; this example demonstrates the use of pages (double-buffering). 
; this program uses first 4 pages of video memory by writing 
; some random data to them, and waits for any key press, 
; pressing any key will show all pages one after another. 

name "pages"

org 100h

; set video mode: 
; text mode 80x25, 16 colors, 8 pages 
mov     ah, 0
mov     al, 3
int     10h

; ======== print chars on different pages: 

mov al, '0' ; char to print. 
mov bh, 0 ; page number. 
mov bl, 0000_1110b  ; attributes. 

do_print:

mov cx, 200         ; number of chars to write. 
mov ah, 09h         ; write char function. 
int 10h

inc al  ; next char. 
inc bh  ; next page. 

rol bl, 1  ; select another attribute. 

cmp al, '4'
jb  do_print


; ===== modify pages by writing directly 
;       to video memory: 

push    ds

mov ax, 0b800h
mov ds, ax

; first byte is a color attribute 
; (0f2h = white background, green char), 
; second byte is an ascii code 
; (41h = 'a', 42h = 'b', 43h = 'c'...) 

mov di, 0           ; page 0. 
mov w.[di], 0f241h

add di, 4096        ; page 1. 
mov w.[di], 0f242h

add di, 4096        ; page 2. 
mov w.[di], 0f243h

add di, 4096        ; page 3. 
mov w.[di], 0f244h

pop ds

; ======= show pages one after another: 

mov  al, 0   ; page number 

show_next_page:

mov  ah, 05h ; change page function. 
int  10h

push ax
; wait for any key: 
xor  ax, ax
int  16h
pop  ax

inc  al

cmp  al, 4
jb   show_next_page




ret ; return control to operating system.