; nasmw boot.asm -f bin -o boot.bin
; partcopy boot.bin 0 200 -f0

[ORG 0x7c00]      ; add to offsets
xor ax, ax    ; make it zero
mov ds, ax   ; DS=0
mov ss, ax   ; stack starts at 0
mov sp, 0x9c00   ; 200h past code start

mov ax, 0xb800   ; text video memory
mov es, ax

mov si, msg   ; show text string
call sprint

mov ax, 0xb800   ; look at video mem
mov gs, ax
mov bx, 0x0000   ; ‘W’=57 attrib=0F
mov ax, [gs:bx]

mov  word [reg16], ax ;look at register
call printreg16

hang:
jmp hang

;———————-
dochar:   call cprint         ; print one character
sprint:   lodsb      ; string char to AL
cmp al, 0
jne dochar   ; else, we’re done
add byte [ypos], 1   ;down one row
mov byte [xpos], 0   ;back to left
ret

cprint:   mov ah, 0x0F   ; attrib = white on black
mov cx, ax    ; save char/attribute
movzx ax, byte [ypos]
mov dx, 160   ; 2 bytes (char/attrib)
mul dx      ; for 80 columns
movzx bx, byte [xpos]
shl bx, 1    ; times 2 to skip attrib

mov di, 0        ; start of video memory
add di, ax      ; add y offset
add di, bx      ; add x offset

mov ax, cx        ; restore char/attribute
stosw              ; write char/attribute
add byte [xpos], 1  ; advance to right

ret

;————————————

printreg16:
mov di, outstr16
mov ax, [reg16]
mov si, hexstr
mov cx, 4   ;four places
hexloop:
rol ax, 4   ;leftmost will
mov bx, ax   ; become
and bx, 0x0f   ; rightmost
mov bl, [si + bx];index into hexstr
mov [di], bl
inc di
dec cx
jnz hexloop

mov si, outstr16
call sprint

ret

;————————————

xpos   db 0
ypos   db 0
hexstr   db ‘0123456789ABCDEF’
outstr16   db ‘0000’, 0  ;register value string
reg16   dw    0  ; pass values to printreg16
msg   db “What are you doing, Dave?”, 0
times 510-($-$$) db 0
db 0x55
db 0xAA
;==================================