Getting errors while trying to output a string and set background color using TASM and DOSBox

I'm testing a TASM program to set the background color and output a string. But I'm having errors.

.model small
.stack 100h

.data
    prompt db 13, 10, 'Hello Gaiz!', '$'

.code
    start:

        call color    ; Call the color procedure to set the background color
        call output

        mov ax, 4C00h  ; Exit program
        int 21h
        
output proc
    mov ah, 09h
    lea dx, prompt
    int 21h
    ret
output endp

color proc
    MOV AH, 06h    ; Scroll up function
    XOR AL, AL     ; Clear entire screen
    XOR CX, CX     ; Upper left corner CH=row, CL=column
    MOV DX, 184FH  ; lower right corner DH=row, DL=column 
    MOV BH, 42H   ; Red and Green
    INT 10H
    ret
color endp

end start

When run, random characters and not properly executed.

> mov ah, 09h
> lea dx, prompt
> int 21h

The pointer that you pass to DOS is actually DS:DX. Sadly, you forgot to initialize the DS segment register at program start. For this .EXE program using the small model, DS by default will be pointing at the PSP (Program Segment Prefix). Just add next to your program:

.code
    start:
        mov     ax, @data     ; Setup DS
        mov     ds, ax        ;
        call    color

Alternatively, keep it much simpler

Start developing .COM programs. No troubles with setting up segment registers as they start out equal to each other (CS=DS=ES=SS):

  ORG 256

  call color      ; Call the color procedure to set the background color
  call output
  mov  ax, 4C00h  ; Exit program
  int  21h
        
output:
  mov  ah, 09h
  lea  dx, prompt
  int  21h
  ret

color:
  MOV  AH, 06h    ; Scroll up function
  XOR  AL, AL     ; Clear entire screen
  XOR  CX, CX     ; Upper left corner CH=row, CL=column
  MOV  DX, 184FH  ; lower right corner DH=row, DL=column 
  MOV  BH, 42H    ; Red and Green
  INT  10H
  ret

prompt db 13, 10, 'Hello Gaiz!', '$'