;GC set/reset register
GC_ENABLE_SET_RESET equ 1 ;GC enable set/reset register
;
; Macro to set indexed register INDEX of SC chip to SETTING.
;
SETSC macro INDEX, SETTING
mov dx,SC_INDEX
mov al,INDEX
out dx,al
inc dx
mov al,SETTING
out dx,al
dec dx
endm
;
; Macro to set indexed register INDEX of GC chip to SETTING.
;
SETGC macro INDEX, SETTING
mov dx,GC_INDEX
mov al,INDEX
out dx,al
inc dx
mov al,SETTING
out dx,al
dec dx
endm
;
cseg segment para public CODE#146;
assume cs:cseg
start proc near
;
; Select 640×480 graphics mode.
;
mov ax,012h
int 10h
;
mov ax,EGA_VIDEO_SEGMENT
mov es,ax ;point to video memory
;
; Draw 24 10-scan-line high horizontal bars in green, 10 scan lines apart.
;
SETSC SC_MAP_MASK,02h ;map mask setting enables only
; plane 1, the green plane
sub di,di ;start at beginning of video memory
mov al,0ffh
mov bp,24 ;# bars to draw
HorzBarLoop:
mov cx,80*10 ;# bytes per horizontal bar
rep stosb ;draw bar
add di,80*10 ;point to start of next bar
dec bp
jnz HorzBarLoop
;
; Fill screen with blue, using set/reset to force plane 0 to 1#146;s and all
; other plane to 0#146;s.
;
SETSC SC_MAP_MASK,0fh ;must set map mask to enable all
; planes, so set/reset values can
; be written to memory
SETGC GC_ENABLE_SET_RESET,0fh ;CPU data to all planes will be
; replaced by set/reset value
SETGC GC_SET_RESET,01h ;set/reset value is 0ffh for plane 0
; (the blue plane) and 0 for other
; planes
sub di,di
mov cx,80*480 ;# bytes per screen
mov al,0ffh ;since set/reset is enabled for all
; planes, the CPU data is ignored-
; only the act of writing is
; important
rep stosb ;perform fill (affects all planes)
;
; Turn off set/reset.
;
SETGC GC_ENABLE_SET_RESET,0
;
; Wait for a keystroke.
;
mov ah,1
int 21h
;
; Restore text mode.
;
mov ax,03h
int 10h
;
; Exit to DOS.
;
mov ah,4ch
int 21h
start endp
cseg ends
end start
Manipulating Planes Individually
Listing 25.4 illustrates the use of set/reset to control only some, rather than all, planes. Here, the set/reset circuitry forces plane 2 to 1 and planes 0 and 3 to 0. Because bit 1 of the Enable Set/Reset register is 0, however, set/reset does not affect plane 1; the CPU data goes unchanged to the plane 1 ALU. Consequently, the CPU data can be used to control the value written to plane 1. Given the settings of the other three planes, this means that each bit of CPU data that is 1 generates a brown pixel, and each bit that is 0 generates a red pixel. Writing alternating bytes of 07H and 0E0H, then, creates a vertically striped pattern of brown and red.
In Listing 25.4, note that the vertical bars are 10 and 6 bytes wide, and do not start on byte boundaries. Although set/reset replaces an entire byte of CPU data for a plane, the combination of set/reset for some planes and CPU data for other planes, as in the example above, can be used to control individual pixels.
LISTING 25.4 L25-4.ASM
; Program to illustrate operation of set/reset circuitry in conjunction
; with CPU data to modify setting of memory that already contains data.
; By Michael Abrash.
;
stack segment para stack STACK#146;
db 512 dup(?)
stack ends
;
EGA_VIDEO_SEGMENT equ 0a000h ;EGA display memory segment
;
; EGA register equates.
;
SC_INDEX equ 3c4h ;SC index register
SC_MAP_MASK equ 2 ;SC map mask register
GC_INDEX equ 3ceh ;GC index register
GC_SET_RESET equ 0 ;GC set/reset register
GC_ENABLE_SET_RESET equ 1 ;GC enable set/reset register
;
; Macro to set indexed register INDEX of SC chip to SETTING.
;
SETSC macro INDEX, SETTING
mov dx,SC_INDEX
mov al,INDEX
out dx,al
inc dx
mov al,SETTING
out dx,al
dec dx
endm
;
; Macro to set indexed register INDEX of GC chip to SETTING.
;
SETGC macro INDEX, SETTING
mov dx,GC_INDEX
mov al,INDEX
out dx,al
inc dx
mov al,SETTING
out dx,al
dec dx
endm
;
cseg segment para public CODE#146;
assume cs:cseg
start proc near
;
; Select 640×350 graphics mode.
;
mov ax,010h
int 10h
;
mov ax,EGA_VIDEO_SEGMENT
mov es,ax ;point to video memory
;
; Draw 18 10-scan-line high horizontal bars in green, 10 scan lines apart.
;
SETSC SC_MAP_MASK,02h;map mask setting enables only
; plane 1, the green plane
sub di,di;start at beginning of video memory
mov al,0ffh
mov bp,18;# bars to draw
HorzBarLoop:
mov cx,80*10;# bytes per horizontal bar
rep stosb;draw bar
add di,80*10;point to start of next bar
dec bp
jnz HorzBarLoop
;
; Fill screen with alternating bars of red and brown, using CPU data
; to set plane 1 and set/reset to set planes 0, 2 & 3.
;
SETSC SC_MAP_MASK,0fh ;must set map mask to enable all
; planes, so set/reset values can
; be written to planes 0, 2 & 3
; and CPU data can be written to
; plane 1 (the green plane)
SETGC GC_ENABLE_SET_RESET,0dh ;CPU data to planes 0, 2 & 3 will be
; replaced by set/reset value
SETGC GC_SET_RESET,04h ;set/reset value is 0ffh for plane 2
; (the red plane) and 0 for other
; planes
sub di,di
mov cx,80*350/2 ;# words per screen
mov ax,07e0h ;CPU data controls only plane 1;
; set/reset controls other planes
rep stosw ;perform fill (affects all planes)
;
; Turn off set/reset.
;
SETGC GC_ENABLE_SET_RESET,0
;
; Wait for a keystroke.
;
mov ah,1
int 21h
;
; Restore text mode.
;
mov ax,03h
int 10h
;
; Exit to DOS.
;
mov ah,4ch
int 21h
start endp
cseg ends
end start
|