左边的风声 电影:immediate operand not allowed

data1 segment
datatype union
s db "123456",13,10,'$'
a db 6 dup(?)
datatype ends
data1 ends
code1 segment
assume cs:code1,ds:data1

start:
mov ax,data1 ;初始化段寄存器
mov ds,ax
mov ah,9 ;输出s的值
mov dx,offset s
int 21h
mov a[0],'a'
mov a[1],'b'
mov a[2],'c'
mov a[3],'d'
mov a[4],'e'
mov a[5],'f'
mov ah,9 ;再次输出s的值
mov dx,offset s
int 21h
mov ah,4ch ;程序结束
int 21h

code1 ends
end start

Assembling: sa1.asm
sa1.asm(16): error A2001: immediate operand not allowed
sa1.asm(17): error A2001: immediate operand not allowed
sa1.asm(18): error A2001: immediate operand not allowed
sa1.asm(19): error A2001: immediate operand not allowed
sa1.asm(20): error A2001: immediate operand not allowed
sa1.asm(21): error A2001: immediate operand not allowed

为什么