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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


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Graphics Programming Black Book © 2001 Michael Abrash