CD Actual 15/CDACTUAL15.iso/cdactual/program/asm/PJGRAPH.ZIP/CHAP08.2

Text File | 1989-09-26 | 5.5 KB | 182 lines

; Program to demonstrate the 32 text pages of the VGA/EGA.
;
; To demonstrate the 32 text pages of the VGA, set
; ADAPTER_IS_VGA to 1.
;
; To demonstrate the 32 text pages of the EGA, set
; ADAPTER_IS_VGA to 0.
;
; By Michael Abrash  5/22/88
; Updated 6/29/89.
;

ADAPTER_IS_VGA          equ     0       ;set to 1 to assemble for VGA,
                                        ; 0 to assemble for EGA
PAGE_LENGTH             equ     1000h   ;number of bytes per page
MISCELLANEOUS_OUTPUT_W  equ     3c2h    ;Miscellaneous Output write port
MISCELLANEOUS_OUTPUT_R  equ     3cch    ;Miscellaneous Output read port
                                        ; (available only on VGA)
GC_INDEX                equ     3ceh
GC_MISCELLANEOUS        equ     6
CRTC_INDEX              equ     3d4h
CRTC_START_ADDRESS_HIGH equ     0ch
CRTC_START_ADDRESS_LOW  equ     0dh

stackseg        segment para stack 'STACK'
        db      512 dup (?)
stackseg        ends

code    segment para public 'CODE'
        assume  cs:code, ds:nothing
start   proc    near
;
; Set to text mode.
;
        mov     ax,3
        int     10h
;
; Relocate display memory to A000 by reprogramming
; the Graphics Controller's Miscellaneous register.
;
        mov     dx,GC_INDEX
        mov     al,GC_MISCELLANEOUS
        out     dx,al
        inc     dx
        mov     al,06h
        out     dx,al
;
; Point to display memory.
;
        mov     ax,0a000h
        mov     es,ax
;
; Fill text pages 0-15 (the pages that are accessible when
; the Page bit is 1, the setting after a mode set is performed).
; (Page 0 is filled with "A", page 1 with "B", and so on.)
;
        sub     ax,ax   ;start with page 0
        mov     bl,'A'  ;base letter for pages 0-15
FillPages0_15Loop:
        push    ax
        call    FillPage
        pop     ax
        inc     ax
        cmp     ax,16
        jb6     FillPages0_15Loop
;
; Set the Page bit to 0.
;
if ADAPTER_IS_VGA
        mov     dx,MISCELLANEOUS_OUTPUT_R
        in      al,dx
        and     al,not 20h      ;Page bit (bit 5) -> 0
        mov     dx,MISCELLANEOUS_OUTPUT_W
        out     dx,al
else
        mov     dx,MISCELLANEOUS_OUTPUT_W
        mov     al,87h          ;Page bit (bit 5) -> 0
        out     dx,al
endif
;
; Now fill text pages 16-31 (the pages that are accessible when
; the Page bit is 0).
; (Page 16 is filled with "Q", page 17 with "R", and so on.)
;
        sub     ax,ax   ;start with page 0 in the second set of
                        ; 16 pages; pages in this set are accessed
                        ; exactly as pages 0-15 were
        mov     bl,'Q'  ;base letter for pages 16-31
FillPages16_31Loop:
        push    ax
        call    FillPage
        pop     ax
        inc     ax
        cmp     ax,16
        jb      FillPages16_31Loop
;
; Display each of the text pages in turn, waiting for a key
; before going to the next page. Note that all 32 pages can
; be displayed without changing the Page bit; the Page bit
; only affects CPU accesses, not video data fetching.
;
        sub     ax,ax
ShowPageLoop:
        push    ax
        call    ShowPage
        call    WaitKey
        pop     ax
        inc     ax
        cmp     ax,32
        jb      ShowPageLoop
;
; Done-perform a mode set to reset all paging and clear
; the screen, then return to DOS.
;
        mov     ax,3
        int     10h
        mov     ah,4ch
        int     21h
start   endp
;
; Fills a text page with a corresponding character.
;
; Input:
;       AX = text page
;       BL = character for text page 0 (each text page
;               is filled with character BL + text page #)
;
FillPage        proc    near
        push    ax      ;save the page #
        mov     dx,PAGE_LENGTH
        mul     dx      ;calculate the start offset of the page
                        ; in the display memory segment
        mov     di,ax
        pop     ax      ;get back the page #
        add     al,bl   ;convert it to a unique letter
        mov     ah,7    ;attribute is white
        mov     cx,PAGE_LENGTH/2 ;page length as measured
                                 ; in character/attribute pairs
        cld
        rep     stosw   ;fill the page
        ret
FillPage        endp
;
; Programs the CRTC to display the desired text page.
;
; Input: AX = text page.
;
ShowPage        proc    near
        mov     dx,PAGE_LENGTH/2
        mul     dx              ;start offset of the pages as
                                ; measured in character/attribute
                                ; pairs (which is how the CRTC
                                ; counts in text mode)
        mov     dx,CRTC_INDEX   ;point to the CRTC Index register
        push    ax
        mov     al,CRTC_START_ADDRESS_HIGH
        out     dx,al
        inc     dx              ;point to the CRTC Data register
        mov     al,ah
        out     dx,al           ;set the high start address byte
        dec     dx              ;point to the CRTC Index register
        mov     al,CRTC_START_ADDRESS_LOW
        out     dx,al
        inc     dx              ;point to the CRTC Data register
        pop     ax
        out     dx,al           ;set the low start address byte
        ret
ShowPage        endp
;
; Waits until a key is pressed.
;
WaitKey proc    near
        mov     ah,1
        int     16h
        jz      WaitKey
        sub     ah,ah
        int     16h     ;clear the key
        ret
WaitKey endp

code    ends
        end     start