textmode.asm

; nasm mode13.asm -fbin -omode13.com
SECTION         .text
                org     100h
start:          mov     dx, filenamez
                mov     al, 0
                call    openfile
                jnc     fileopened
                mov     dx, filenotopeneds
                call    printstr
                jmp     filedone
fileopened:     mov     dx, fileopeneds
                call    printstr
filedone:       call    waitkey
                call    exit
                jmp     $

waitkey:        mov     ax, 0
                int     16h             ; Wait for keyboard
                ret

exit:           mov     ax, 3h
                int     10h             ; Switch to text mode
                int     20h             ; Terminate
                ret

; Print $ terminated string at ds:dx
printstr:       mov     ah, 9
                int     21h
                ret

; cx = x, dx = y
; color holds palette index
putpixel:       mov     ax, 0xa000
                mov     es, ax
                mov     ax, dx
                mov     bx, dx
                shl     ax, 8
                shl     bx, 6
                add     ax, bx
                add     ax, cx
                mov     di, ax
                mov     al, byte [color]
                mov     [es:di], al
                ret

; al = access mode (0, 1, 2)/(read/write/read&write)
; ds:dx = pointer to asciiz file name
; returns ax for file handle
openfile:       mov     ah, 0x3d
                int     21h
                ret

SECTION         .bss
color:          resb   1

SECTION         .data
filenamez:      db      "mode13.asm", 0
fileopeneds:    db      "File successfully opened.$", 0
filenotopeneds: db      "Failed to open file.$", 0