how to print char on the screen

Question

I try to make '#' left side of the screen and the other side will be '$' but it's not stop on 25*40

.MODEL TINY

.CODE

.STARTUP

  CLD
  MOV AX, 0B800H ;for open screen
  MOV ES, AX    
  MOV DI, 0
  MOV CX, 25*40  ;try to divide screen 
  MOV AX, 5C23H
  REP STOSW

  MOV AX, 0B800H
  MOV DS, AX
  MOV SI,0  
  MOV CX, 25*40
  MOV AX, 6F24H
  REP STOSW

.EXIT
END

Answer 1

.MODEL TINY

.CODE
.STARTUP

  CLD
  MOV AX, 0B800H ;for open screen
  MOV ES, AX
  XOR DI, DI     ; DI = 0
  MOV DX, 25     ; lines counter
line_loop:
  MOV CX, 40
  MOV AX, 5C23H
  REP STOSW
  MOV CX, 40
  MOV AX, 6F24H
  REP STOSW
  DEC DX
  JNZ line_loop

.EXIT
END

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For fun and to exercise the power of xor for you, shorter variant of code (try to step over it in debugger or head and understand how it works):

  ...
  XOR DI, DI     ; DI = 0
  MOV DX, 50     ; 50 half-lines to fill
  MOV AX, 5C23H  ; start with '#'
half_line_loop:
  MOV CX, 40
  REP STOSW
  XOR AX, 3307h  ; 5C23 xor 3307 = 6F24, 6F24 xor 3307 = 5C23
  DEC DX
  JNZ half_line_loop
  ...